Fix spelling mistake in README

Also, the executable component uses a dependency that keeps having security vulnerabilities.

.gitignore

@@ -2,7 +2,10 @@
 .settings
 .project
 target/
+library/target/
+executable/target/
 *.iml
 .idea
-template.yaml
+library/dependency-reduced-pom.xml
-dependency-reduced-pom.xml
+executable/dependency-reduced-pom.xml
+executable/template.yaml

README.md

@@ -1,14 +1,20 @@
 # README
 
-This Java software package contains the backend classes used in the R package [largeRCRF](https://github.com/jatherrien/largeRCRF). 
+This repository contains the largeRCRF Java library, containing all of the logic used for training the random forests. This provides the Jar file used in the R package [largeRCRF](https://github.com/jatherrien/largeRCRF).
 
-On its own it's not useful, but you're free to integrate it into your own projects (as long as you follow the terms of the GPL-3 license), or extend it. More documentation will be added later on how to extend it, but for now if you want an idea I suggest you take a look at the `MeanResponseCombiner` and `WeightedVarianceSplitFinder` classes, which is a small example of a regression random forest implementation. 
+Most users interested in training random competing risks forests should use the [R package component](https://github.com/jatherrien/largeRCRF); the content in this repository is only useful for advanced users. 
 
-If you've made an extension or modification to the package and would like to integrate it into the R package component, build the project in Maven with `mvn clean package` and copy the `largeRCRF-1.0-SNAPSHOT.jar` file now found in the `target/` directory into the `inst/java/` directory for the R package (delete the previous jar file). Then just build the R package, possibly with your modifications in the R code, with `R> devtools::build()`.
+## License
 
-If you have any questions on how to integrate this code with your own, how to integrate it with the R project, or anything else related to this project, please feel free to either [email me](mailto:joelt@sfu.ca) or create an Issue. 
+You're free to use / modify / redistribute the project, as long as you follow the terms of the GPL-3 license. 
 
-A small project allowing this code to be called directly outside of R will be released soon.
+## Extending
+
+Documentation on how to extend the library to add support for other types of random forests will eventually be added, but for now if you're interested in that I suggest you take a look at the `MeanResponseCombiner` and `WeightedVarianceSplitFinder` classes to see how some basic regression random forests were introduced. 
+
+If you've made a modification to the package and would like to integrate it into the R package component, build the project in Maven with `mvn clean package`, then just copy `target/largeRCRF-library-1.0-SNAPSHOT.jar` into the `inst/java/` directory for the R package, replacing the previous Jar file there. Then build the R package, possibly with your modifications to the code there, with `R> devtools::build()`.
+
+Please don't take the current lack of documentation as a sign that I oppose others extending or modifying the project; if you have any questions on running, extending, integrating with R, or anything else related to this project, please don't hesitate to either [email me](mailto:joelt@sfu.ca) or create an Issue. Most likely my answers to your questions will end up forming the basis for any documentation written. 
 
 ## System Requirements
 
@@ -17,5 +23,3 @@ You need:
 * A Java runtime version 1.8 or greater
 * Maven to build the project
 
-
-

pom.xml

@@ -4,8 +4,8 @@
          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
 
-    <groupId>ca.joeltherrien</groupId>
+    <groupId>ca.joeltherrien.ca</groupId>
-    <artifactId>largeRCRF</artifactId>
+    <artifactId>largeRCRF-library</artifactId>
     <version>1.0-SNAPSHOT</version>
 
     <properties>
@@ -60,7 +60,8 @@
         <plugins>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-shade-plugin</artifactId>
+		<artifactId>maven-shade-plugin</artifactId>
+		<version>3.2.1</version>
                 <executions>
                     <execution>
                         <phase>package</phase>
@@ -85,7 +86,7 @@
             <configuration>
                 <rulesets>
                     <!-- Custom local file system rule set -->
-                    <ruleset>${project.basedir}/pmd-rules.xml</ruleset>
+                    <ruleset>pmd-rules.xml</ruleset>
                 </rulesets>
             </configuration>
         </plugin>
@@ -94,4 +95,4 @@
 
 
 
-</project>
+</project>

src/main/java/ca/joeltherrien/randomforest/CovariateRow.java

@@ -21,12 +21,13 @@ import lombok.Getter;
 import lombok.RequiredArgsConstructor;
 
 import java.io.Serializable;
-import java.util.HashMap;
+import java.util.*;
-import java.util.List;
+import java.util.stream.Collectors;
-import java.util.Map;
 
 @RequiredArgsConstructor
-public class CovariateRow implements Serializable {
+public class CovariateRow implements Serializable, Cloneable {
+
+    private static final long serialVersionUID = 1L;
 
     private final Covariate.Value[] valueArray;
 
@@ -46,6 +47,14 @@ public class CovariateRow implements Serializable {
         return "CovariateRow " + this.id;
     }
 
+    @Override
+    public CovariateRow clone() {
+        // shallow clone, which is fine. I want a new array, but the values don't need to be copied
+        final Covariate.Value[] copyValueArray = this.valueArray.clone();
+
+        return new CovariateRow(copyValueArray, this.id);
+    }
+
     public static CovariateRow createSimple(Map<String, String> simpleMap, List<Covariate> covariateList, int id){
         final Covariate.Value[] valueArray = new Covariate.Value[covariateList.size()];
         final Map<String, Covariate> covariateMap = new HashMap<>();
@@ -64,4 +73,27 @@ public class CovariateRow implements Serializable {
         return new CovariateRow(valueArray, id);
     }
 
+    /**
+     * Used for variable importance; takes a List of CovariateRows and permute one of the Covariates.
+     *
+     * @param covariateRows The List of CovariateRows to scramble. Note that the originals won't be modified.
+     * @param covariateToScramble The Covariate to scramble on.
+     * @param random The source of randomness to use. If not present, one will be created.
+     * @return A List of CovariateRows where the specified covariate was scrambled. These are different objects from the ones provided.
+     */
+    public static List<CovariateRow> scrambleCovariateValues(List<? extends CovariateRow> covariateRows, Covariate covariateToScramble, Optional<Random> random){
+        final List<CovariateRow> permutedCovariateRowList = new ArrayList<>(covariateRows);
+        Collections.shuffle(permutedCovariateRowList, random.orElse(new Random())); // without replacement
+
+        final List<CovariateRow> clonedRowList = covariateRows.stream().map(CovariateRow::clone).collect(Collectors.toList());
+
+        final int covariateToScrambleIndex = covariateToScramble.getIndex();
+        for(int i=0; i < covariateRows.size(); i++){
+            clonedRowList.get(i).valueArray[covariateToScrambleIndex] = permutedCovariateRowList.get(i).valueArray[covariateToScrambleIndex];
+        }
+
+        return clonedRowList;
+
+    }
+
 }

src/main/java/ca/joeltherrien/randomforest/Row.java

@@ -32,9 +32,6 @@ public class Row<Y> extends CovariateRow {
 	    this.response = response;
     }
 
-
-
-
 	public Y getResponse() {
 		return this.response;
 	}

src/main/java/ca/joeltherrien/randomforest/covariates/Covariate.java

@@ -49,6 +49,8 @@ public interface Covariate<V> extends Serializable, Comparable<Covariate> {
         return getIndex() - other.getIndex();
     }
 
+    boolean haveNASplitPenalty();
+
     interface Value<V> extends Serializable{
 
         Covariate<V> getParent();

src/main/java/ca/joeltherrien/randomforest/covariates/bool/BooleanCovariate.java

@@ -25,9 +25,12 @@ import lombok.Getter;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Random;
+import java.util.stream.Collectors;
 
 public final class BooleanCovariate implements Covariate<Boolean> {
 
+    private static final long serialVersionUID = 1L;
+
     @Getter
     private final String name;
 
@@ -38,14 +41,26 @@ public final class BooleanCovariate implements Covariate<Boolean> {
 
     private final BooleanSplitRule splitRule; // there's only one possible rule for BooleanCovariates.
 
-    public BooleanCovariate(String name, int index){
+    private final boolean haveNASplitPenalty;
+    @Override
+    public boolean haveNASplitPenalty(){
+        // penalty would add worthless computational time if there are no NAs
+        return hasNAs && haveNASplitPenalty;
+    }
+
+    public BooleanCovariate(String name, int index, boolean haveNASplitPenalty){
         this.name = name;
         this.index = index;
-        splitRule = new BooleanSplitRule(this);
+        this.splitRule = new BooleanSplitRule(this);
+        this.haveNASplitPenalty = haveNASplitPenalty;
     }
 
     @Override
     public <Y> Iterator<Split<Y, Boolean>> generateSplitRuleUpdater(List<Row<Y>> data, int number, Random random) {
+        if(hasNAs){
+            data = data.stream().filter(row -> !row.getValueByIndex(index).isNA()).collect(Collectors.toList());
+        }
+
         return new SingletonIterator<>(this.splitRule.applyRule(data));
     }
 
@@ -84,6 +99,8 @@ public final class BooleanCovariate implements Covariate<Boolean> {
 
     public class BooleanValue implements Value<Boolean>{
 
+        private static final long serialVersionUID = 1L;
+
         private final Boolean value;
 
         private BooleanValue(final Boolean value){

src/main/java/ca/joeltherrien/randomforest/covariates/bool/BooleanSplitRule.java

@@ -21,6 +21,8 @@ import ca.joeltherrien.randomforest.covariates.SplitRule;
 
 public class BooleanSplitRule implements SplitRule<Boolean> {
 
+    private static final long serialVersionUID = 1L;
+
     private final int parentCovariateIndex;
 
     public BooleanSplitRule(BooleanCovariate parent){

src/main/java/ca/joeltherrien/randomforest/covariates/factor/FactorCovariate.java

@@ -23,9 +23,12 @@ import lombok.EqualsAndHashCode;
 import lombok.Getter;
 
 import java.util.*;
+import java.util.stream.Collectors;
 
 public final class FactorCovariate implements Covariate<String> {
 
+    private static final long serialVersionUID = 1L;
+
     @Getter
     private final String name;
 
@@ -38,8 +41,15 @@ public final class FactorCovariate implements Covariate<String> {
 
     private boolean hasNAs;
 
+    private final boolean haveNASplitPenalty;
+    @Override
+    public boolean haveNASplitPenalty(){
+        // penalty would add worthless computational time if there are no NAs
+        return hasNAs && haveNASplitPenalty;
+    }
 
-    public FactorCovariate(final String name, final int index, List<String> levels){
+
+    public FactorCovariate(final String name, final int index, List<String> levels, final boolean haveNASplitPenalty){
         this.name = name;
         this.index = index;
         this.factorLevels = new HashMap<>();
@@ -61,12 +71,22 @@ public final class FactorCovariate implements Covariate<String> {
         this.numberOfPossiblePairings = numberOfPossiblePairingsTemp-1;
 
         this.naValue = new FactorValue(null);
+
+        this.haveNASplitPenalty = haveNASplitPenalty;
     }
 
 
 
     @Override
     public <Y> Iterator<Split<Y, String>> generateSplitRuleUpdater(List<Row<Y>> data, int number, Random random) {
+        if(hasNAs()){
+            data = data.stream().filter(row -> !row.getCovariateValue(this).isNA()).collect(Collectors.toList());
+        }
+
+        if(number == 0){ // nsplit = 0 => try every possibility, although we limit it to the number of observations.
+            number = data.size();
+        }
+
         final Set<Split<Y, String>> splits = new HashSet<>();
 
         // This is to ensure we don't get stuck in an infinite loop for small factors
@@ -122,6 +142,8 @@ public final class FactorCovariate implements Covariate<String> {
     @EqualsAndHashCode
     public final class FactorValue implements Covariate.Value<String>{
 
+        private static final long serialVersionUID = 1L;
+
         private final String value;
 
         private FactorValue(final String value){

src/main/java/ca/joeltherrien/randomforest/covariates/factor/FactorSplitRule.java

@@ -25,6 +25,8 @@ import java.util.Set;
 @EqualsAndHashCode
 public final class FactorSplitRule implements SplitRule<String> {
 
+    private static final long serialVersionUID = 1L;
+
     private final int parentCovariateIndex;
     private final Set<String> leftSideValues;
 

src/main/java/ca/joeltherrien/randomforest/covariates/numeric/NumericCovariate.java

@@ -37,6 +37,8 @@ import java.util.stream.Stream;
 @ToString
 public final class NumericCovariate implements Covariate<Double> {
 
+    private static final long serialVersionUID = 1L;
+
     @Getter
     private final String name;
 
@@ -45,6 +47,13 @@ public final class NumericCovariate implements Covariate<Double> {
 
     private boolean hasNAs = false;
 
+    private final boolean haveNASplitPenalty;
+    @Override
+    public boolean haveNASplitPenalty(){
+        // penalty would add worthless computational time if there are no NAs
+        return hasNAs && haveNASplitPenalty;
+    }
+
     @Override
     public <Y> NumericSplitRuleUpdater<Y> generateSplitRuleUpdater(List<Row<Y>> data, int number, Random random) {
         Stream<Row<Y>> stream = data.stream();
@@ -122,6 +131,8 @@ public final class NumericCovariate implements Covariate<Double> {
     @EqualsAndHashCode
     public class NumericValue implements Covariate.Value<Double>{
 
+        private static final long serialVersionUID = 1L;
+
         private final Double value; // may be null
 
         private NumericValue(final Double value){

src/main/java/ca/joeltherrien/randomforest/covariates/numeric/NumericSplitRule.java

@@ -23,10 +23,12 @@ import lombok.EqualsAndHashCode;
 @EqualsAndHashCode
 public class NumericSplitRule implements SplitRule<Double> {
 
+    private static final long serialVersionUID = 1L;
+
     private final int parentCovariateIndex;
     private final double threshold;
 
-    NumericSplitRule(NumericCovariate parent, final double threshold){
+    public NumericSplitRule(NumericCovariate parent, final double threshold){
         this.parentCovariateIndex = parent.getIndex();
         this.threshold = threshold;
     }

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/CompetingRiskFunctions.java

@@ -26,6 +26,8 @@ import java.util.List;
 @Builder
 public class CompetingRiskFunctions implements Serializable {
 
+    private static final long serialVersionUID = 1L;
+
     private final List<RightContinuousStepFunction> causeSpecificHazards;
     private final List<RightContinuousStepFunction> cumulativeIncidenceCurves;
 

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/CompetingRiskResponse.java

@@ -23,6 +23,8 @@ import java.io.Serializable;
 @Data
 public class CompetingRiskResponse implements Serializable {
 
+    private static final long serialVersionUID = 1L;
+
     private final int delta;
     private final double u;
 

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/CompetingRiskResponseWithCensorTime.java

@@ -26,6 +26,9 @@ import lombok.EqualsAndHashCode;
 @EqualsAndHashCode(callSuper = true)
 @Data
 public final class CompetingRiskResponseWithCensorTime extends CompetingRiskResponse {
+
+    private static final long serialVersionUID = 1L;
+
     private final double c;
 
     public CompetingRiskResponseWithCensorTime(int delta, double u, double c) {

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/CompetingRiskUtils.java

@@ -16,9 +16,11 @@
 
 package ca.joeltherrien.randomforest.responses.competingrisk;
 
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
 import ca.joeltherrien.randomforest.utils.StepFunction;
 
 import java.util.*;
+import java.util.stream.IntStream;
 import java.util.stream.Stream;
 
 public class CompetingRiskUtils {
@@ -116,6 +118,44 @@ public class CompetingRiskUtils {
 
     }
 
+    /**
+     * Calculate the Integrated Brier Score error on a list of responses and predictions.
+     *
+     * @param responses A List of responses
+     * @param predictions The corresponding List of predictions.
+     * @param censoringDistribution The censoring distribution.
+     * @param eventOfFocus The event we are calculating the error for.
+     * @param integrationUpperBound The upper bound to integrate to.
+     * @param isParallel Whether we should use parallel streams or not (provided because of bugs on a particular system).
+     * @return
+     */
+    public static double[] calculateIBSError(final List<CompetingRiskResponse> responses,
+                                                     List<CompetingRiskFunctions> predictions,
+                                                     Optional<RightContinuousStepFunction> censoringDistribution,
+                                                     int eventOfFocus,
+                                                     double integrationUpperBound,
+                                                     boolean isParallel){
+
+        if(responses.size() != predictions.size()){
+            throw new IllegalArgumentException("Length of responses and predictions must be equal.");
+        }
+
+        final IBSCalculator calculator = new IBSCalculator(censoringDistribution);
+
+        IntStream stream = IntStream.range(0, responses.size());
+
+        if(isParallel){
+            stream = stream.parallel();
+        }
+
+        return stream.mapToDouble(i -> {
+            CompetingRiskResponse response = responses.get(i);
+            RightContinuousStepFunction cif = predictions.get(i).getCumulativeIncidenceFunction(eventOfFocus);
+
+            return calculator.calculateError(response, cif, eventOfFocus, integrationUpperBound);
+        }).toArray();
+    }
+
 
     public static CompetingRiskSetsImpl calculateSetsEfficiently(final List<CompetingRiskResponse> initialLeftHand,
                                                                  final List<CompetingRiskResponse> initialRightHand,

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/IBSCalculator.java

@@ -0,0 +1,82 @@
+package ca.joeltherrien.randomforest.responses.competingrisk;
+
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
+
+import java.util.Optional;
+
+/**
+ * Used to calculate the Integrated Brier Score. See Section 4.2 of "Random survival forests for competing risks" by Ishwaran.
+ *
+ */
+public class IBSCalculator {
+
+    private final Optional<RightContinuousStepFunction> censoringDistribution;
+
+    public IBSCalculator(RightContinuousStepFunction censoringDistribution){
+        this.censoringDistribution = Optional.of(censoringDistribution);
+    }
+
+    public IBSCalculator(){
+        this.censoringDistribution = Optional.empty();
+    }
+
+    public IBSCalculator(Optional<RightContinuousStepFunction> censoringDistribution){
+        this.censoringDistribution = censoringDistribution;
+    }
+
+    public double calculateError(CompetingRiskResponse response, RightContinuousStepFunction cif, int eventOfInterest, double integrationUpperBound){
+
+        // return integral of weights*(I(response.getU() <= times & response.getDelta() == eventOfInterest) - cif(times))^2
+        // Note that if we don't have weights, just treat them all as one (i.e. don't bother multiplying)
+
+        RightContinuousStepFunction functionToIntegrate = cif;
+
+        if(response.getDelta() == eventOfInterest){
+            final RightContinuousStepFunction observedFunction = new RightContinuousStepFunction(new double[]{response.getU()}, new double[]{1.0}, 0.0);
+            functionToIntegrate = RightContinuousStepFunction.biOperation(observedFunction, functionToIntegrate, (a, b) -> (a - b) * (a - b));
+        } else{
+            functionToIntegrate = functionToIntegrate.unaryOperation(a -> a*a);
+        }
+
+        if(censoringDistribution.isPresent()){
+            final RightContinuousStepFunction weights = calculateWeights(response, censoringDistribution.get());
+            functionToIntegrate = RightContinuousStepFunction.biOperation(weights, functionToIntegrate, (a, b) -> a*b);
+
+            // the censoring weights go to 0 after the response is censored, so we can speed up results by only integrating
+            // prior to the censor times
+            if(response.isCensored()){
+                integrationUpperBound = Math.min(integrationUpperBound, response.getU());
+            }
+
+        }
+
+        return functionToIntegrate.integrate(0.0, integrationUpperBound);
+    }
+
+    private RightContinuousStepFunction calculateWeights(CompetingRiskResponse response, RightContinuousStepFunction censoringDistribution){
+        final double recordedTime = response.getU();
+
+        // Function(t) = firstPart(t) + secondPart(t)/thirdPart(t) where:
+            // firstPart(t) = I(recordedTime <= t & !response.isCensored()) / censoringDistribution.evaluate(recordedTime);
+            // secondPart(t) = I(recordedTime > t) = 1 - I(recordedTime <= t)
+            // thirdPart(t) = censoringDistribution.evaluate(t)
+
+        final RightContinuousStepFunction secondPart = new RightContinuousStepFunction(new double[]{recordedTime}, new double[]{0.0}, 1.0);
+        RightContinuousStepFunction result = RightContinuousStepFunction.biOperation(secondPart, censoringDistribution,
+                (second, third) -> second / third);
+
+        if(!response.isCensored()){
+            final RightContinuousStepFunction firstPart = new RightContinuousStepFunction(
+                    new double[]{recordedTime},
+                    new double[]{1.0 / censoringDistribution.evaluate(recordedTime)},
+                    0.0);
+
+            result = RightContinuousStepFunction.biOperation(firstPart, result, Double::sum);
+        }
+
+       return result;
+
+    }
+
+
+}

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/combiner/CompetingRiskFunctionCombiner.java

@@ -17,17 +17,17 @@
 package ca.joeltherrien.randomforest.responses.competingrisk.combiner;
 
 import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskFunctions;
-import ca.joeltherrien.randomforest.tree.ResponseCombiner;
+import ca.joeltherrien.randomforest.tree.ForestResponseCombiner;
-import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
+import ca.joeltherrien.randomforest.tree.IntermediateCombinedResponse;
-import ca.joeltherrien.randomforest.utils.Utils;
 import lombok.RequiredArgsConstructor;
 
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
 @RequiredArgsConstructor
-public class CompetingRiskFunctionCombiner implements ResponseCombiner<CompetingRiskFunctions, CompetingRiskFunctions> {
+public class CompetingRiskFunctionCombiner implements ForestResponseCombiner<CompetingRiskFunctions, CompetingRiskFunctions> {
+
+    private static final long serialVersionUID = 1L;
 
     private final int[] events;
     private final double[] times; // We may restrict ourselves to specific times.
@@ -55,72 +55,22 @@ public class CompetingRiskFunctionCombiner implements ResponseCombiner<Competing
                     ).sorted().distinct().toArray();
         }
 
-        final double n = responses.size();
+        final IntermediateCompetingRisksFunctionsTimesKnown intermediateResult = new IntermediateCompetingRisksFunctionsTimesKnown(responses.size(), this.events, timesToUse);
-
-        final double[] survivalY = new double[timesToUse.length];
-        final double[][] csCHFY = new double[events.length][timesToUse.length];
-        final double[][] cifY = new double[events.length][timesToUse.length];
-
-        /*
-            We're going to try to efficiently put our predictions together -
-                Assumptions - for each event on a response, the hazard and CIF functions share the same x points
-
-            Plan - go through the time on each response and make use of that so that when we search for a time index
-            to evaluate the function at, we don't need to re-search the earlier times.
-
-         */
-
-
-        for(final CompetingRiskFunctions currentFunctions : responses){
-            final double[] survivalXPoints = currentFunctions.getSurvivalCurve().getX();
-            final double[][] eventSpecificXPoints = new double[events.length][];
-
-            for(final int event : events){
-                eventSpecificXPoints[event-1] = currentFunctions.getCumulativeIncidenceFunction(event)
-                        .getX();
-            }
-
-            int previousSurvivalIndex = 0;
-            final int[] previousEventSpecificIndex = new int[events.length]; // relying on 0 being default value
-
-            for(int i=0; i<timesToUse.length; i++){
-                final double time = timesToUse[i];
-
-                // Survival curve
-                final int survivalTimeIndex = Utils.binarySearchLessThan(previousSurvivalIndex, survivalXPoints.length, survivalXPoints, time);
-                survivalY[i] = survivalY[i] + currentFunctions.getSurvivalCurve().evaluateByIndex(survivalTimeIndex) / n;
-                previousSurvivalIndex = Math.max(survivalTimeIndex, 0); // if our current time is less than the smallest time in xPoints then binarySearchLessThan returned a -1.
-                // -1's not an issue for evaluateByIndex, but it is an issue for the next time binarySearchLessThan is called.
-
-                // CHFs and CIFs
-                for(final int event : events){
-                    final double[] xPoints = eventSpecificXPoints[event-1];
-                    final int eventTimeIndex = Utils.binarySearchLessThan(previousEventSpecificIndex[event-1], xPoints.length,
-                            xPoints, time);
-                    csCHFY[event-1][i] = csCHFY[event-1][i] + currentFunctions.getCauseSpecificHazardFunction(event)
-                            .evaluateByIndex(eventTimeIndex) / n;
-                    cifY[event-1][i] = cifY[event-1][i] + currentFunctions.getCumulativeIncidenceFunction(event)
-                            .evaluateByIndex(eventTimeIndex) / n;
-
-                    previousEventSpecificIndex[event-1] = Math.max(eventTimeIndex, 0);
-                }
-            }
 
+        for(CompetingRiskFunctions input : responses){
+            intermediateResult.processNewInput(input);
         }
 
-        final RightContinuousStepFunction survivalFunction = new RightContinuousStepFunction(timesToUse, survivalY, 1.0);
+        return intermediateResult.transformToOutput();
-        final List<RightContinuousStepFunction> causeSpecificCumulativeHazardFunctionList = new ArrayList<>(events.length);
+    }
-        final List<RightContinuousStepFunction> cumulativeIncidenceFunctionList = new ArrayList<>(events.length);
 
-        for(final int event : events){
+    @Override
-            causeSpecificCumulativeHazardFunctionList.add(event-1, new RightContinuousStepFunction(timesToUse, csCHFY[event-1], 0));
+    public IntermediateCombinedResponse<CompetingRiskFunctions, CompetingRiskFunctions> startIntermediateCombinedResponse(int countInputs) {
-            cumulativeIncidenceFunctionList.add(event-1, new RightContinuousStepFunction(timesToUse, cifY[event-1], 0));
+        if(this.times != null){
+            return new IntermediateCompetingRisksFunctionsTimesKnown(countInputs, this.events, this.times);
         }
 
-        return CompetingRiskFunctions.builder()
+        // TODO - implement
-                .causeSpecificHazards(causeSpecificCumulativeHazardFunctionList)
+        throw new RuntimeException("startIntermediateCombinedResponse when times is unknown is not yet implemented");
-                .cumulativeIncidenceCurves(cumulativeIncidenceFunctionList)
-                .survivalCurve(survivalFunction)
-                .build();
     }
 }

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/combiner/CompetingRiskResponseCombiner.java

@@ -35,6 +35,8 @@ import java.util.List;
  */
 public class CompetingRiskResponseCombiner implements ResponseCombiner<CompetingRiskResponse, CompetingRiskFunctions> {
 
+    private static final long serialVersionUID = 1L;
+
     private final int[] events;
 
     public CompetingRiskResponseCombiner(final int[] events){

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/combiner/IntermediateCompetingRisksFunctionsTimesKnown.java

@@ -0,0 +1,118 @@
+package ca.joeltherrien.randomforest.responses.competingrisk.combiner;
+
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskFunctions;
+import ca.joeltherrien.randomforest.tree.IntermediateCombinedResponse;
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
+import ca.joeltherrien.randomforest.utils.Utils;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class IntermediateCompetingRisksFunctionsTimesKnown implements IntermediateCombinedResponse<CompetingRiskFunctions, CompetingRiskFunctions> {
+
+    private double expectedN;
+    private final int[] events;
+    private final double[] timesToUse;
+    private int actualN;
+
+    private final double[] survivalY;
+    private final double[][] csCHFY;
+    private final double[][] cifY;
+
+    public IntermediateCompetingRisksFunctionsTimesKnown(int n, int[] events, double[] timesToUse){
+        this.expectedN = n;
+        this.events = events;
+        this.timesToUse = timesToUse;
+        this.actualN = 0;
+
+        this.survivalY = new double[timesToUse.length];
+        this.csCHFY = new double[events.length][timesToUse.length];
+        this.cifY = new double[events.length][timesToUse.length];
+    }
+
+    @Override
+    public void processNewInput(CompetingRiskFunctions input) {
+        /*
+            We're going to try to efficiently put our predictions together -
+                Assumptions - for each event on a response, the hazard and CIF functions share the same x points
+
+            Plan - go through the time on each response and make use of that so that when we search for a time index
+            to evaluate the function at, we don't need to re-search the earlier times.
+
+         */
+
+        this.actualN++;
+
+        final double[] survivalXPoints = input.getSurvivalCurve().getX();
+        final double[][] eventSpecificXPoints = new double[events.length][];
+
+        for(final int event : events){
+            eventSpecificXPoints[event-1] = input.getCumulativeIncidenceFunction(event)
+                    .getX();
+        }
+
+        int previousSurvivalIndex = 0;
+        final int[] previousEventSpecificIndex = new int[events.length]; // relying on 0 being default value
+
+        for(int i=0; i<timesToUse.length; i++){
+            final double time = timesToUse[i];
+
+            // Survival curve
+            final int survivalTimeIndex = Utils.binarySearchLessThan(previousSurvivalIndex, survivalXPoints.length, survivalXPoints, time);
+            survivalY[i] = survivalY[i] + input.getSurvivalCurve().evaluateByIndex(survivalTimeIndex) / expectedN;
+            previousSurvivalIndex = Math.max(survivalTimeIndex, 0); // if our current time is less than the smallest time in xPoints then binarySearchLessThan returned a -1.
+            // -1's not an issue for evaluateByIndex, but it is an issue for the next time binarySearchLessThan is called.
+
+            // CHFs and CIFs
+            for(final int event : events){
+                final double[] xPoints = eventSpecificXPoints[event-1];
+                final int eventTimeIndex = Utils.binarySearchLessThan(previousEventSpecificIndex[event-1], xPoints.length,
+                        xPoints, time);
+                csCHFY[event-1][i] = csCHFY[event-1][i] + input.getCauseSpecificHazardFunction(event)
+                        .evaluateByIndex(eventTimeIndex) / expectedN;
+                cifY[event-1][i] = cifY[event-1][i] + input.getCumulativeIncidenceFunction(event)
+                        .evaluateByIndex(eventTimeIndex) / expectedN;
+
+                previousEventSpecificIndex[event-1] = Math.max(eventTimeIndex, 0);
+            }
+        }
+    }
+
+    @Override
+    public CompetingRiskFunctions transformToOutput() {
+        rescaleOutput();
+
+        final RightContinuousStepFunction survivalFunction = new RightContinuousStepFunction(timesToUse, survivalY, 1.0);
+        final List<RightContinuousStepFunction> causeSpecificCumulativeHazardFunctionList = new ArrayList<>(events.length);
+        final List<RightContinuousStepFunction> cumulativeIncidenceFunctionList = new ArrayList<>(events.length);
+
+        for(final int event : events){
+            causeSpecificCumulativeHazardFunctionList.add(event-1, new RightContinuousStepFunction(timesToUse, csCHFY[event-1], 0));
+            cumulativeIncidenceFunctionList.add(event-1, new RightContinuousStepFunction(timesToUse, cifY[event-1], 0));
+        }
+
+        return CompetingRiskFunctions.builder()
+                .causeSpecificHazards(causeSpecificCumulativeHazardFunctionList)
+                .cumulativeIncidenceCurves(cumulativeIncidenceFunctionList)
+                .survivalCurve(survivalFunction)
+                .build();
+    }
+
+    private void rescaleOutput() {
+        rescaleArray(actualN, this.survivalY);
+
+        for(int event : events){
+            rescaleArray(actualN, this.cifY[event - 1]);
+            rescaleArray(actualN, this.csCHFY[event - 1]);
+        }
+
+        this.expectedN = actualN;
+
+    }
+
+    private void rescaleArray(double newN, double[] array){
+        for(int i=0; i<array.length; i++){
+            array[i] = array[i] * (this.expectedN / newN);
+        }
+    }
+}

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/splitfinder/GrayLogRankSplitFinder.java

@@ -28,6 +28,7 @@ import java.util.List;
  *
  */
 public class GrayLogRankSplitFinder extends CompetingRiskSplitFinder<CompetingRiskResponseWithCensorTime> {
+    private static final long serialVersionUID = 1L;
 
     private final int[] eventsOfFocus;
     private final int[] events;

src/main/java/ca/joeltherrien/randomforest/responses/competingrisk/splitfinder/LogRankSplitFinder.java

@@ -28,6 +28,7 @@ import java.util.List;
  *
  */
 public class LogRankSplitFinder extends CompetingRiskSplitFinder<CompetingRiskResponse> {
+    private static final long serialVersionUID = 1L;
 
     private final int[] eventsOfFocus;
     private final int[] events;

src/main/java/ca/joeltherrien/randomforest/responses/regression/MeanResponseCombiner.java

@@ -16,7 +16,8 @@
 
 package ca.joeltherrien.randomforest.responses.regression;
 
-import ca.joeltherrien.randomforest.tree.ResponseCombiner;
+import ca.joeltherrien.randomforest.tree.ForestResponseCombiner;
+import ca.joeltherrien.randomforest.tree.IntermediateCombinedResponse;
 
 import java.util.List;
 
@@ -24,7 +25,8 @@ import java.util.List;
  * Returns the Mean value of a group of Doubles.
  *
  */
-public class MeanResponseCombiner implements ResponseCombiner<Double, Double> {
+public class MeanResponseCombiner implements ForestResponseCombiner<Double, Double> {
+    private static final long serialVersionUID = 1L;
 
     @Override
     public Double combine(List<Double> responses) {
@@ -34,5 +36,39 @@ public class MeanResponseCombiner implements ResponseCombiner<Double, Double> {
 
     }
 
+    @Override
+    public IntermediateCombinedResponse<Double, Double> startIntermediateCombinedResponse(int countInputs) {
+        return new MeanIntermediateCombinedResponse(countInputs);
+    }
+
+    public static class MeanIntermediateCombinedResponse implements IntermediateCombinedResponse<Double, Double>{
+
+        private double expectedN;
+        private int actualN;
+        private double currentMean;
+
+        public MeanIntermediateCombinedResponse(int n){
+            this.expectedN = n;
+            this.actualN = 0;
+            this.currentMean = 0.0;
+        }
+
+        @Override
+        public void processNewInput(Double input) {
+            this.currentMean = this.currentMean + input / expectedN;
+            this.actualN ++;
+        }
+
+        @Override
+        public Double transformToOutput() {
+            // rescale if necessary
+            this.currentMean = this.currentMean * (this.expectedN / (double) actualN);
+            this.expectedN = actualN;
+
+            return currentMean;
+        }
+
+
+    }
 
 }

src/main/java/ca/joeltherrien/randomforest/responses/regression/WeightedVarianceSplitFinder.java

@@ -27,6 +27,7 @@ import java.util.List;
 import java.util.stream.Collectors;
 
 public class WeightedVarianceSplitFinder implements SplitFinder<Double> {
+    private static final long serialVersionUID = 1L;
 
     private Double getScore(Set leftHand, Set rightHand) {
 

src/main/java/ca/joeltherrien/randomforest/tree/Forest.java

@@ -17,31 +17,18 @@
 package ca.joeltherrien.randomforest.tree;
 
 import ca.joeltherrien.randomforest.CovariateRow;
-import ca.joeltherrien.randomforest.covariates.Covariate;
-import lombok.Builder;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.TreeMap;
 import java.util.stream.Collectors;
 
-@Builder
+public abstract class Forest<O, FO> {
-public class Forest<O, FO> { // O = output of trees, FO = forest output. In practice O == FO, even in competing risk & survival settings
 
-    private final List<Tree<O>> trees;
+    public abstract FO evaluate(CovariateRow row);
-    private final ResponseCombiner<O, FO> treeResponseCombiner;
+    public abstract FO evaluateOOB(CovariateRow row);
-    private final List<Covariate> covariateList;
+    public abstract Iterable<Tree<O>> getTrees();
-
+    public abstract int getNumberOfTrees();
-    public FO evaluate(CovariateRow row){
-
-        return treeResponseCombiner.combine(
-                trees.stream()
-                .map(node -> node.evaluate(row))
-                .collect(Collectors.toList())
-        );
-
-    }
 
     /**
      * Used primarily in the R package interface to avoid R loops; and for easier parallelization.
@@ -93,21 +80,6 @@ public class Forest<O, FO> { // O = output of trees, FO = forest output. In prac
                 .collect(Collectors.toList());
     }
 
-    public FO evaluateOOB(CovariateRow row){
-
-        return treeResponseCombiner.combine(
-          trees.stream()
-          .filter(tree -> !tree.idInBootstrapSample(row.getId()))
-          .map(node -> node.evaluate(row))
-          .collect(Collectors.toList())
-        );
-
-    }
-
-    public List<Tree<O>> getTrees(){
-        return Collections.unmodifiableList(trees);
-    }
-
     public Map<Integer, Integer> findSplitsByCovariate(){
         final Map<Integer, Integer> countMap = new TreeMap<>();
 
@@ -158,4 +130,5 @@ public class Forest<O, FO> { // O = output of trees, FO = forest output. In prac
         return countTerminalNodes;
     }
 
+
 }

src/main/java/ca/joeltherrien/randomforest/tree/ForestResponseCombiner.java

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2019 Joel Therrien.
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ *
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package ca.joeltherrien.randomforest.tree;
+
+public interface ForestResponseCombiner<I, O>  extends ResponseCombiner<I, O>{
+
+    IntermediateCombinedResponse<I, O> startIntermediateCombinedResponse(int countInputs);
+
+}

src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java

@@ -38,7 +38,7 @@ public class ForestTrainer<Y, TO, FO> {
 
     private final TreeTrainer<Y, TO> treeTrainer;
     private final List<Covariate> covariates;
-    private final ResponseCombiner<TO, FO> treeResponseCombiner;
+    private final ForestResponseCombiner<TO, FO> treeResponseCombiner;
     private final List<Row<Y>> data;
 
     // number of trees to try
@@ -57,10 +57,10 @@ public class ForestTrainer<Y, TO, FO> {
      *                      in which case its trees are combined with the new one.
      * @return A trained forest.
      */
-    public Forest<TO, FO> trainSerialInMemory(Optional<Forest<TO, FO>> initialForest){
+    public OnlineForest<TO, FO> trainSerialInMemory(Optional<Forest<TO, FO>> initialForest){
 
         final List<Tree<TO>> trees = new ArrayList<>(ntree);
-        initialForest.ifPresent(forest -> trees.addAll(forest.getTrees()));
+        initialForest.ifPresent(forest -> forest.getTrees().forEach(trees::add));
 
         final Bootstrapper<Row<Y>> bootstrapper = new Bootstrapper<>(data);
 
@@ -77,11 +77,9 @@ public class ForestTrainer<Y, TO, FO> {
             System.out.println("Finished");
         }
 
-
+        return OnlineForest.<TO, FO>builder()
-        return Forest.<TO, FO>builder()
                 .treeResponseCombiner(treeResponseCombiner)
                 .trees(trees)
-                .covariateList(covariates)
                 .build();
 
     }
@@ -94,7 +92,7 @@ public class ForestTrainer<Y, TO, FO> {
      *                      There cannot be existing trees if the initial forest is
      *                      specified.
      */
-    public void trainSerialOnDisk(Optional<Forest<TO, FO>> initialForest){
+    public OfflineForest<TO, FO> trainSerialOnDisk(Optional<Forest<TO, FO>> initialForest){
         // First we need to see how many trees there currently are
         final File folder = new File(saveTreeLocation);
         if(!folder.exists()){
@@ -115,17 +113,14 @@ public class ForestTrainer<Y, TO, FO> {
         final AtomicInteger treeCount; // tracks how many trees are finished
         // Using an AtomicInteger is overkill for serial code, but this lets us reuse TreeSavedWorker
         if(initialForest.isPresent()){
-            final List<Tree<TO>> initialTrees = initialForest.get().getTrees();
+            int j=0;
-
+            for(final Tree<TO> tree : initialForest.get().getTrees()){
-            for(int j=0; j<initialTrees.size(); j++){
                 final String filename = "tree-" + (j+1) + ".tree";
-                final Tree<TO> tree = initialTrees.get(j);
-
                 saveTree(tree, filename);
-
+                j++;
             }
 
-            treeCount = new AtomicInteger(initialTrees.size());
+            treeCount = new AtomicInteger(j);
         } else{
             treeCount = new AtomicInteger(treeFiles.length);
         }
@@ -153,6 +148,8 @@ public class ForestTrainer<Y, TO, FO> {
             System.out.println("Finished");
         }
 
+        return new OfflineForest<>(folder, treeResponseCombiner);
+
     }
 
     /**
@@ -162,7 +159,7 @@ public class ForestTrainer<Y, TO, FO> {
      *                      in which case its trees are combined with the new one.
      * @param threads The number of trees to train at once.
      */
-    public Forest<TO, FO> trainParallelInMemory(Optional<Forest<TO, FO>> initialForest, int threads){
+    public OnlineForest<TO, FO> trainParallelInMemory(Optional<Forest<TO, FO>> initialForest, int threads){
 
         // create a list that is pre-specified in size (I can call the .set method at any index < ntree without
         // the earlier indexes being filled.
@@ -170,11 +167,12 @@ public class ForestTrainer<Y, TO, FO> {
 
         final int startingCount;
         if(initialForest.isPresent()){
-            final List<Tree<TO>> initialTrees = initialForest.get().getTrees();
+            int j = 0;
-            for(int j=0; j<initialTrees.size(); j++) {
+            for(final Tree<TO> tree : initialForest.get().getTrees()){
-                trees.set(j, initialTrees.get(j));
+                trees.set(j, tree);
+                j++;
             }
-            startingCount = initialTrees.size();
+            startingCount = initialForest.get().getNumberOfTrees();
         }
         else{
             startingCount = 0;
@@ -219,7 +217,7 @@ public class ForestTrainer<Y, TO, FO> {
             System.out.println("\nFinished");
         }
 
-        return Forest.<TO, FO>builder()
+        return OnlineForest.<TO, FO>builder()
                 .treeResponseCombiner(treeResponseCombiner)
                 .trees(trees)
                 .build();
@@ -235,7 +233,7 @@ public class ForestTrainer<Y, TO, FO> {
      *                      specified.
      * @param threads The number of trees to train at once.
      */
-    public void trainParallelOnDisk(Optional<Forest<TO, FO>> initialForest, int threads){
+    public OfflineForest<TO, FO> trainParallelOnDisk(Optional<Forest<TO, FO>> initialForest, int threads){
         // First we need to see how many trees there currently are
         final File folder = new File(saveTreeLocation);
         if(!folder.exists()){
@@ -255,17 +253,14 @@ public class ForestTrainer<Y, TO, FO> {
 
         final AtomicInteger treeCount; // tracks how many trees are finished
         if(initialForest.isPresent()){
-            final List<Tree<TO>> initialTrees = initialForest.get().getTrees();
+            int j=0;
-
+            for(final Tree<TO> tree : initialForest.get().getTrees()){
-            for(int j=0; j<initialTrees.size(); j++){
                 final String filename = "tree-" + (j+1) + ".tree";
-                final Tree<TO> tree = initialTrees.get(j);
-
                 saveTree(tree, filename);
-
+                j++;
             }
 
-            treeCount = new AtomicInteger(initialTrees.size());
+            treeCount = new AtomicInteger(j);
         } else{
             treeCount = new AtomicInteger(treeFiles.length);
         }
@@ -309,6 +304,8 @@ public class ForestTrainer<Y, TO, FO> {
             System.out.println("\nFinished");
         }
 
+        return new OfflineForest<>(folder, treeResponseCombiner);
+
     }
 
     private Tree<TO> trainTree(final Bootstrapper<Row<Y>> bootstrapper, Random random){

src/main/java/ca/joeltherrien/randomforest/tree/IntermediateCombinedResponse.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2019 Joel Therrien.
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ *
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package ca.joeltherrien.randomforest.tree;
+
+/**
+ * Similar to ResponseCombiner, but an IntermediateCombinedResponse represents the intermediate state of a single output in the process of being combined.
+ * This class is only used in OfflineForests where we can only load one Tree in memory at a time.
+ *
+ */
+public interface IntermediateCombinedResponse<I, O> {
+
+    void processNewInput(I input);
+
+    O transformToOutput();
+
+}

src/main/java/ca/joeltherrien/randomforest/tree/OfflineForest.java

@@ -0,0 +1,198 @@
+/*
+ * Copyright (c) 2019 Joel Therrien.
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ *
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package ca.joeltherrien.randomforest.tree;
+
+import ca.joeltherrien.randomforest.CovariateRow;
+import ca.joeltherrien.randomforest.utils.IterableOfflineTree;
+import lombok.AllArgsConstructor;
+
+import java.io.File;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+@AllArgsConstructor
+public class OfflineForest<O, FO> extends Forest<O, FO> {
+
+    private final File[] treeFiles;
+    private final ForestResponseCombiner<O, FO> treeResponseCombiner;
+
+    public OfflineForest(File treeDirectoryPath, ForestResponseCombiner<O, FO> treeResponseCombiner){
+        this.treeResponseCombiner = treeResponseCombiner;
+
+        if(!treeDirectoryPath.isDirectory()){
+            throw new IllegalArgumentException("treeDirectoryPath must point to a directory!");
+        }
+
+        this.treeFiles = treeDirectoryPath.listFiles((file, s) -> s.endsWith(".tree"));
+    }
+
+    @Override
+    public FO evaluate(CovariateRow row) {
+        final List<O> predictedOutputs = new ArrayList<>(treeFiles.length);
+        for(final Tree<O> tree : getTrees()){
+            final O prediction = tree.evaluate(row);
+            predictedOutputs.add(prediction);
+        }
+
+        return treeResponseCombiner.combine(predictedOutputs);
+    }
+
+    @Override
+    public FO evaluateOOB(CovariateRow row) {
+        final List<O> predictedOutputs = new ArrayList<>(treeFiles.length);
+        for(final Tree<O> tree : getTrees()){
+            if(!tree.idInBootstrapSample(row.getId())){
+                final O prediction = tree.evaluate(row);
+                predictedOutputs.add(prediction);
+            }
+        }
+
+        return treeResponseCombiner.combine(predictedOutputs);
+    }
+
+
+    @Override
+    public List<FO> evaluate(List<? extends CovariateRow> rowList){
+        final List<IntermediateCombinedResponse<O, FO>> intermediatePredictions =
+                IntStream.range(0, rowList.size())
+                .mapToObj(i -> treeResponseCombiner.startIntermediateCombinedResponse(treeFiles.length))
+                .collect(Collectors.toList());
+
+        final Iterator<Tree<O>> treeIterator = getTrees().iterator();
+        for(int treeId = 0; treeId < treeFiles.length; treeId++){
+            final Tree<O> currentTree = treeIterator.next();
+
+            IntStream.range(0, rowList.size()).parallel().forEach(
+                    rowId -> {
+                        final CovariateRow row = rowList.get(rowId);
+                        final O prediction = currentTree.evaluate(row);
+                        intermediatePredictions.get(rowId).processNewInput(prediction);
+                    }
+            );
+        }
+
+        return intermediatePredictions.stream().parallel()
+                .map(intPred -> intPred.transformToOutput())
+                .collect(Collectors.toList());
+    }
+
+    @Override
+    public List<FO> evaluateSerial(List<? extends CovariateRow> rowList){
+        final List<IntermediateCombinedResponse<O, FO>> intermediatePredictions =
+                IntStream.range(0, rowList.size())
+                        .mapToObj(i -> treeResponseCombiner.startIntermediateCombinedResponse(treeFiles.length))
+                        .collect(Collectors.toList());
+
+        final Iterator<Tree<O>> treeIterator = getTrees().iterator();
+        for(int treeId = 0; treeId < treeFiles.length; treeId++){
+            final Tree<O> currentTree = treeIterator.next();
+
+            IntStream.range(0, rowList.size()).sequential().forEach(
+                    rowId -> {
+                        final CovariateRow row = rowList.get(rowId);
+                        final O prediction = currentTree.evaluate(row);
+                        intermediatePredictions.get(rowId).processNewInput(prediction);
+                    }
+            );
+        }
+
+        return intermediatePredictions.stream().sequential()
+                .map(intPred -> intPred.transformToOutput())
+                .collect(Collectors.toList());
+    }
+
+
+    @Override
+    public List<FO> evaluateOOB(List<? extends CovariateRow> rowList){
+        final List<IntermediateCombinedResponse<O, FO>> intermediatePredictions =
+                IntStream.range(0, rowList.size())
+                        .mapToObj(i -> treeResponseCombiner.startIntermediateCombinedResponse(treeFiles.length))
+                        .collect(Collectors.toList());
+
+        final Iterator<Tree<O>> treeIterator = getTrees().iterator();
+        for(int treeId = 0; treeId < treeFiles.length; treeId++){
+            final Tree<O> currentTree = treeIterator.next();
+
+            IntStream.range(0, rowList.size()).parallel().forEach(
+                    rowId -> {
+                        final CovariateRow row = rowList.get(rowId);
+                        if(!currentTree.idInBootstrapSample(row.getId())){
+                            final O prediction = currentTree.evaluate(row);
+                            intermediatePredictions.get(rowId).processNewInput(prediction);
+                        }
+                        // else do nothing; when we get the final output it will get scaled for the smaller N
+                    }
+            );
+        }
+
+        return intermediatePredictions.stream().parallel()
+                .map(intPred -> intPred.transformToOutput())
+                .collect(Collectors.toList());
+    }
+
+    @Override
+    public List<FO> evaluateSerialOOB(List<? extends CovariateRow> rowList){
+        final List<IntermediateCombinedResponse<O, FO>> intermediatePredictions =
+                IntStream.range(0, rowList.size())
+                        .mapToObj(i -> treeResponseCombiner.startIntermediateCombinedResponse(treeFiles.length))
+                        .collect(Collectors.toList());
+
+        final Iterator<Tree<O>> treeIterator = getTrees().iterator();
+        for(int treeId = 0; treeId < treeFiles.length; treeId++){
+            final Tree<O> currentTree = treeIterator.next();
+
+            IntStream.range(0, rowList.size()).sequential().forEach(
+                    rowId -> {
+                        final CovariateRow row = rowList.get(rowId);
+                        if(!currentTree.idInBootstrapSample(row.getId())){
+                            final O prediction = currentTree.evaluate(row);
+                            intermediatePredictions.get(rowId).processNewInput(prediction);
+                        }
+                        // else do nothing; when we get the final output it will get scaled for the smaller N
+                    }
+            );
+        }
+
+        return intermediatePredictions.stream().sequential()
+                .map(intPred -> intPred.transformToOutput())
+                .collect(Collectors.toList());
+    }
+
+    @Override
+    public Iterable<Tree<O>> getTrees() {
+        return new IterableOfflineTree<>(treeFiles);
+    }
+
+    @Override
+    public int getNumberOfTrees() {
+        return treeFiles.length;
+    }
+
+    public OnlineForest<O, FO> createOnlineCopy(){
+        final List<Tree<O>> allTrees = new ArrayList<>(getNumberOfTrees());
+        getTrees().forEach(allTrees::add);
+
+        return OnlineForest.<O, FO>builder()
+                .trees(allTrees)
+                .treeResponseCombiner(treeResponseCombiner)
+                .build();
+    }
+}
+

src/main/java/ca/joeltherrien/randomforest/tree/OnlineForest.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2019 Joel Therrien.
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ *
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package ca.joeltherrien.randomforest.tree;
+
+import ca.joeltherrien.randomforest.CovariateRow;
+import lombok.Builder;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+
+@Builder
+public class OnlineForest<O, FO> extends Forest<O, FO> { // O = output of trees, FO = forest output. In practice O == FO, even in competing risk & survival settings
+
+    private final List<Tree<O>> trees;
+    private final ResponseCombiner<O, FO> treeResponseCombiner;
+
+    @Override
+    public FO evaluate(CovariateRow row){
+
+        return treeResponseCombiner.combine(
+                trees.stream()
+                .map(node -> node.evaluate(row))
+                .collect(Collectors.toList())
+        );
+
+    }
+
+    @Override
+    public FO evaluateOOB(CovariateRow row){
+
+        return treeResponseCombiner.combine(
+          trees.stream()
+          .filter(tree -> !tree.idInBootstrapSample(row.getId()))
+          .map(node -> node.evaluate(row))
+          .collect(Collectors.toList())
+        );
+
+    }
+
+    @Override
+    public List<Tree<O>> getTrees(){
+        return Collections.unmodifiableList(trees);
+    }
+
+    @Override
+    public int getNumberOfTrees() {
+        return trees.size();
+    }
+
+
+}

src/main/java/ca/joeltherrien/randomforest/tree/SplitAndScore.java

@@ -17,15 +17,13 @@
 package ca.joeltherrien.randomforest.tree;
 
 import lombok.AllArgsConstructor;
-import lombok.Getter;
+import lombok.Data;
 
 @AllArgsConstructor
+@Data
 public class SplitAndScore<Y, V> {
 
-    @Getter
+    private Split<Y, V> split;
-    private final Split<Y, V> split;
+    private Double score;
-
-    @Getter
-    private final Double score;
 
 }

src/main/java/ca/joeltherrien/randomforest/tree/SplitNode.java

@@ -29,6 +29,7 @@ import java.util.List;
 @ToString
 @Getter
 public class SplitNode<Y> implements Node<Y> {
+    private static final long serialVersionUID = 1L;
 
     private final Node<Y> leftHand;
     private final Node<Y> rightHand;

src/main/java/ca/joeltherrien/randomforest/tree/TerminalNode.java

@@ -27,6 +27,7 @@ import java.util.List;
 @RequiredArgsConstructor
 @ToString
 public class TerminalNode<Y> implements Node<Y> {
+    private static final long serialVersionUID = 1L;
 
     private final Y responseValue;
 

src/main/java/ca/joeltherrien/randomforest/tree/Tree.java

@@ -23,6 +23,7 @@ import java.util.Arrays;
 import java.util.List;
 
 public class Tree<Y> implements Node<Y> {
+    private static final long serialVersionUID = 1L;
 
     @Getter
     private final Node<Y> rootNode;

src/main/java/ca/joeltherrien/randomforest/tree/TreeTrainer.java

@@ -17,7 +17,9 @@
 package ca.joeltherrien.randomforest.tree;
 
 import ca.joeltherrien.randomforest.Row;
+import ca.joeltherrien.randomforest.VisibleForTesting;
 import ca.joeltherrien.randomforest.covariates.Covariate;
+import ca.joeltherrien.randomforest.utils.SingletonIterator;
 import lombok.AccessLevel;
 import lombok.AllArgsConstructor;
 import lombok.Builder;
@@ -72,31 +74,12 @@ public class TreeTrainer<Y, O> {
 
             }
 
-
             // Now that we have the best split; we need to handle any NAs that were dropped off
             final double probabilityLeftHand = (double) bestSplit.leftHand.size() /
                     (double) (bestSplit.leftHand.size() + bestSplit.rightHand.size());
 
             // Assign missing values to the split if necessary
-            if(covariates.get(bestSplit.getSplitRule().getParentCovariateIndex()).hasNAs()){
+            bestSplit = randomlyAssignNAs(data, bestSplit, random);
-                bestSplit = bestSplit.modifiableClone(); // the lists in bestSplit are otherwise usually unmodifiable lists
-
-                for(Row<Y> row : data) {
-                    final int covariateIndex = bestSplit.getSplitRule().getParentCovariateIndex();
-
-                    if(row.getValueByIndex(covariateIndex).isNA()) {
-                        final boolean randomDecision = random.nextDouble() <= probabilityLeftHand;
-
-                        if(randomDecision){
-                            bestSplit.getLeftHand().add(row);
-                        }
-                        else{
-                            bestSplit.getRightHand().add(row);
-                        }
-
-                    }
-                }
-            }
 
             final Node<O> leftNode;
             final Node<O> rightNode;
@@ -144,7 +127,8 @@ public class TreeTrainer<Y, O> {
         return splitCovariates;
     }
 
-    private Split<Y, ?> findBestSplitRule(List<Row<Y>> data, List<Covariate> covariatesToTry, Random random){
+    @VisibleForTesting
+    public Split<Y, ?> findBestSplitRule(List<Row<Y>> data, List<Covariate> covariatesToTry, Random random){
 
         SplitAndScore<Y, ?> bestSplitAndScore = null;
         final SplitFinder noGenericSplitFinder = splitFinder; // cause Java generics are sometimes too frustrating
@@ -157,10 +141,32 @@ public class TreeTrainer<Y, O> {
                 continue;
             }
 
-            final SplitAndScore<Y, ?> candidateSplitAndScore = noGenericSplitFinder.findBestSplit(iterator);
+            SplitAndScore<Y, ?> candidateSplitAndScore = noGenericSplitFinder.findBestSplit(iterator);
 
-            if(candidateSplitAndScore != null && (bestSplitAndScore == null ||
+
-                    candidateSplitAndScore.getScore() > bestSplitAndScore.getScore())) {
+            if(candidateSplitAndScore == null){
+                continue;
+            }
+
+            // This score was based on splitting only non-NA values. However, there might be a similar covariate we are also considering
+            // that is just as good at splitting but has less NAs; we should thus penalize the split score for variables with NAs
+            // We do this by randomly assigning the NAs and then recalculating the split score on the best split we already have.
+            //
+            // We only have to penalize the score though if we know it's possible that this might be the best split. If it's not,
+            // then we can skip the computations.
+            final boolean mayBeGoodSplit = bestSplitAndScore == null ||
+                    candidateSplitAndScore.getScore() > bestSplitAndScore.getScore();
+            if(mayBeGoodSplit && covariate.haveNASplitPenalty()){
+                Split<Y, ?> candiateSplitWithNAs = randomlyAssignNAs(data, candidateSplitAndScore.getSplit(), random);
+                final Iterator<Split<Y, ?>> newSplitWithRandomNAs = new SingletonIterator<>(candiateSplitWithNAs);
+                final double newScore = splitFinder.findBestSplit(newSplitWithRandomNAs).getScore();
+
+                // There's a chance that NAs might add noise to *improve* the score; but we want to ensure we penalize it.
+                // Thus we only change the score if its worse.
+                candidateSplitAndScore.setScore(Math.min(newScore, candidateSplitAndScore.getScore()));
+            }
+
+            if(bestSplitAndScore == null || candidateSplitAndScore.getScore() > bestSplitAndScore.getScore()) {
                 bestSplitAndScore = candidateSplitAndScore;
             }
 
@@ -174,6 +180,38 @@ public class TreeTrainer<Y, O> {
 
     }
 
+    private <V> Split<Y, V> randomlyAssignNAs(List<Row<Y>> data, Split<Y, V> existingSplit, Random random){
+
+        // Now that we have the best split; we need to handle any NAs that were dropped off
+        final double probabilityLeftHand = (double) existingSplit.leftHand.size() /
+                (double) (existingSplit.leftHand.size() + existingSplit.rightHand.size());
+
+
+        final int covariateIndex = existingSplit.getSplitRule().getParentCovariateIndex();
+
+        // Assign missing values to the split if necessary
+        if(covariates.get(existingSplit.getSplitRule().getParentCovariateIndex()).hasNAs()){
+            existingSplit = existingSplit.modifiableClone(); // the lists in bestSplit are otherwise usually unmodifiable lists
+
+            for(Row<Y> row : data) {
+                if(row.getValueByIndex(covariateIndex).isNA()) {
+                    final boolean randomDecision = random.nextDouble() <= probabilityLeftHand;
+
+                    if(randomDecision){
+                        existingSplit.getLeftHand().add(row);
+                    }
+                    else{
+                        existingSplit.getRightHand().add(row);
+                    }
+
+                }
+            }
+        }
+
+        return existingSplit;
+
+    }
+
     private boolean nodeIsPure(List<Row<Y>> data){
         if(!checkNodePurity){
             return false;

src/main/java/ca/joeltherrien/randomforest/tree/vimp/ErrorCalculator.java

@@ -0,0 +1,24 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import java.util.List;
+
+/**
+ * Simple interface for VariableImportanceCalculator; takes in a List of observed responses and a List of predictions
+ * and produces an average error measure.
+ *
+ * @param <Y> The class of the responses.
+ * @param <P> The class of the predictions.
+ */
+public interface ErrorCalculator<Y, P>{
+
+    /**
+     * Compares the observed responses with the predictions to produce an average error measure.
+     * Lower errors should indicate a better model fit.
+     *
+     * @param responses
+     * @param predictions
+     * @return
+     */
+    double averageError(List<Y> responses, List<P> predictions);
+
+}

src/main/java/ca/joeltherrien/randomforest/tree/vimp/IBSErrorCalculatorWrapper.java

@@ -0,0 +1,63 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskFunctions;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskResponse;
+import ca.joeltherrien.randomforest.responses.competingrisk.IBSCalculator;
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
+
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * Implements ErrorCalculator; essentially just wraps around IBSCalculator to fit into VariableImportanceCalculator.
+ *
+ */
+public class IBSErrorCalculatorWrapper implements ErrorCalculator<CompetingRiskResponse, CompetingRiskFunctions> {
+
+    private final IBSCalculator calculator;
+    private final int[] events;
+    private final double integrationUpperBound;
+    private final double[] eventWeights;
+
+    public IBSErrorCalculatorWrapper(IBSCalculator calculator, int[] events, double integrationUpperBound, double[] eventWeights) {
+        this.calculator = calculator;
+        this.events = events;
+        this.integrationUpperBound = integrationUpperBound;
+        this.eventWeights = eventWeights;
+    }
+
+    public IBSErrorCalculatorWrapper(IBSCalculator calculator, int[] events, double integrationUpperBound) {
+        this.calculator = calculator;
+        this.events = events;
+        this.integrationUpperBound = integrationUpperBound;
+        this.eventWeights = new double[events.length];
+
+        Arrays.fill(this.eventWeights, 1.0); // default is to just sum all errors together
+
+    }
+
+    @Override
+    public double averageError(List<CompetingRiskResponse> responses, List<CompetingRiskFunctions> predictions) {
+        final double[] errors = new double[events.length];
+        final double n = responses.size();
+
+        for(int i=0; i < responses.size(); i++){
+            final CompetingRiskResponse response = responses.get(i);
+            final CompetingRiskFunctions prediction = predictions.get(i);
+
+            for(int k=0; k < this.events.length; k++){
+                final int event = this.events[k];
+                final RightContinuousStepFunction cif = prediction.getCumulativeIncidenceFunction(event);
+                errors[k] += calculator.calculateError(response, cif, event, integrationUpperBound) / n;
+            }
+
+        }
+
+        double totalError = 0.0;
+        for(int k=0; k < this.events.length; k++){
+            totalError += this.eventWeights[k] * errors[k];
+        }
+
+        return totalError;
+    }
+}

src/main/java/ca/joeltherrien/randomforest/tree/vimp/RegressionErrorCalculator.java

@@ -0,0 +1,23 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import java.util.List;
+
+public class RegressionErrorCalculator implements ErrorCalculator<Double, Double>{
+
+    @Override
+    public double averageError(List<Double> responses, List<Double> predictions) {
+        double mean = 0.0;
+        final double n = responses.size();
+
+        for(int i=0; i<responses.size(); i++){
+            final double response = responses.get(i);
+            final double prediction = predictions.get(i);
+
+            final double difference = response - prediction;
+
+            mean += difference * difference / n;
+        }
+
+        return mean;
+    }
+}

src/main/java/ca/joeltherrien/randomforest/tree/vimp/VariableImportanceCalculator.java

@@ -0,0 +1,117 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import ca.joeltherrien.randomforest.CovariateRow;
+import ca.joeltherrien.randomforest.Row;
+import ca.joeltherrien.randomforest.covariates.Covariate;
+import ca.joeltherrien.randomforest.tree.Tree;
+
+import java.util.List;
+import java.util.Optional;
+import java.util.Random;
+import java.util.stream.Collectors;
+
+
+public class VariableImportanceCalculator<Y, P> {
+
+    private final ErrorCalculator<Y, P> errorCalculator;
+    private final List<Tree<P>> trees;
+    private final List<Row<Y>> observations;
+
+    private final boolean isTrainingSet; // If true, then we use out-of-bag predictions
+    private final double[] baselineErrors;
+
+    public VariableImportanceCalculator(
+            ErrorCalculator<Y, P> errorCalculator,
+            List<Tree<P>> trees,
+            List<Row<Y>> observations,
+            boolean isTrainingSet
+            ){
+        this.errorCalculator = errorCalculator;
+        this.trees = trees;
+        this.observations = observations;
+        this.isTrainingSet = isTrainingSet;
+
+
+        try {
+
+            this.baselineErrors = new double[trees.size()];
+            for (int i = 0; i < baselineErrors.length; i++) {
+                final Tree<P> tree = trees.get(i);
+                final List<Row<Y>> oobSubset = getAppropriateSubset(observations, tree); // may not actually be OOB depending on isTrainingSet
+                final List<Y> responses = oobSubset.stream().map(Row::getResponse).collect(Collectors.toList());
+
+                this.baselineErrors[i] = errorCalculator.averageError(responses, makePredictions(oobSubset, tree));
+            }
+
+        } catch(Exception e){
+            e.printStackTrace();
+            throw e;
+        }
+
+    }
+
+    /**
+     * Returns an array of importance values for every Tree for the given Covariate.
+     *
+     * @param covariate The Covariate to scramble.
+     * @param random
+     * @return
+     */
+    public double[] calculateVariableImportanceRaw(Covariate covariate, Optional<Random> random){
+
+        final double[] vimp = new double[trees.size()];
+        for(int i = 0; i < vimp.length; i++){
+            final Tree<P> tree = trees.get(i);
+            final List<Row<Y>> oobSubset = getAppropriateSubset(observations, tree); // may not actually be OOB depending on isTrainingSet
+            final List<Y> responses = oobSubset.stream().map(Row::getResponse).collect(Collectors.toList());
+            final List<CovariateRow> scrambledValues = CovariateRow.scrambleCovariateValues(oobSubset, covariate, random);
+
+            final double error = errorCalculator.averageError(responses, makePredictions(scrambledValues, tree));
+
+            vimp[i] = error - this.baselineErrors[i];
+        }
+
+        return vimp;
+    }
+
+    public double calculateVariableImportanceZScore(Covariate covariate, Optional<Random> random){
+        final double[] vimpArray = calculateVariableImportanceRaw(covariate, random);
+
+        double mean = 0.0;
+        double variance = 0.0;
+        final double numTrees = vimpArray.length;
+
+        for(double vimp : vimpArray){
+            mean += vimp / numTrees;
+        }
+        for(double vimp : vimpArray){
+            variance += (vimp - mean)*(vimp - mean) / (numTrees - 1.0);
+        }
+
+        final double standardError = Math.sqrt(variance / numTrees);
+
+        return mean / standardError;
+    }
+
+
+
+    // Assume rowList has already been filtered for OOB
+    private List<P> makePredictions(List<? extends CovariateRow> rowList, Tree<P> tree){
+        return rowList.stream()
+                .map(tree::evaluate)
+                .collect(Collectors.toList());
+    }
+
+    private List<Row<Y>> getAppropriateSubset(List<Row<Y>> initialList, Tree<P> tree){
+        if(!isTrainingSet){
+            return initialList; // no need to make any subsets
+        }
+
+        return initialList.stream()
+                .filter(row -> !tree.idInBootstrapSample(row.getId()))
+                .collect(Collectors.toList());
+
+    }
+
+
+}

src/main/java/ca/joeltherrien/randomforest/utils/DataUtils.java

@@ -16,9 +16,7 @@
 
 package ca.joeltherrien.randomforest.utils;
 
-import ca.joeltherrien.randomforest.tree.Forest;
+import ca.joeltherrien.randomforest.tree.*;
-import ca.joeltherrien.randomforest.tree.ResponseCombiner;
-import ca.joeltherrien.randomforest.tree.Tree;
 
 import java.io.*;
 import java.util.*;
@@ -27,12 +25,17 @@ import java.util.zip.GZIPOutputStream;
 
 public class DataUtils {
 
-    public static <O, FO> Forest<O, FO> loadForest(File folder, ResponseCombiner<O, FO> treeResponseCombiner) throws IOException, ClassNotFoundException {
+    public static <O, FO> OnlineForest<O, FO> loadOnlineForest(File folder, ResponseCombiner<O, FO> treeResponseCombiner) throws IOException, ClassNotFoundException {
         if(!folder.isDirectory()){
             throw new IllegalArgumentException("Tree directory must be a directory!");
         }
 
         final File[] treeFiles = folder.listFiles((file, s) -> s.endsWith(".tree"));
+
+        return loadOnlineForest(treeFiles, treeResponseCombiner);
+    }
+
+    public static <O, FO> OnlineForest<O, FO> loadOnlineForest(File[] treeFiles, ResponseCombiner<O, FO> treeResponseCombiner) throws IOException, ClassNotFoundException {
         final List<File> treeFileList = Arrays.asList(treeFiles);
 
         Collections.sort(treeFileList, Comparator.comparing(File::getName));
@@ -48,16 +51,16 @@ public class DataUtils {
 
         }
 
-        return Forest.<O, FO>builder()
+        return OnlineForest.<O, FO>builder()
                 .trees(treeList)
                 .treeResponseCombiner(treeResponseCombiner)
                 .build();
 
     }
 
-    public static <O, FO> Forest<O, FO> loadForest(String folder, ResponseCombiner<O, FO> treeResponseCombiner) throws IOException, ClassNotFoundException {
+    public static <O, FO> OnlineForest<O, FO> loadOnlineForest(String folder, ResponseCombiner<O, FO> treeResponseCombiner) throws IOException, ClassNotFoundException {
         final File directory = new File(folder);
-        return loadForest(directory, treeResponseCombiner);
+        return loadOnlineForest(directory, treeResponseCombiner);
     }
 
     public static void saveObject(Serializable object, String filename) throws IOException {

src/main/java/ca/joeltherrien/randomforest/utils/DiscontinuousStepFunction.java

@@ -1,113 +0,0 @@
-/*
- * Copyright (c) 2019 Joel Therrien.
- *     This program is free software: you can redistribute it and/or modify
- *     it under the terms of the GNU General Public License as published by
- *     the Free Software Foundation, either version 3 of the License, or
- *     (at your option) any later version.
- *
- *     This program is distributed in the hope that it will be useful,
- *     but WITHOUT ANY WARRANTY; without even the implied warranty of
- *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- *     GNU General Public License for more details.
- *
- *     You should have received a copy of the GNU General Public License
- *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-
-package ca.joeltherrien.randomforest.utils;
-
-/**
- * Represents a function represented by discrete points. However, the function may be right-continuous or left-continuous
- * at a given point, with no consistency. This function tracks that.
- */
-public final class DiscontinuousStepFunction extends StepFunction {
-
-    private final double[] y;
-    private final boolean[] isLeftContinuous;
-
-    /**
-     * Represents the value that should be returned by evaluate if there are points prior to the time the function is being evaluated at.
-     *
-     * Map be null.
-     */
-    private final double defaultY;
-
-    public DiscontinuousStepFunction(double[] x, double[] y, boolean[] isLeftContinuous, double defaultY) {
-        super(x);
-        this.y = y;
-        this.isLeftContinuous = isLeftContinuous;
-        this.defaultY = defaultY;
-    }
-
-    @Override
-    public double evaluate(double time){
-        int index = Utils.binarySearchLessThan(0, x.length, x, time);
-        if(index < 0){
-            return defaultY;
-        }
-        else{
-            if(x[index] == time){
-                return evaluateByIndex(index);
-            }
-            else{
-                return y[index];
-            }
-        }
-    }
-
-
-    @Override
-    public double evaluatePrevious(double time){
-        int index = Utils.binarySearchLessThan(0, x.length, x, time) - 1;
-        if(index < 0){
-            return defaultY;
-        }
-        else{
-            if(x[index] == time){
-                return evaluateByIndex(index);
-            }
-            else{
-                return y[index];
-            }
-        }
-    }
-
-    @Override
-    public double evaluateByIndex(int i) {
-        if(isLeftContinuous[i]){
-            i -= 1;
-        }
-
-        if(i < 0){
-            return defaultY;
-        }
-
-        return y[i];
-    }
-
-    @Override
-    public String toString(){
-        final StringBuilder builder = new StringBuilder();
-        builder.append("Default point: ");
-        builder.append(defaultY);
-        builder.append("\n");
-
-        for(int i=0; i<x.length; i++){
-            builder.append("x:");
-            builder.append(x[i]);
-            builder.append('\t');
-
-            if(isLeftContinuous[i]){
-                builder.append("*y:");
-            }
-            else{
-                builder.append("y*:");
-            }
-            builder.append(y[i]);
-            builder.append("\n");
-        }
-
-        return builder.toString();
-    }
-
-}

src/main/java/ca/joeltherrien/randomforest/utils/IterableOfflineTree.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2019 Joel Therrien.
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ *
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package ca.joeltherrien.randomforest.utils;
+
+import ca.joeltherrien.randomforest.tree.Tree;
+import lombok.RequiredArgsConstructor;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.util.Iterator;
+import java.util.zip.GZIPInputStream;
+
+@RequiredArgsConstructor
+public class IterableOfflineTree<Y> implements Iterable<Tree<Y>> {
+
+    private final File[] treeFiles;
+
+    @Override
+    public Iterator<Tree<Y>> iterator() {
+        return new OfflineTreeIterator<>(treeFiles);
+    }
+
+    @RequiredArgsConstructor
+    public static class OfflineTreeIterator<Y> implements Iterator<Tree<Y>>{
+        private final File[] treeFiles;
+        private int position = 0;
+
+        @Override
+        public boolean hasNext() {
+            return position < treeFiles.length;
+        }
+
+        @Override
+        public Tree<Y> next() {
+            final File treeFile = treeFiles[position];
+            position++;
+
+
+            try {
+                final ObjectInputStream inputStream= new ObjectInputStream(new GZIPInputStream(new FileInputStream(treeFile)));
+                final Tree<Y> tree = (Tree) inputStream.readObject();
+                return tree;
+            } catch (IOException | ClassNotFoundException e) {
+                e.printStackTrace();
+                throw new RuntimeException("Failed to load tree for " + treeFile.toString());
+            }
+
+        }
+    }
+
+
+}

src/main/java/ca/joeltherrien/randomforest/utils/LeftContinuousStepFunction.java

@@ -1,129 +0,0 @@
-/*
- * Copyright (c) 2019 Joel Therrien.
- *     This program is free software: you can redistribute it and/or modify
- *     it under the terms of the GNU General Public License as published by
- *     the Free Software Foundation, either version 3 of the License, or
- *     (at your option) any later version.
- *
- *     This program is distributed in the hope that it will be useful,
- *     but WITHOUT ANY WARRANTY; without even the implied warranty of
- *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- *     GNU General Public License for more details.
- *
- *     You should have received a copy of the GNU General Public License
- *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-
-package ca.joeltherrien.randomforest.utils;
-
-import java.util.List;
-import java.util.ListIterator;
-
-/**
- * Represents a function represented by discrete points. We assume that the function is a stepwise left-continuous
- * function, constant at the value of the previous encountered point.
- *
- */
-public final class LeftContinuousStepFunction extends StepFunction {
-
-    private final double[] y;
-
-    /**
-     * Represents the value that should be returned by evaluate if there are points prior to the time the function is being evaluated at.
-     *
-     * Map be null.
-     */
-    private final double defaultY;
-
-    public LeftContinuousStepFunction(double[] x, double[] y, double defaultY) {
-        super(x);
-        this.y = y;
-        this.defaultY = defaultY;
-    }
-
-    /**
-     * This isn't a formal constructor because of limitations with abstract classes.
-     *
-     * @param pointList
-     * @param defaultY
-     * @return
-     */
-    public static LeftContinuousStepFunction constructFromPoints(final List<Point> pointList, final double defaultY){
-
-        final double[] x = new double[pointList.size()];
-        final double[] y = new double[pointList.size()];
-
-        final ListIterator<Point> pointIterator = pointList.listIterator();
-        while(pointIterator.hasNext()){
-            final int index = pointIterator.nextIndex();
-            final Point currentPoint = pointIterator.next();
-
-            x[index] = currentPoint.getTime();
-            y[index] = currentPoint.getY();
-        }
-
-        return new LeftContinuousStepFunction(x, y, defaultY);
-
-    }
-
-    @Override
-    public double evaluate(double time){
-        int index = Utils.binarySearchLessThan(0, x.length, x, time);
-        if(index < 0){
-            return defaultY;
-        }
-        else{
-            if(x[index] == time){
-                return evaluateByIndex(index-1);
-            }
-            else{
-                return y[index];
-            }
-        }
-    }
-
-    @Override
-    public double evaluatePrevious(double time){
-        int index = Utils.binarySearchLessThan(0, x.length, x, time) - 1;
-        if(index < 0){
-            return defaultY;
-        }
-        else{
-            if(x[index] == time){
-                return evaluateByIndex(index-1);
-            }
-            else{
-                return y[index];
-            }
-        }
-    }
-
-    @Override
-    public double evaluateByIndex(int i) {
-        if(i < 0){
-            return defaultY;
-        }
-
-        return y[i];
-    }
-
-
-    @Override
-    public String toString(){
-        final StringBuilder builder = new StringBuilder();
-        builder.append("Default point: ");
-        builder.append(defaultY);
-        builder.append("\n");
-
-        for(int i=0; i<x.length; i++){
-            builder.append("x:");
-            builder.append(x[i]);
-            builder.append("\ty:");
-            builder.append(y[i]);
-            builder.append("\n");
-        }
-
-        return builder.toString();
-    }
-
-}

src/main/java/ca/joeltherrien/randomforest/utils/Point.java

@@ -26,6 +26,8 @@ import java.io.Serializable;
  */
 @Data
 public class Point implements Serializable {
+    private static final long serialVersionUID = 1L;
+
     private final double time;
     private final double y;
 }

src/main/java/ca/joeltherrien/randomforest/utils/RUtils.java

@@ -188,4 +188,24 @@ public final class RUtils {
         return responses;
     }
 
+    public static List<Object> produceSublist(List<Object> initialList, int[] indices){
+        final List<Object> newList = new ArrayList<>(indices.length);
+
+        for(int i : indices){
+            newList.add(initialList.get(i));
+        }
+
+        return newList;
+    }
+
+    public static File[] getTreeFileArray(String folderPath, int endingId){
+        final File[] fileArray = new File[endingId];
+
+        for(int i = 1; i <= endingId; i++){
+            fileArray[i-1] = new File(folderPath + "/tree-" + i + ".tree");
+        }
+
+        return fileArray;
+    }
+
 }

src/main/java/ca/joeltherrien/randomforest/utils/RightContinuousStepFunction.java

@@ -16,8 +16,14 @@
 
 package ca.joeltherrien.randomforest.utils;
 
+import lombok.Getter;
+
+import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.ListIterator;
+import java.util.function.DoubleBinaryOperator;
+import java.util.function.DoubleUnaryOperator;
 
 /**
  * Represents a function represented by discrete points. We assume that the function is a stepwise right-continuous
@@ -26,13 +32,16 @@ import java.util.ListIterator;
  */
 public final class RightContinuousStepFunction extends StepFunction {
 
+    private static final long serialVersionUID = 1L;
+
     private final double[] y;
 
     /**
-     * Represents the value that should be returned by evaluate if there are points prior to the time the function is being evaluated at.
+     * Represents the value that should be returned by evaluate if there are no points prior to the time the function is being evaluated at.
      *
-     * Map be null.
+     * May not be null.
      */
+    @Getter
     private final double defaultY;
 
     public RightContinuousStepFunction(double[] x, double[] y, double defaultY) {
@@ -127,7 +136,12 @@ public final class RightContinuousStepFunction extends StepFunction {
         }
 
         if(to < from){
-            return integrate(to, from);
+            return -integrate(to, from);
+        }
+
+        // Edge case - no points; just defaultY
+        if(this.x.length == 0){
+            return (to - from) * this.defaultY;
         }
 
         double summation = 0.0;
@@ -170,7 +184,7 @@ public final class RightContinuousStepFunction extends StepFunction {
             final double currentTime = xPoints[i];
             final double currentHeight = evaluateByIndex(i);
 
-            if(i == xPoints.length-1 || xPoints[i+1] > to){
+            if(i == xPoints.length-1 || xPoints[i+1] >= to){
                 summation += currentHeight * (to - currentTime);
                 return summation;
             }
@@ -186,5 +200,76 @@ public final class RightContinuousStepFunction extends StepFunction {
 
     }
 
+    public RightContinuousStepFunction unaryOperation(DoubleUnaryOperator operator){
+        final double newDefaultY = operator.applyAsDouble(this.defaultY);
+        final double[] newY = Arrays.stream(this.getY()).map(operator).toArray();
+
+        return new RightContinuousStepFunction(this.getX(), newY, newDefaultY);
+    }
+
+
+    public static RightContinuousStepFunction biOperation(RightContinuousStepFunction funLeft,
+                                                            RightContinuousStepFunction funRight,
+                                                            DoubleBinaryOperator operator){
+
+        final double newDefaultY = operator.applyAsDouble(funLeft.defaultY, funRight.defaultY);
+        final double[] leftX = funLeft.x;
+        final double[] rightX = funRight.x;
+
+        final List<Point> combinedPoints = new ArrayList<>(leftX.length + rightX.length);
+
+        // These indexes represent the times that have *already* been processed.
+        // They start at -1 because we already processed the defaultY values.
+        int indexLeft = -1;
+        int indexRight = -1;
+
+        // This while-loop will keep going until one of the functions reaches the ends of its points
+        while(indexLeft < leftX.length-1 && indexRight < rightX.length-1){
+            final double time;
+            if(leftX[indexLeft+1] < rightX[indexRight+1]){
+                indexLeft += 1;
+
+                time = leftX[indexLeft];
+                combinedPoints.add(new Point(time, operator.applyAsDouble(funLeft.evaluateByIndex(indexLeft), funRight.evaluateByIndex(indexRight))));
+            }
+            else if(leftX[indexLeft+1] > rightX[indexRight+1]){
+                indexRight += 1;
+
+                time = rightX[indexRight];
+                combinedPoints.add(new Point(time, operator.applyAsDouble(funLeft.evaluateByIndex(indexLeft), funRight.evaluateByIndex(indexRight))));
+            }
+            else{ // equal times
+                indexLeft += 1;
+                indexRight += 1;
+
+                time = leftX[indexLeft];
+                combinedPoints.add(new Point(time, operator.applyAsDouble(funLeft.evaluateByIndex(indexLeft), funRight.evaluateByIndex(indexRight))));
+            }
+        }
+
+        // At this point, at least one of function left or function right has reached the end of its points
+
+        // This while-loop occurring implies that functionRight can not move forward anymore
+        while(indexLeft < leftX.length-1){
+            indexLeft += 1;
+
+            final double time = leftX[indexLeft];
+            combinedPoints.add(new Point(time, operator.applyAsDouble(funLeft.evaluateByIndex(indexLeft), funRight.evaluateByIndex(indexRight))));
+
+        }
+
+        // This while-loop occurring implies that functionLeft can not move forward anymore
+        while(indexRight < rightX.length-1){
+            indexRight += 1;
+
+            final double time = rightX[indexRight];
+            combinedPoints.add(new Point(time, operator.applyAsDouble(funLeft.evaluateByIndex(indexLeft), funRight.evaluateByIndex(indexRight))));
+
+        }
+
+
+        return RightContinuousStepFunction.constructFromPoints(combinedPoints, newDefaultY);
+
+    }
 
 }

src/main/java/ca/joeltherrien/randomforest/utils/Utils.java

@@ -20,7 +20,7 @@ import java.util.*;
 
 public final class Utils {
 
-    public static StepFunction estimateOneMinusECDF(final double[] times){
+    public static RightContinuousStepFunction estimateOneMinusECDF(final double[] times){
         Arrays.sort(times);
 
         final Map<Double, Integer> timeCounterMap = new HashMap<>();

src/main/java/ca/joeltherrien/randomforest/utils/VeryDiscontinuousStepFunction.java

@@ -1,80 +0,0 @@
-/*
- * Copyright (c) 2019 Joel Therrien.
- *     This program is free software: you can redistribute it and/or modify
- *     it under the terms of the GNU General Public License as published by
- *     the Free Software Foundation, either version 3 of the License, or
- *     (at your option) any later version.
- *
- *     This program is distributed in the hope that it will be useful,
- *     but WITHOUT ANY WARRANTY; without even the implied warranty of
- *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- *     GNU General Public License for more details.
- *
- *     You should have received a copy of the GNU General Public License
- *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-
-package ca.joeltherrien.randomforest.utils;
-
-/**
- * Represents a step function represented by discrete points. However, there may be individual time values that has
- * a y value that doesn't belong to a particular 'step'.
- */
-public final class VeryDiscontinuousStepFunction implements MathFunction {
-
-    private final double[] x;
-    private final double[] yAt;
-    private final double[] yRight;
-
-    /**
-     * Represents the value that should be returned by evaluate if there are points prior to the time the function is being evaluated at.
-     *
-     * Map be null.
-     */
-    private final double defaultY;
-
-    public VeryDiscontinuousStepFunction(double[] x, double[] yAt, double[] yRight, double defaultY) {
-        this.x = x;
-        this.yAt = yAt;
-        this.yRight = yRight;
-        this.defaultY = defaultY;
-    }
-
-    @Override
-    public double evaluate(double time){
-        int index = Utils.binarySearchLessThan(0, x.length, x, time);
-        if(index < 0){
-            return defaultY;
-        }
-        else{
-            if(x[index] == time){
-                return yAt[index];
-            }
-            else{ // time > x[index]
-                return yRight[index];
-            }
-        }
-    }
-
-
-    @Override
-    public String toString(){
-        final StringBuilder builder = new StringBuilder();
-        builder.append("Default point: ");
-        builder.append(defaultY);
-        builder.append("\n");
-
-        for(int i=0; i<x.length; i++){
-            builder.append("x:");
-            builder.append(x[i]);
-            builder.append("\tyAt:");
-            builder.append(yAt[i]);
-            builder.append("\tyRight:");
-            builder.append(yRight[i]);
-            builder.append("\n");
-        }
-
-        return builder.toString();
-    }
-
-}

src/test/java/ca/joeltherrien/randomforest/TestDeterministicForests.java

@@ -45,20 +45,20 @@ public class TestDeterministicForests {
 
         int index = 0;
         for(int j=0; j<5; j++){
-            final NumericCovariate numericCovariate = new NumericCovariate("numeric"+j, index);
+            final NumericCovariate numericCovariate = new NumericCovariate("numeric"+j, index, false);
             covariateList.add(numericCovariate);
             index++;
         }
 
         for(int j=0; j<5; j++){
-            final BooleanCovariate booleanCovariate = new BooleanCovariate("boolean"+j, index);
+            final BooleanCovariate booleanCovariate = new BooleanCovariate("boolean"+j, index, false);
             covariateList.add(booleanCovariate);
             index++;
         }
 
         final List<String> levels = Utils.easyList("cat", "dog", "mouse");
         for(int j=0; j<5; j++){
-            final FactorCovariate factorCovariate = new FactorCovariate("factor"+j, index, levels);
+            final FactorCovariate factorCovariate = new FactorCovariate("factor"+j, index, levels, false);
             covariateList.add(factorCovariate);
             index++;
         }
@@ -214,14 +214,14 @@ public class TestDeterministicForests {
 
         forestTrainer5Trees.trainSerialOnDisk(Optional.empty());
         forestTrainer10Trees.trainSerialOnDisk(Optional.empty());
-        final Forest<Double, Double> forestSerial = DataUtils.loadForest(saveTreeFile, new MeanResponseCombiner());
+        final Forest<Double, Double> forestSerial = DataUtils.loadOnlineForest(saveTreeFile, new MeanResponseCombiner());
         TestUtils.removeFolder(saveTreeFile);
         verifyTwoForestsEqual(testData, referenceForest, forestSerial);
 
 
         forestTrainer5Trees.trainParallelOnDisk(Optional.empty(), 4);
         forestTrainer10Trees.trainParallelOnDisk(Optional.empty(), 4);
-        final Forest<Double, Double> forestParallel = DataUtils.loadForest(saveTreeFile, new MeanResponseCombiner());
+        final Forest<Double, Double> forestParallel = DataUtils.loadOnlineForest(saveTreeFile, new MeanResponseCombiner());
         TestUtils.removeFolder(saveTreeFile);
         verifyTwoForestsEqual(testData, referenceForest, forestParallel);
 
@@ -259,7 +259,7 @@ public class TestDeterministicForests {
 
         for(int k=0; k<3; k++){
             forestTrainer.trainSerialOnDisk(Optional.empty());
-            final Forest<Double, Double> replicantForest = DataUtils.loadForest(saveTreeFile, responseCombiner);
+            final Forest<Double, Double> replicantForest = DataUtils.loadOnlineForest(saveTreeFile, responseCombiner);
             TestUtils.removeFolder(saveTreeFile);
             verifyTwoForestsEqual(testData, referenceForest, replicantForest);
         }
@@ -274,7 +274,7 @@ public class TestDeterministicForests {
 
         for(int k=0; k<3; k++){
             forestTrainer.trainParallelOnDisk(Optional.empty(), 4);
-            final Forest<Double, Double> replicantForest = DataUtils.loadForest(saveTreeFile, responseCombiner);
+            final Forest<Double, Double> replicantForest = DataUtils.loadOnlineForest(saveTreeFile, responseCombiner);
             TestUtils.removeFolder(saveTreeFile);
             verifyTwoForestsEqual(testData, referenceForest, replicantForest);
         }

src/test/java/ca/joeltherrien/randomforest/TestProvidingInitialForest.java

@@ -20,8 +20,8 @@ import ca.joeltherrien.randomforest.covariates.Covariate;
 import ca.joeltherrien.randomforest.covariates.numeric.NumericCovariate;
 import ca.joeltherrien.randomforest.responses.regression.MeanResponseCombiner;
 import ca.joeltherrien.randomforest.responses.regression.WeightedVarianceSplitFinder;
-import ca.joeltherrien.randomforest.tree.Forest;
 import ca.joeltherrien.randomforest.tree.ForestTrainer;
+import ca.joeltherrien.randomforest.tree.OnlineForest;
 import ca.joeltherrien.randomforest.tree.Tree;
 import ca.joeltherrien.randomforest.tree.TreeTrainer;
 import ca.joeltherrien.randomforest.utils.DataUtils;
@@ -39,12 +39,12 @@ import static org.junit.jupiter.api.Assertions.*;
 
 public class TestProvidingInitialForest {
 
-    private Forest<Double, Double> initialForest;
+    private OnlineForest<Double, Double> initialForest;
     private List<Covariate> covariateList;
     private List<Row<Double>> data;
 
     public TestProvidingInitialForest(){
-        covariateList = Collections.singletonList(new NumericCovariate("x", 0));
+        covariateList = Collections.singletonList(new NumericCovariate("x", 0, false));
 
         data = Utils.easyList(
                 Row.createSimple(Utils.easyMap("x", "1.0"), covariateList, 1, 1.0),
@@ -107,8 +107,8 @@ public class TestProvidingInitialForest {
     public void testSerialInMemory(){
         final ForestTrainer<Double, Double, Double> forestTrainer = getForestTrainer(null, 20);
 
-        final Forest<Double, Double> newForest = forestTrainer.trainSerialInMemory(Optional.of(initialForest));
+        final OnlineForest<Double, Double> newForest = forestTrainer.trainSerialInMemory(Optional.of(initialForest));
-        assertEquals(20, newForest.getTrees().size());
+        assertEquals(20, newForest.getNumberOfTrees());
 
         for(Tree<Double> initialTree : initialForest.getTrees()){
             assertTrue(newForest.getTrees().contains(initialTree));
@@ -124,8 +124,8 @@ public class TestProvidingInitialForest {
     public void testParallelInMemory(){
         final ForestTrainer<Double, Double, Double> forestTrainer = getForestTrainer(null, 20);
 
-        final Forest<Double, Double> newForest = forestTrainer.trainParallelInMemory(Optional.of(initialForest), 2);
+        final OnlineForest<Double, Double> newForest = forestTrainer.trainParallelInMemory(Optional.of(initialForest), 2);
-        assertEquals(20, newForest.getTrees().size());
+        assertEquals(20, newForest.getNumberOfTrees());
 
         for(Tree<Double> initialTree : initialForest.getTrees()){
             assertTrue(newForest.getTrees().contains(initialTree));
@@ -149,11 +149,11 @@ public class TestProvidingInitialForest {
         forestTrainer.trainParallelOnDisk(Optional.of(initialForest), 2);
 
         assertEquals(20, directory.listFiles().length);
-        final Forest<Double, Double> newForest = DataUtils.loadForest(directory, new MeanResponseCombiner());
+        final OnlineForest<Double, Double> newForest = DataUtils.loadOnlineForest(directory, new MeanResponseCombiner());
 
 
 
-        assertEquals(20, newForest.getTrees().size());
+        assertEquals(20, newForest.getNumberOfTrees());
 
         final List<String> newForestTreesAsStrings = newForest.getTrees().stream()
                 .map(tree -> tree.toString()).collect(Collectors.toList());
@@ -179,9 +179,9 @@ public class TestProvidingInitialForest {
 
         assertEquals(20, directory.listFiles().length);
 
-        final Forest<Double, Double> newForest = DataUtils.loadForest(directory, new MeanResponseCombiner());
+        final OnlineForest<Double, Double> newForest = DataUtils.loadOnlineForest(directory, new MeanResponseCombiner());
 
-        assertEquals(20, newForest.getTrees().size());
+        assertEquals(20, newForest.getNumberOfTrees());
 
         final List<String> newForestTreesAsStrings = newForest.getTrees().stream()
                 .map(tree -> tree.toString()).collect(Collectors.toList());
@@ -198,7 +198,7 @@ public class TestProvidingInitialForest {
         it's not clear if the forest being provided is the same one that trees were saved from.
      */
     @Test
-    public void verifyExceptions(){
+    public void testExceptions(){
         final String filePath = "src/test/resources/trees/";
         final File directory = new File(filePath);
         if(directory.exists()){

src/test/java/ca/joeltherrien/randomforest/TestSavingLoading.java

@@ -24,11 +24,10 @@ import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskRespons
 import ca.joeltherrien.randomforest.responses.competingrisk.combiner.CompetingRiskFunctionCombiner;
 import ca.joeltherrien.randomforest.responses.competingrisk.combiner.CompetingRiskResponseCombiner;
 import ca.joeltherrien.randomforest.responses.competingrisk.splitfinder.LogRankSplitFinder;
-import ca.joeltherrien.randomforest.tree.Forest;
+import ca.joeltherrien.randomforest.tree.*;
-import ca.joeltherrien.randomforest.tree.ForestTrainer;
-import ca.joeltherrien.randomforest.tree.TreeTrainer;
 import ca.joeltherrien.randomforest.utils.DataUtils;
 import ca.joeltherrien.randomforest.utils.ResponseLoader;
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
 import ca.joeltherrien.randomforest.utils.Utils;
 import org.junit.jupiter.api.Test;
 
@@ -47,10 +46,10 @@ public class TestSavingLoading {
 
     public List<Covariate> getCovariates(){
         return Utils.easyList(
-                new NumericCovariate("ageatfda", 0),
+                new NumericCovariate("ageatfda", 0, false),
-                new BooleanCovariate("idu", 1),
+                new BooleanCovariate("idu", 1, false),
-                new BooleanCovariate("black", 2),
+                new BooleanCovariate("black", 2, false),
-                new NumericCovariate("cd4nadir", 3)
+                new NumericCovariate("cd4nadir", 3, false)
         );
     }
 
@@ -119,16 +118,21 @@ public class TestSavingLoading {
         assertTrue(directory.isDirectory());
         assertEquals(NTREE, directory.listFiles().length);
 
-        final Forest<CompetingRiskFunctions, CompetingRiskFunctions> forest = DataUtils.loadForest(directory, new CompetingRiskFunctionCombiner(new int[]{1,2}, null));
+        final CompetingRiskFunctionCombiner treeResponseCombiner = new CompetingRiskFunctionCombiner(new int[]{1,2}, null);
+        final OnlineForest<CompetingRiskFunctions, CompetingRiskFunctions> onlineForest = DataUtils.loadOnlineForest(directory, treeResponseCombiner);
+        final OfflineForest<CompetingRiskFunctions, CompetingRiskFunctions> offlineForest = new OfflineForest<>(directory, treeResponseCombiner);
 
         final CovariateRow predictionRow = getPredictionRow(covariates);
 
-        final CompetingRiskFunctions functions = forest.evaluate(predictionRow);
+        final CompetingRiskFunctions functionsOnline = onlineForest.evaluate(predictionRow);
-        assertNotNull(functions);
+        assertNotNull(functionsOnline);
-        assertTrue(functions.getCumulativeIncidenceFunction(1).getX().length > 2);
+        assertTrue(functionsOnline.getCumulativeIncidenceFunction(1).getX().length > 2);
+
+        final CompetingRiskFunctions functionsOffline = offlineForest.evaluate(predictionRow);
+        assertTrue(competingFunctionsEqual(functionsOffline, functionsOnline));
 
 
-        assertEquals(NTREE, forest.getTrees().size());
+        assertEquals(NTREE, onlineForest.getTrees().size());
 
         TestUtils.removeFolder(directory);
 
@@ -159,17 +163,22 @@ public class TestSavingLoading {
         assertEquals(NTREE, directory.listFiles().length);
 
 
-
+        final CompetingRiskFunctionCombiner treeResponseCombiner = new CompetingRiskFunctionCombiner(new int[]{1,2}, null);
-        final Forest<CompetingRiskFunctions, CompetingRiskFunctions> forest = DataUtils.loadForest(directory, new CompetingRiskFunctionCombiner(new int[]{1,2}, null));
+        final OnlineForest<CompetingRiskFunctions, CompetingRiskFunctions> onlineForest = DataUtils.loadOnlineForest(directory, treeResponseCombiner);
+        final OfflineForest<CompetingRiskFunctions, CompetingRiskFunctions> offlineForest = new OfflineForest<>(directory, treeResponseCombiner);
 
         final CovariateRow predictionRow = getPredictionRow(covariates);
 
-        final CompetingRiskFunctions functions = forest.evaluate(predictionRow);
+        final CompetingRiskFunctions functionsOnline = onlineForest.evaluate(predictionRow);
-        assertNotNull(functions);
+        assertNotNull(functionsOnline);
-        assertTrue(functions.getCumulativeIncidenceFunction(1).getX().length > 2);
+        assertTrue(functionsOnline.getCumulativeIncidenceFunction(1).getX().length > 2);
 
 
-        assertEquals(NTREE, forest.getTrees().size());
+        final CompetingRiskFunctions functionsOffline = offlineForest.evaluate(predictionRow);
+        assertTrue(competingFunctionsEqual(functionsOffline, functionsOnline));
+
+
+        assertEquals(NTREE, onlineForest.getTrees().size());
 
         TestUtils.removeFolder(directory);
 
@@ -177,6 +186,64 @@ public class TestSavingLoading {
 
     }
 
+    /*
+        We don't implement equals() methods on the below mentioned classes because then we'd need to implement an
+        appropriate hashCode() method that's consistent with the equals(), and we only need plain equals() for
+        these tests.
+     */
+
+    private boolean competingFunctionsEqual(CompetingRiskFunctions f1 ,CompetingRiskFunctions f2){
+        if(!functionsEqual(f1.getSurvivalCurve(), f2.getSurvivalCurve())){
+            return false;
+        }
+
+        for(int i=1; i<=2; i++){
+            if(!functionsEqual(f1.getCauseSpecificHazardFunction(i), f2.getCauseSpecificHazardFunction(i))){
+                return false;
+            }
+            if(!functionsEqual(f1.getCumulativeIncidenceFunction(i), f2.getCumulativeIncidenceFunction(i))){
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private boolean functionsEqual(RightContinuousStepFunction f1, RightContinuousStepFunction f2){
+
+        final double[] f1X = f1.getX();
+        final double[] f2X = f2.getX();
+
+        final double[] f1Y = f1.getY();
+        final double[] f2Y = f2.getY();
+
+        // first compare array lengths
+        if(f1X.length != f2X.length){
+            return false;
+        }
+        if(f1Y.length != f2Y.length){
+            return false;
+        }
+
+        // TODO - better comparisons of doubles. I don't really care too much though as this equals method is only being used in tests
+        final double delta = 0.000001;
+
+        if(Math.abs(f1.getDefaultY() - f2.getDefaultY()) > delta){
+            return false;
+        }
+
+        for(int i=0; i < f1X.length; i++){
+            if(Math.abs(f1X[i] - f2X[i]) > delta){
+                return false;
+            }
+            if(Math.abs(f1Y[i] - f2Y[i]) > delta){
+                return false;
+            }
+        }
+
+        return true;
+
+    }
 
 
 }

src/test/java/ca/joeltherrien/randomforest/TestUtils.java

@@ -156,7 +156,7 @@ public class TestUtils {
     }
 
     @Test
-    public void reduceListToSize(){
+    public void testReduceListToSize(){
         final List<Integer> testList = Utils.easyList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
         final Random random = new Random();
         for(int i=0; i<100; i++) { // want to test many times to ensure it doesn't work just due to randomness

src/test/java/ca/joeltherrien/randomforest/competingrisk/IBSCalculatorTest.java

@@ -0,0 +1,164 @@
+package ca.joeltherrien.randomforest.competingrisk;
+
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskFunctions;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskResponse;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskUtils;
+import ca.joeltherrien.randomforest.responses.competingrisk.IBSCalculator;
+import ca.joeltherrien.randomforest.utils.Point;
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
+import ca.joeltherrien.randomforest.utils.Utils;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class IBSCalculatorTest {
+
+    private final RightContinuousStepFunction cif;
+
+    public IBSCalculatorTest(){
+        this.cif = RightContinuousStepFunction.constructFromPoints(
+                Utils.easyList(
+                    new Point(1.0, 0.1),
+                    new Point(2.0, 0.2),
+                    new Point(3.0, 0.3),
+                    new Point(4.0, 0.8)
+                ), 0.0
+        );
+    }
+
+    /*
+    R code to get these results:
+
+    predicted_cif <- stepfun(1:4, c(0, 0.1, 0.2, 0.3, 0.8))
+    weights <- 1
+    recorded_time <- 2.0
+    recorded_status <- 1.0
+    event_of_interest <- 2
+    times <- 0:4
+
+    errors <- weights * ( as.integer(recorded_time <= times & recorded_status == event_of_interest) - predicted_cif(times))^2
+    sum(errors)
+
+
+    and run again with event_of_interest <- 1
+
+
+    Note that in the R code I only evaluate up to 4, while in the Java code I integrate up to 5
+    This is because the evaluation at 4 is giving the area of the rectangle from 4 to 5.
+
+     */
+
+    @Test
+    public void testResultsWithoutCensoringDistribution(){
+        final IBSCalculator calculator = new IBSCalculator();
+
+        final double errorDifferentEvent = calculator.calculateError(
+                new CompetingRiskResponse(1, 2.0),
+                this.cif,
+                2,
+                5.0);
+
+        assertEquals(0.78, errorDifferentEvent, 0.000001);
+
+        final double errorSameEvent = calculator.calculateError(
+                new CompetingRiskResponse(1, 2.0),
+                this.cif,
+                1,
+                5.0);
+
+        assertEquals(1.18, errorSameEvent, 0.000001);
+
+    }
+
+    @Test
+    public void testResultsWithCensoringDistribution(){
+        final RightContinuousStepFunction censorSurvivalFunction = RightContinuousStepFunction.constructFromPoints(
+                Utils.easyList(
+                        new Point(0.0, 0.75),
+                        new Point(1.0, 0.5),
+                        new Point(3.0, 0.25),
+                        new Point(5.0, 0)
+                        ), 1.0
+        );
+
+        final IBSCalculator calculator = new IBSCalculator(censorSurvivalFunction);
+
+        final double errorDifferentEvent = calculator.calculateError(
+                new CompetingRiskResponse(1, 2.0),
+                this.cif,
+                2,
+                5.0);
+
+        assertEquals(1.56, errorDifferentEvent, 0.000001);
+
+        final double errorSameEvent = calculator.calculateError(
+                new CompetingRiskResponse(1, 2.0),
+                this.cif,
+                1,
+                5.0);
+
+        assertEquals(2.36, errorSameEvent, 0.000001);
+
+    }
+
+    @Test
+    public void testStaticFunction(){
+        final RightContinuousStepFunction censorSurvivalFunction = RightContinuousStepFunction.constructFromPoints(
+                Utils.easyList(
+                        new Point(0.0, 0.75),
+                        new Point(1.0, 0.5),
+                        new Point(3.0, 0.25),
+                        new Point(5.0, 0)
+                ), 1.0
+        );
+
+        final List<CompetingRiskResponse> responseList = Utils.easyList(
+                new CompetingRiskResponse(1, 2.0),
+                new CompetingRiskResponse(1, 2.0));
+
+        // for predictions; we'll construct an improper CompetingRisksFunctions
+        final RightContinuousStepFunction trivialFunction = RightContinuousStepFunction.constructFromPoints(
+                Utils.easyList(new Point(1.0, 0.0)),
+                1.0);
+
+        final CompetingRiskFunctions prediction = CompetingRiskFunctions.builder()
+                .survivalCurve(trivialFunction)
+                .causeSpecificHazards(Utils.easyList(trivialFunction, trivialFunction))
+                .cumulativeIncidenceCurves(Utils.easyList(this.cif, trivialFunction))
+                .build();
+
+        final List<CompetingRiskFunctions> predictionList = Utils.easyList(prediction, prediction);
+
+        double[] errorParallel = CompetingRiskUtils.calculateIBSError(
+                responseList,
+                predictionList,
+                Optional.of(censorSurvivalFunction),
+                1,
+                5.0,
+                true);
+
+        double[] errorSerial = CompetingRiskUtils.calculateIBSError(
+                responseList,
+                predictionList,
+                Optional.of(censorSurvivalFunction),
+                1,
+                5.0,
+                false);
+
+        assertEquals(responseList.size(), errorParallel.length);
+        assertEquals(responseList.size(), errorSerial.length);
+
+        assertEquals(2.36, errorParallel[0], 0.000001);
+        assertEquals(2.36, errorParallel[1], 0.000001);
+
+        assertEquals(2.36, errorSerial[0], 0.000001);
+        assertEquals(2.36, errorSerial[1], 0.000001);
+
+    }
+
+
+
+}

src/test/java/ca/joeltherrien/randomforest/competingrisk/TestCompetingRisk.java

@@ -53,10 +53,10 @@ public class TestCompetingRisk {
 
     public List<Covariate> getCovariates(){
         return Utils.easyList(
-                new NumericCovariate("ageatfda", 0),
+                new NumericCovariate("ageatfda", 0, false),
-                new BooleanCovariate("idu", 1),
+                new BooleanCovariate("idu", 1, false),
-                new BooleanCovariate("black", 2),
+                new BooleanCovariate("black", 2, false),
-                new NumericCovariate("cd4nadir", 3)
+                new NumericCovariate("cd4nadir", 3, false)
         );
     }
 
@@ -109,8 +109,8 @@ public class TestCompetingRisk {
 
         // by only using BooleanCovariates (only one split rule) we can guarantee identical results with randomForestSRC on one tree.
         final List<Covariate> covariates = Utils.easyList(
-                new BooleanCovariate("idu", 0),
+                new BooleanCovariate("idu", 0, false),
-                new BooleanCovariate("black", 1)
+                new BooleanCovariate("black", 1, false)
         );
 
         final List<Row<CompetingRiskResponse>> dataset = getData(covariates, "src/test/resources/wihs.bootstrapped.csv");
@@ -210,8 +210,8 @@ public class TestCompetingRisk {
     public void testLogRankSplitFinderTwoBooleans() throws IOException {
         // by only using BooleanCovariates (only one split rule) we can guarantee identical results with randomForestSRC on one tree.
         final List<Covariate> covariates = Utils.easyList(
-                new BooleanCovariate("idu", 0),
+                new BooleanCovariate("idu", 0, false),
-                new BooleanCovariate("black", 1)
+                new BooleanCovariate("black", 1, false)
         );
 
 
@@ -259,7 +259,7 @@ public class TestCompetingRisk {
     }
 
     @Test
-    public void verifyDataset() throws IOException {
+    public void testDataset() throws IOException {
         final List<Covariate> covariates = getCovariates();
 
         final List<Row<CompetingRiskResponse>> dataset = getData(covariates, DEFAULT_FILEPATH);

src/test/java/ca/joeltherrien/randomforest/competingrisk/TestCompetingRiskErrorRateCalculator.java

@@ -16,10 +16,11 @@
 
 package ca.joeltherrien.randomforest.competingrisk;
 
-import ca.joeltherrien.randomforest.Row;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskFunctions;
-import ca.joeltherrien.randomforest.covariates.Covariate;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskResponse;
-import ca.joeltherrien.randomforest.responses.competingrisk.*;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskUtils;
 import ca.joeltherrien.randomforest.tree.Forest;
+import ca.joeltherrien.randomforest.tree.OnlineForest;
 import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
 import ca.joeltherrien.randomforest.utils.StepFunction;
 import ca.joeltherrien.randomforest.utils.Utils;
@@ -30,8 +31,6 @@ import java.util.List;
 
 import static ca.joeltherrien.randomforest.TestUtils.closeEnough;
 import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.when;
 
 public class TestCompetingRiskErrorRateCalculator {
 
@@ -48,7 +47,7 @@ public class TestCompetingRiskErrorRateCalculator {
 
         final int event = 1;
 
-        final Forest<CompetingRiskFunctions, CompetingRiskFunctions> fakeForest = Forest.<CompetingRiskFunctions, CompetingRiskFunctions>builder().build();
+        final Forest<CompetingRiskFunctions, CompetingRiskFunctions> fakeForest = OnlineForest.<CompetingRiskFunctions, CompetingRiskFunctions>builder().build();
 
         final double naiveConcordance = CompetingRiskUtils.calculateConcordance(responseList, mortalityArray, event);
 

src/test/java/ca/joeltherrien/randomforest/competingrisk/TestLogRankSplitFinder.java

@@ -46,7 +46,7 @@ public class TestLogRankSplitFinder {
     public static Data<CompetingRiskResponse> loadData(String filename) throws IOException {
 
         final List<Covariate> covariates = Utils.easyList(
-                new NumericCovariate("x2", 0)
+                new NumericCovariate("x2", 0, false)
         );
 
         final List<Row<CompetingRiskResponse>> rows = TestUtils.loadData(covariates, new ResponseLoader.CompetingRisksResponseLoader("delta", "u"), filename);

src/test/java/ca/joeltherrien/randomforest/competingrisk/TestMathFunctions.java

@@ -16,7 +16,6 @@
 
 package ca.joeltherrien.randomforest.competingrisk;
 
-import ca.joeltherrien.randomforest.utils.LeftContinuousStepFunction;
 import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
 import org.junit.jupiter.api.Test;
 
@@ -31,13 +30,6 @@ public class TestMathFunctions {
         return new RightContinuousStepFunction(time, y, 0.1);
     }
 
-    private LeftContinuousStepFunction generateLeftContinuousStepFunction(){
-        final double[] time = new double[]{1.0, 2.0, 3.0};
-        final double[] y = new double[]{-1.0, 1.0, 0.5};
-
-        return new LeftContinuousStepFunction(time, y, 0.1);
-    }
-
     @Test
     public void testRightContinuousStepFunction(){
         final RightContinuousStepFunction function = generateRightContinuousStepFunction();
@@ -56,21 +48,5 @@ public class TestMathFunctions {
 
     }
 
-    @Test
-    public void testLeftContinuousStepFunction(){
-        final LeftContinuousStepFunction function = generateLeftContinuousStepFunction();
-
-        assertEquals(0.1, function.evaluate(0.5));
-        assertEquals(0.1, function.evaluate(1.0));
-        assertEquals(-1.0, function.evaluate(2.0));
-        assertEquals(1.0, function.evaluate(3.0));
-
-
-        assertEquals(0.1, function.evaluate(0.6));
-        assertEquals(-1.0, function.evaluate(1.1));
-        assertEquals(1.0, function.evaluate(2.1));
-        assertEquals(0.5, function.evaluate(3.1));
-
-    }
 
 }

src/test/java/ca/joeltherrien/randomforest/covariates/FactorCovariateTest.java

@@ -17,12 +17,15 @@
 package ca.joeltherrien.randomforest.covariates;
 
 
+import ca.joeltherrien.randomforest.Row;
 import ca.joeltherrien.randomforest.covariates.factor.FactorCovariate;
+import ca.joeltherrien.randomforest.tree.Split;
 import ca.joeltherrien.randomforest.utils.Utils;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.function.Executable;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Random;
 
@@ -31,7 +34,7 @@ import static org.junit.jupiter.api.Assertions.*;
 public class FactorCovariateTest {
 
     @Test
-    void verifyEqualLevels() {
+    public void testEqualLevels() {
         final FactorCovariate petCovariate = createTestCovariate();
 
         final FactorCovariate.FactorValue dog1 = petCovariate.createValue("DOG");
@@ -53,7 +56,7 @@ public class FactorCovariateTest {
     }
 
     @Test
-    void verifyBadLevelException(){
+    public void testBadLevelException(){
         final FactorCovariate petCovariate = createTestCovariate();
         final Executable badCode = () -> petCovariate.createValue("vulcan");
 
@@ -61,25 +64,169 @@ public class FactorCovariateTest {
     }
 
     @Test
-    void testAllSubsets(){
+    public void testAllSubsets(){
+        final int n = 2*3; // ensure that n is a multiple of 3 for the test
         final FactorCovariate petCovariate = createTestCovariate();
+        final List<Row<Double>> data = generateSampleData(petCovariate, n);
 
-        final List<SplitRule<String>> splitRules = new ArrayList<>();
+        final List<Split<Double, String>> splits = new ArrayList<>();
 
-        petCovariate.generateSplitRuleUpdater(null, 100, new Random())
+        petCovariate.generateSplitRuleUpdater(data, 100, new Random())
-                .forEachRemaining(split -> splitRules.add(split.getSplitRule()));
+                .forEachRemaining(split -> splits.add(split));
 
-        assertEquals(splitRules.size(), 3);
+        assertEquals(splits.size(), 3);
 
-        // TODO verify the contents of the split rules
+        // These are the 3 possibilities
+        boolean dog_catmouse = false;
+        boolean cat_dogmouse = false;
+        boolean mouse_dogcat = false;
 
+        for(Split<Double, String> split : splits){
+            List<Row<Double>> smallerHand;
+            List<Row<Double>> largerHand;
+
+            if(split.getLeftHand().size() < split.getRightHand().size()){
+                smallerHand = split.getLeftHand();
+                largerHand = split.getRightHand();
+            } else{
+                smallerHand = split.getRightHand();
+                largerHand = split.getLeftHand();
+            }
+
+            // There should be exactly one distinct value in the smaller list
+            assertEquals(n/3, smallerHand.size());
+            assertEquals(1,
+                    smallerHand.stream()
+                            .map(row -> row.getCovariateValue(petCovariate).getValue())
+                            .distinct()
+                            .count()
+            );
+
+            // There should be exactly two distinct values in the smaller list
+            assertEquals(2*n/3, largerHand.size());
+            assertEquals(2,
+                    largerHand.stream()
+                            .map(row -> row.getCovariateValue(petCovariate).getValue())
+                            .distinct()
+                            .count()
+            );
+
+            switch(smallerHand.get(0).getCovariateValue(petCovariate).getValue()){
+                case "DOG":
+                    dog_catmouse = true;
+                case "CAT":
+                    cat_dogmouse = true;
+                case "MOUSE":
+                    mouse_dogcat = true;
+            }
+
+        }
+
+        assertTrue(dog_catmouse);
+        assertTrue(cat_dogmouse);
+        assertTrue(mouse_dogcat);
+
+    }
+
+    /*
+     * There was a bug where if number==0 in generateSplitRuleUpdater, then the result was empty.
+     */
+    @Test
+    public void testNumber0Subsets(){
+        final int n = 2*3; // ensure that n is a multiple of 3 for the test
+        final FactorCovariate petCovariate = createTestCovariate();
+        final List<Row<Double>> data = generateSampleData(petCovariate, n);
+
+        final List<Split<Double, String>> splits = new ArrayList<>();
+
+        petCovariate.generateSplitRuleUpdater(data, 0, new Random())
+                .forEachRemaining(split -> splits.add(split));
+
+        assertEquals(splits.size(), 3);
+
+        // These are the 3 possibilities
+        boolean dog_catmouse = false;
+        boolean cat_dogmouse = false;
+        boolean mouse_dogcat = false;
+
+        for(Split<Double, String> split : splits){
+            List<Row<Double>> smallerHand;
+            List<Row<Double>> largerHand;
+
+            if(split.getLeftHand().size() < split.getRightHand().size()){
+                smallerHand = split.getLeftHand();
+                largerHand = split.getRightHand();
+            } else{
+                smallerHand = split.getRightHand();
+                largerHand = split.getLeftHand();
+            }
+
+            // There should be exactly one distinct value in the smaller list
+            assertEquals(n/3, smallerHand.size());
+            assertEquals(1,
+                    smallerHand.stream()
+                    .map(row -> row.getCovariateValue(petCovariate).getValue())
+                    .distinct()
+                    .count()
+            );
+
+            // There should be exactly two distinct values in the smaller list
+            assertEquals(2*n/3, largerHand.size());
+            assertEquals(2,
+                    largerHand.stream()
+                            .map(row -> row.getCovariateValue(petCovariate).getValue())
+                            .distinct()
+                            .count()
+            );
+
+            switch(smallerHand.get(0).getCovariateValue(petCovariate).getValue()){
+                case "DOG":
+                    dog_catmouse = true;
+                case "CAT":
+                    cat_dogmouse = true;
+                case "MOUSE":
+                    mouse_dogcat = true;
+            }
+
+        }
+
+        assertTrue(dog_catmouse);
+        assertTrue(cat_dogmouse);
+        assertTrue(mouse_dogcat);
+
+    }
+
+    @Test
+    public void testSpitRuleUpdaterWithNAs(){
+        // When some NAs were present calling generateSplitRuleUpdater caused an exception.
+
+        final FactorCovariate covariate = createTestCovariate();
+        final List<Row<Double>> sampleData = generateSampleData(covariate, 10);
+        sampleData.add(Row.createSimple(Utils.easyMap("pet", "NA"), Collections.singletonList(covariate), 11, 5.0));
+
+        covariate.generateSplitRuleUpdater(sampleData, 0, new Random());
+
+        // Test passes if no exception has occurred.
     }
 
 
     private FactorCovariate createTestCovariate(){
         final List<String> levels = Utils.easyList("DOG", "CAT", "MOUSE");
 
-        return new FactorCovariate("pet", 0, levels);
+        return new FactorCovariate("pet", 0, levels, false);
+    }
+
+    private List<Row<Double>> generateSampleData(Covariate covariate, int n){
+        final List<Covariate> covariateList = Collections.singletonList(covariate);
+        final List<Row<Double>> dataList = new ArrayList<>(n);
+
+        final String[] levels = new String[]{"DOG", "CAT", "MOUSE"};
+
+        for(int i=0; i<n; i++){
+            dataList.add(Row.createSimple(Utils.easyMap("pet", levels[i % 3]), covariateList, 1, 1.0));
+        }
+
+        return dataList;
     }
 
 

src/test/java/ca/joeltherrien/randomforest/covariates/NumericCovariateTest.java

@@ -70,7 +70,7 @@ public class NumericCovariateTest {
 
     @Test
     public void testNumericCovariateDeterministic(){
-        final NumericCovariate covariate = new NumericCovariate("x", 0);
+        final NumericCovariate covariate = new NumericCovariate("x", 0, false);
 
         final List<Row<Double>> dataset = createTestDataset(covariate);
 
@@ -158,7 +158,7 @@ public class NumericCovariateTest {
 
     @Test
     public void testNumericSplitRuleUpdaterWithIndexes(){
-        final NumericCovariate covariate = new NumericCovariate("x", 0);
+        final NumericCovariate covariate = new NumericCovariate("x", 0, false);
 
         final List<Row<Double>> dataset = createTestDataset(covariate);
 
@@ -223,7 +223,7 @@ public class NumericCovariateTest {
      */
     @Test
     public void testNumericSplitRuleUpdaterWithIndexesAllMissingData(){
-        final NumericCovariate covariate = new NumericCovariate("x", 0);
+        final NumericCovariate covariate = new NumericCovariate("x", 0, false);
         final List<Row<Double>> dataset = createTestDatasetMissingValues(covariate);
         final NumericSplitRuleUpdater<Double> updater = covariate.generateSplitRuleUpdater(dataset, 5, new Random());
 

src/test/java/ca/joeltherrien/randomforest/nas/TestNAs.java

@@ -18,31 +18,34 @@ package ca.joeltherrien.randomforest.nas;
 
 import ca.joeltherrien.randomforest.Row;
 import ca.joeltherrien.randomforest.covariates.Covariate;
+import ca.joeltherrien.randomforest.covariates.bool.BooleanCovariate;
+import ca.joeltherrien.randomforest.covariates.factor.FactorCovariate;
 import ca.joeltherrien.randomforest.covariates.numeric.NumericCovariate;
 import ca.joeltherrien.randomforest.responses.regression.MeanResponseCombiner;
 import ca.joeltherrien.randomforest.responses.regression.WeightedVarianceSplitFinder;
+import ca.joeltherrien.randomforest.tree.Split;
 import ca.joeltherrien.randomforest.tree.TreeTrainer;
 import ca.joeltherrien.randomforest.utils.Utils;
 import org.junit.jupiter.api.Test;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Random;
 
+import static org.junit.jupiter.api.Assertions.*;
+
 public class TestNAs {
 
-    private List<Row<Double>> generateData(List<Covariate> covariates){
+    private List<Row<Double>> generateData1(List<Covariate> covariates){
         final List<Row<Double>> dataList = new ArrayList<>();
 
-
         // We must include an NA for one of the values
-        dataList.add(Row.createSimple(Utils.easyMap("x", "NA"), covariates, 1, 5.0));
+        dataList.add(Row.createSimple(Utils.easyMap("x", "NA", "y", "true", "z", "green"), covariates, 1, 5.0));
-        dataList.add(Row.createSimple(Utils.easyMap("x", "1"), covariates, 1, 6.0));
+        dataList.add(Row.createSimple(Utils.easyMap("x", "1", "y", "NA", "z", "blue"), covariates, 2, 6.0));
-        dataList.add(Row.createSimple(Utils.easyMap("x", "2"), covariates, 1, 5.5));
+        dataList.add(Row.createSimple(Utils.easyMap("x", "2", "y", "true", "z", "NA"), covariates, 3, 5.5));
-        dataList.add(Row.createSimple(Utils.easyMap("x", "7"), covariates, 1, 0.0));
+        dataList.add(Row.createSimple(Utils.easyMap("x", "7", "y", "false", "z", "green"), covariates, 4, 0.0));
-        dataList.add(Row.createSimple(Utils.easyMap("x", "8"), covariates, 1, 1.0));
+        dataList.add(Row.createSimple(Utils.easyMap("x", "8", "y", "true", "z", "blue"), covariates, 5, 1.0));
-        dataList.add(Row.createSimple(Utils.easyMap("x", "8.4"), covariates, 1, 1.0));
+        dataList.add(Row.createSimple(Utils.easyMap("x", "8.4", "y", "false", "z", "yellow"), covariates, 6, 1.0));
 
 
         return dataList;
@@ -54,14 +57,19 @@ public class TestNAs {
         // but NumericSplitRuleUpdater had unmodifiable lists when creating the split.
         // This bug verifies that this no longer causes a crash
 
-        final List<Covariate> covariates = Collections.singletonList(new NumericCovariate("x", 0));
+        final List<Covariate> covariates = Utils.easyList(
-        final List<Row<Double>> dataset = generateData(covariates);
+                new NumericCovariate("x", 0, false),
+                new BooleanCovariate("y", 1, true),
+                new FactorCovariate("z", 2, Utils.easyList("green", "blue", "yellow"), true)
+                );
+        final List<Row<Double>> dataset = generateData1(covariates);
 
         final TreeTrainer<Double, Double> treeTrainer = TreeTrainer.<Double, Double>builder()
                 .checkNodePurity(false)
                 .covariates(covariates)
                 .numberOfSplits(0)
                 .nodeSize(1)
+                .mtry(3)
                 .maxNodeDepth(1000)
                 .splitFinder(new WeightedVarianceSplitFinder())
                 .responseCombiner(new MeanResponseCombiner())
@@ -70,6 +78,87 @@ public class TestNAs {
         treeTrainer.growTree(dataset, new Random(123));
 
         // As long as no exception occurs, we passed
+    }
+
+    private List<Row<Double>> generateData2(List<Covariate> covariates){
+        final List<Row<Double>> dataList = new ArrayList<>();
+        // Idea - when ignoring NAs, BadVar gives a perfect split.
+        // GoodVar is slightly worse than BadVar when NAs are excluded.
+        // However, BadVar has a ton of NAs that will degrade its performance.
+        dataList.add(Row.createSimple(
+                Utils.easyMap("BadVar", "-1.0", "GoodVar", "true") // GoodVars one error
+                , covariates, 1, 5.0)
+        );
+        dataList.add(Row.createSimple(
+                Utils.easyMap("BadVar", "NA", "GoodVar", "false")
+                , covariates, 2, 5.0)
+        );
+        dataList.add(Row.createSimple(
+                Utils.easyMap("BadVar", "NA", "GoodVar", "false")
+                , covariates, 3, 5.0)
+        );
+        dataList.add(Row.createSimple(
+                Utils.easyMap("BadVar", "0.5", "GoodVar", "true")
+                , covariates, 4, 10.0)
+        );
+        dataList.add(Row.createSimple(
+                Utils.easyMap("BadVar", "NA", "GoodVar", "true")
+                , covariates, 5, 10.0)
+        );
+        dataList.add(Row.createSimple(
+                Utils.easyMap("BadVar", "NA", "GoodVar", "true")
+                , covariates, 6, 10.0)
+        );
+
+        return dataList;
+    }
+
+    @Test
+    // Test that the NA penalty works when selecting a best split.
+    public void testNAPenalty(){
+        final List<Covariate> covariates1 = Utils.easyList(
+          new NumericCovariate("BadVar", 0, true),
+          new BooleanCovariate("GoodVar", 1, false)
+        );
+
+        final List<Row<Double>> dataList1 = generateData2(covariates1);
+
+        final TreeTrainer<Double, Double> treeTrainer1 = TreeTrainer.<Double, Double>builder()
+                .checkNodePurity(false)
+                .covariates(covariates1)
+                .numberOfSplits(0)
+                .nodeSize(1)
+                .mtry(2)
+                .maxNodeDepth(1000)
+                .splitFinder(new WeightedVarianceSplitFinder())
+                .responseCombiner(new MeanResponseCombiner())
+                .build();
+
+        final Split<Double, ?> bestSplit1 = treeTrainer1.findBestSplitRule(dataList1, covariates1, new Random(123));
+        assertEquals(1, bestSplit1.getSplitRule().getParentCovariateIndex()); // 1 corresponds to GoodVar
+
+        // Run again without the penalty; verify that we get different results
+
+        final List<Covariate> covariates2 = Utils.easyList(
+                new NumericCovariate("BadVar", 0, false),
+                new BooleanCovariate("GoodVar", 1, false)
+        );
+
+        final List<Row<Double>> dataList2 = generateData2(covariates2);
+
+        final TreeTrainer<Double, Double> treeTrainer2 = TreeTrainer.<Double, Double>builder()
+                .checkNodePurity(false)
+                .covariates(covariates2)
+                .numberOfSplits(0)
+                .nodeSize(1)
+                .mtry(2)
+                .maxNodeDepth(1000)
+                .splitFinder(new WeightedVarianceSplitFinder())
+                .responseCombiner(new MeanResponseCombiner())
+                .build();
+
+        final Split<Double, ?> bestSplit2 = treeTrainer2.findBestSplitRule(dataList2, covariates2, new Random(123));
+        assertEquals(0, bestSplit2.getSplitRule().getParentCovariateIndex()); // 1 corresponds to GoodVar
 
 
     }

src/test/java/ca/joeltherrien/randomforest/tree/vimp/IBSErrorCalculatorWrapperTest.java

@@ -0,0 +1,143 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskFunctions;
+import ca.joeltherrien.randomforest.responses.competingrisk.CompetingRiskResponse;
+import ca.joeltherrien.randomforest.responses.competingrisk.IBSCalculator;
+import ca.joeltherrien.randomforest.utils.Point;
+import ca.joeltherrien.randomforest.utils.RightContinuousStepFunction;
+import ca.joeltherrien.randomforest.utils.Utils;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class IBSErrorCalculatorWrapperTest {
+
+    /*
+        We already have tests for the IBSCalculator, so these tests are concerned with making sure we correctly average
+        the errors together, not that we fully test the production of each error under different scenarios (like
+        providing / not providing a censoring distribution).
+     */
+
+    private final double integrationUpperBound = 5.0;
+
+    private final List<CompetingRiskResponse> responses;
+    private final List<CompetingRiskFunctions> functions;
+
+
+    private final double[][] errors;
+
+    public IBSErrorCalculatorWrapperTest(){
+        this.responses = Utils.easyList(
+                new CompetingRiskResponse(0, 2.0),
+                new CompetingRiskResponse(0, 3.0),
+                new CompetingRiskResponse(1, 1.0),
+                new CompetingRiskResponse(1, 1.5),
+                new CompetingRiskResponse(2, 3.0),
+                new CompetingRiskResponse(2, 4.0)
+        );
+
+        final RightContinuousStepFunction cif1 = RightContinuousStepFunction.constructFromPoints(Utils.easyList(
+                new Point(1.0, 0.25),
+                new Point(1.5, 0.45)
+        ), 0.0);
+
+        final RightContinuousStepFunction cif2 = RightContinuousStepFunction.constructFromPoints(Utils.easyList(
+                new Point(3.0, 0.25),
+                new Point(4.0, 0.45)
+        ), 0.0);
+
+        // This function is for the unused CHFs and survival curve
+        // If we see infinities or NaNs popping up in our output we should look here.
+        final RightContinuousStepFunction emptyFun = RightContinuousStepFunction.constructFromPoints(Utils.easyList(
+                new Point(0.0, Double.NaN)
+                ), Double.NEGATIVE_INFINITY
+        );
+
+        final CompetingRiskFunctions function = CompetingRiskFunctions.builder()
+                .cumulativeIncidenceCurves(Utils.easyList(cif1, cif2))
+                .causeSpecificHazards(Utils.easyList(emptyFun, emptyFun))
+                .survivalCurve(emptyFun)
+                .build();
+
+        // Same prediction for every response.
+        this.functions = Utils.easyList(function, function, function, function, function, function);
+
+        final IBSCalculator calculator = new IBSCalculator();
+        this.errors = new double[2][6];
+
+        for(int event : new int[]{1, 2}){
+            for(int i=0; i<6; i++){
+                this.errors[event-1][i] = calculator.calculateError(
+                        responses.get(i), function.getCumulativeIncidenceFunction(event),
+                        event, integrationUpperBound
+                );
+            }
+        }
+
+    }
+
+    @Test
+    public void testOneEventOne(){
+        final IBSErrorCalculatorWrapper wrapper = new IBSErrorCalculatorWrapper(new IBSCalculator(), new int[]{1},
+                this.integrationUpperBound);
+
+        final double error = wrapper.averageError(this.responses, this.functions);
+        double expectedError = 0.0;
+        for(int i=0; i<6; i++){
+            expectedError += errors[0][i] / 6.0;
+        }
+
+        assertEquals(expectedError, error, 0.00000001);
+    }
+
+    @Test
+    public void testOneEventTwo(){
+        final IBSErrorCalculatorWrapper wrapper = new IBSErrorCalculatorWrapper(new IBSCalculator(), new int[]{2},
+                this.integrationUpperBound);
+
+        final double error = wrapper.averageError(this.responses, this.functions);
+        double expectedError = 0.0;
+        for(int i=0; i<6; i++){
+            expectedError += errors[1][i] / 6.0;
+        }
+
+        assertEquals(expectedError, error, 0.00000001);
+    }
+
+    @Test
+    public void testTwoEventsNoWeights(){
+        final IBSErrorCalculatorWrapper wrapper = new IBSErrorCalculatorWrapper(new IBSCalculator(), new int[]{1, 2},
+                this.integrationUpperBound);
+
+        final double error = wrapper.averageError(this.responses, this.functions);
+        double expectedError1 = 0.0;
+        double expectedError2 = 0.0;
+
+        for(int i=0; i<6; i++){
+            expectedError1 += errors[0][i] / 6.0;
+            expectedError2 += errors[1][i] / 6.0;
+        }
+
+        assertEquals(expectedError1 + expectedError2, error, 0.00000001);
+    }
+
+    @Test
+    public void testTwoEventsWithWeights(){
+        final IBSErrorCalculatorWrapper wrapper = new IBSErrorCalculatorWrapper(new IBSCalculator(), new int[]{1, 2},
+                this.integrationUpperBound, new double[]{1.0, 2.0});
+
+        final double error = wrapper.averageError(this.responses, this.functions);
+        double expectedError1 = 0.0;
+        double expectedError2 = 0.0;
+
+        for(int i=0; i<6; i++){
+            expectedError1 += errors[0][i] / 6.0;
+            expectedError2 += errors[1][i] / 6.0;
+        }
+
+        assertEquals(1.0 * expectedError1 + 2.0 * expectedError2, error, 0.00000001);
+    }
+
+}

src/test/java/ca/joeltherrien/randomforest/tree/vimp/RegressionErrorCalculatorTest.java

@@ -0,0 +1,26 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import ca.joeltherrien.randomforest.utils.Utils;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class RegressionErrorCalculatorTest {
+
+    private final RegressionErrorCalculator calculator = new RegressionErrorCalculator();
+
+    @Test
+    public void testRegressionErrorCalculator(){
+        final List<Double> responses = Utils.easyList(1.0, 1.5, 0.0, 3.0);
+        final List<Double> predictions = Utils.easyList(1.5, 1.7, 0.1, 2.9);
+
+        // Differences are 0.5, 0.2, -0.1, 0.1
+        // Squared: 0.25, 0.04, 0.01, 0.01
+
+        assertEquals((0.25 + 0.04 + 0.01 + 0.01)/4.0, calculator.averageError(responses, predictions), 0.000000001);
+
+    }
+
+}

src/test/java/ca/joeltherrien/randomforest/tree/vimp/TestVariableImportanceCalculator.java

@@ -0,0 +1,484 @@
+package ca.joeltherrien.randomforest.tree.vimp;
+
+import ca.joeltherrien.randomforest.Row;
+import ca.joeltherrien.randomforest.covariates.Covariate;
+import ca.joeltherrien.randomforest.covariates.bool.BooleanCovariate;
+import ca.joeltherrien.randomforest.covariates.bool.BooleanSplitRule;
+import ca.joeltherrien.randomforest.covariates.factor.FactorCovariate;
+import ca.joeltherrien.randomforest.covariates.numeric.NumericCovariate;
+import ca.joeltherrien.randomforest.covariates.numeric.NumericSplitRule;
+import ca.joeltherrien.randomforest.responses.regression.MeanResponseCombiner;
+import ca.joeltherrien.randomforest.responses.regression.WeightedVarianceSplitFinder;
+import ca.joeltherrien.randomforest.tree.*;
+import ca.joeltherrien.randomforest.utils.Utils;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Optional;
+import java.util.Random;
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class TestVariableImportanceCalculator {
+
+    /*
+        Since the logic for VariableImportanceCalculator is generic, it will be much easier to test under a regression
+        setting.
+     */
+
+    // We'l have a very simple Forest of two trees
+    private final OnlineForest<Double, Double> forest;
+
+
+    private final List<Covariate> covariates;
+    private final List<Row<Double>> rowList;
+
+    /*
+        Long setup process; forest is manually constructed so that we can be exactly sure on our variable importance.
+
+     */
+    public TestVariableImportanceCalculator(){
+        final BooleanCovariate booleanCovariate = new BooleanCovariate("x", 0, false);
+        final NumericCovariate numericCovariate = new NumericCovariate("y", 1, false);
+        final FactorCovariate factorCovariate = new FactorCovariate("z", 2,
+                Utils.easyList("red", "blue", "green"), false);
+
+        this.covariates = Utils.easyList(booleanCovariate, numericCovariate, factorCovariate);
+
+        final TreeTrainer<Double, Double> treeTrainer = TreeTrainer.<Double, Double>builder()
+            .responseCombiner(new MeanResponseCombiner())
+            .splitFinder(new WeightedVarianceSplitFinder())
+             .numberOfSplits(0)
+             .nodeSize(1)
+             .maxNodeDepth(100)
+             .mtry(3)
+             .checkNodePurity(false)
+             .covariates(this.covariates)
+             .build();
+
+        /*
+            Plan for data - BooleanCovariate is split on first and has the largest impact.
+            NumericCovariate is at second level and has more minimal impact.
+            FactorCovariate is useless and never used.
+            Our tree (we'll duplicate it for testing OOB errors) will have a depth of 1. (0 based).
+         */
+
+        final Tree<Double> tree1 = makeTree(covariates, 0.0, new int[]{1,2,3,4});
+        final Tree<Double> tree2 = makeTree(covariates, 2.0, new int[]{5,6,7,8});
+
+        this.forest = OnlineForest.<Double, Double>builder()
+                .trees(Utils.easyList(tree1, tree2))
+                .treeResponseCombiner(new MeanResponseCombiner())
+                .build();
+
+        // formula; boolean high adds 100; high numeric adds 10
+        // This row list should have a baseline error of 0.0
+        this.rowList = Utils.easyList(
+                Row.createSimple(Utils.easyMap(
+                        "x", "false",
+                        "y", "0.0",
+                        "z", "red"),
+                        covariates, 1, 0.0
+                ),
+                Row.createSimple(Utils.easyMap(
+                        "x", "false",
+                        "y", "10.0",
+                        "z", "blue"),
+                        covariates, 2, 10.0
+                ),
+                Row.createSimple(Utils.easyMap(
+                        "x", "true",
+                        "y", "0.0",
+                        "z", "red"),
+                        covariates, 3, 100.0
+                ),
+                Row.createSimple(Utils.easyMap(
+                        "x", "true",
+                        "y", "10.0",
+                        "z", "green"),
+                        covariates, 4, 110.0
+                ),
+
+                Row.createSimple(Utils.easyMap(
+                        "x", "false",
+                        "y", "0.0",
+                        "z", "red"),
+                        covariates, 5, 0.0
+                ),
+                Row.createSimple(Utils.easyMap(
+                        "x", "false",
+                        "y", "10.0",
+                        "z", "blue"),
+                        covariates, 6, 10.0
+                ),
+                Row.createSimple(Utils.easyMap(
+                        "x", "true",
+                        "y", "0.0",
+                        "z", "red"),
+                        covariates, 7, 100.0
+                ),
+                Row.createSimple(Utils.easyMap(
+                        "x", "true",
+                        "y", "10.0",
+                        "z", "green"),
+                        covariates, 8, 110.0
+                )
+        );
+    }
+
+    private Tree<Double> makeTree(List<Covariate> covariates, double offset, int[] indices){
+        // Naming convention - xyTerminal where x and y are low/high denotes whether BooleanCovariate(x) is low/high and
+        // whether NumericCovariate(y) is low/high.
+        final TerminalNode<Double> lowLowTerminal = new TerminalNode<>(0.0 + offset, 5);
+        final TerminalNode<Double> lowHighTerminal = new TerminalNode<>(10.0 + offset, 5);
+        final TerminalNode<Double> highLowTerminal = new TerminalNode<>(100.0 + offset, 5);
+        final TerminalNode<Double> highHighTerminal = new TerminalNode<>(110.0 + offset, 5);
+
+        final SplitNode<Double> lowSplitNode = SplitNode.<Double>builder()
+                .leftHand(lowLowTerminal)
+                .rightHand(lowHighTerminal)
+                .probabilityNaLeftHand(0.5)
+                .splitRule(new NumericSplitRule((NumericCovariate) covariates.get(1), 5.0))
+                .build();
+
+        final SplitNode<Double> highSplitNode = SplitNode.<Double>builder()
+                .leftHand(highLowTerminal)
+                .rightHand(highHighTerminal)
+                .probabilityNaLeftHand(0.5)
+                .splitRule(new NumericSplitRule((NumericCovariate) covariates.get(1), 5.0))
+                .build();
+
+        final SplitNode<Double> rootSplitNode = SplitNode.<Double>builder()
+                .leftHand(lowSplitNode)
+                .rightHand(highSplitNode)
+                .probabilityNaLeftHand(0.5)
+                .splitRule(new BooleanSplitRule((BooleanCovariate) covariates.get(0)))
+                .build();
+
+        return new Tree<>(rootSplitNode, indices);
+
+    }
+
+    // Experiment with random seeds to first examine what a split does so we know what to expect
+    /*
+    public static void main(String[] args){
+
+        // Behaviour for OOB
+        final List<Integer> ints1 = IntStream.range(5, 9).boxed().collect(Collectors.toList());
+        final List<Integer> ints2 = IntStream.range(1, 5).boxed().collect(Collectors.toList());
+
+        final Random random = new Random(123);
+        Collections.shuffle(ints1, random);
+        Collections.shuffle(ints2, random);
+
+        System.out.println(ints1);
+        System.out.println(ints2);
+        // [5, 6, 8, 7]
+        // [3, 4, 1, 2]
+
+
+        // Behaviour for no-OOB
+        final List<Integer> fullInts1 = IntStream.range(1, 9).boxed().collect(Collectors.toList());
+        final List<Integer> fullInts2 = IntStream.range(1, 9).boxed().collect(Collectors.toList());
+        final Random fullIntsRandom = new Random(123);
+
+        Collections.shuffle(fullInts1, fullIntsRandom);
+        Collections.shuffle(fullInts2, fullIntsRandom);
+        System.out.println(fullInts1);
+        System.out.println(fullInts2);
+        // [1, 4, 8, 2, 5, 3, 7, 6]
+        // [6, 1, 4, 7, 5, 2, 8, 3]
+
+    }
+    */
+
+
+    private double[] difference(double[] a, double[] b){
+        final double[] results = new double[a.length];
+
+        for(int i = 0; i < a.length; i++){
+            results[i] = a[i] - b[i];
+        }
+
+        return results;
+    }
+
+    private void assertDoubleEquals(double[] expected, double[] actual){
+        assertEquals(expected.length, actual.length, "Lengths of arrays should be equal");
+
+        for(int i=0; i < expected.length; i++){
+            assertEquals(expected[i], actual[i], 0.0000001, "Difference at " + i);
+        }
+
+    }
+
+
+
+    @Test
+    public void testVariableImportanceOnXNoOOB(){
+        // x is the BooleanCovariate
+
+        final VariableImportanceCalculator<Double, Double> calculator = new VariableImportanceCalculator<>(
+                new RegressionErrorCalculator(),
+                this.forest.getTrees(),
+                this.rowList,
+                false
+        );
+
+        final Covariate covariate = this.covariates.get(0);
+
+        double importance[] = calculator.calculateVariableImportanceRaw(covariate, Optional.of(new Random(123)));
+
+        final double[] expectedBaselineError = {0.0, 4.0}; // first tree is accurate, second tree is not
+
+        // [1, 4, 8, 2, 5, 3, 7, 6]
+        final List<Double> permutedPredictionsTree1 = Utils.easyList(
+                0., 110., 100., 10., 0., 110., 100., 10.
+        );
+
+        // [6, 1, 4, 7, 5, 2, 8, 3]
+        // Actual: [F, F, T, T, F, F, T, T]
+        // Seen:   [F, F, T, T, F, F, T, T]
+        // Difference: 0 all around; random chance
+        final List<Double> permutedPredictionsTree2 = Utils.easyList(
+                2., 12., 102., 112., 2., 12., 102., 112.
+        );
+
+        final List<Double> observedValues = this.rowList.stream().map(Row::getResponse).collect(Collectors.toList());
+
+        final double[] expectedError = new double[2];
+
+        expectedError[0] = new RegressionErrorCalculator().averageError(observedValues, permutedPredictionsTree1);
+        expectedError[1] = new RegressionErrorCalculator().averageError(observedValues, permutedPredictionsTree2);
+
+        final double[] expectedVimp = difference(expectedError, expectedBaselineError);
+
+        assertDoubleEquals(expectedVimp, importance);
+
+        final double expectedVimpMean = (expectedVimp[0] + expectedVimp[1]) / 2.0;
+        final double expectedVimpVar = (Math.pow(expectedVimp[0] - expectedVimpMean, 2) + Math.pow(expectedVimp[1] - expectedVimpMean, 2)) / 1.0;
+        final double expectedVimpStandardError = Math.sqrt(expectedVimpVar / 2.0);
+        final double expectedZScore = expectedVimpMean / expectedVimpStandardError;
+
+        final double actualZScore = calculator.calculateVariableImportanceZScore(covariate, Optional.of(new Random(123)));
+
+        assertEquals(expectedZScore, actualZScore, 0.000001, "Z scores must match");
+    }
+
+
+
+    @Test
+    public void testVariableImportanceOnXOOB(){
+        // x is the BooleanCovariate
+
+        final VariableImportanceCalculator<Double, Double> calculator = new VariableImportanceCalculator<>(
+                new RegressionErrorCalculator(),
+                this.forest.getTrees(),
+                this.rowList,
+                true
+        );
+
+        final Covariate covariate = this.covariates.get(0);
+
+        double importance[] = calculator.calculateVariableImportanceRaw(covariate, Optional.of(new Random(123)));
+
+        final double[] expectedBaselineError = {0.0, 4.0}; // first tree is accurate, second tree is not
+
+        // [5, 6, 8, 7]
+        // Actual: [F, F, T, T]
+        // Seen:   [F, F, T, T]
+        // Difference: No differences
+        final List<Double> permutedPredictionsTree1 = Utils.easyList(
+                0., 10., 100., 110.
+        );
+
+        // [3, 4, 1, 2]
+        // Actual: [F, F, T, T]
+        // Seen:   [T, T, F, F]
+        // Difference: +100, +100, -100, -100
+        final List<Double> permutedPredictionsTree2 = Utils.easyList(
+                102., 112., 2., 12.
+        );
+
+        final List<Double> observedValues = this.rowList.stream().map(Row::getResponse).collect(Collectors.toList());
+        final List<Double> tree1OOBValues = observedValues.subList(4, 8);
+        final List<Double> tree2OOBValues = observedValues.subList(0, 4);
+
+        final double[] expectedError = new double[2];
+
+        expectedError[0] = new RegressionErrorCalculator().averageError(tree1OOBValues, permutedPredictionsTree1);
+        expectedError[1] = new RegressionErrorCalculator().averageError(tree2OOBValues, permutedPredictionsTree2);
+
+        final double[] expectedVimp = difference(expectedError, expectedBaselineError);
+
+        assertDoubleEquals(expectedVimp, importance);
+
+        final double expectedVimpMean = (expectedVimp[0] + expectedVimp[1]) / 2.0;
+        final double expectedVimpVar = (Math.pow(expectedVimp[0] - expectedVimpMean, 2) + Math.pow(expectedVimp[1] - expectedVimpMean, 2)) / 1.0;
+        final double expectedVimpStandardError = Math.sqrt(expectedVimpVar / 2.0);
+        final double expectedZScore = expectedVimpMean / expectedVimpStandardError;
+
+        final double actualZScore = calculator.calculateVariableImportanceZScore(covariate, Optional.of(new Random(123)));
+
+        assertEquals(expectedZScore, actualZScore, 0.000001, "Z scores must match");
+    }
+
+
+    @Test
+    public void testVariableImportanceOnYNoOOB(){
+        // y is the NumericCovariate
+
+        final VariableImportanceCalculator<Double, Double> calculator = new VariableImportanceCalculator<>(
+                new RegressionErrorCalculator(),
+                this.forest.getTrees(),
+                this.rowList,
+                false
+        );
+
+        final Covariate covariate = this.covariates.get(1);
+
+        double importance[] = calculator.calculateVariableImportanceRaw(covariate, Optional.of(new Random(123)));
+
+        final double[] expectedBaselineError = {0.0, 4.0}; // first tree is accurate, second tree is not
+
+        // [1, 4, 8, 2, 5, 3, 7, 6]
+        // Actual:     [F, T, F, T, F, T, F, T]
+        // Seen:       [F, T, T, T, F, F, F, T]
+        // Difference: [=, =, +, =, =, -, =, =]x10
+        final List<Double> permutedPredictionsTree1 = Utils.easyList(
+                0., 10., 110., 110., 0., 0., 100., 110.
+        );
+
+        // [6, 1, 4, 7, 5, 2, 8, 3]
+        // Actual:     [F, T, F, T, F, T, F, T]
+        // Seen:       [T, F, T, F, F, T, T, F]
+        // Difference: [+, -, +, -, =, =, +, -]
+        final List<Double> permutedPredictionsTree2 = Utils.easyList(
+                12., 2., 112., 102., 2., 12., 112., 102.
+        );
+
+        final List<Double> observedValues = this.rowList.stream().map(Row::getResponse).collect(Collectors.toList());
+
+        final double[] expectedError = new double[2];
+
+        expectedError[0] = new RegressionErrorCalculator().averageError(observedValues, permutedPredictionsTree1);
+        expectedError[1] = new RegressionErrorCalculator().averageError(observedValues, permutedPredictionsTree2);
+
+        final double[] expectedVimp = difference(expectedError, expectedBaselineError);
+
+        assertDoubleEquals(expectedVimp, importance);
+
+        final double expectedVimpMean = (expectedVimp[0] + expectedVimp[1]) / 2.0;
+        final double expectedVimpVar = (Math.pow(expectedVimp[0] - expectedVimpMean, 2) + Math.pow(expectedVimp[1] - expectedVimpMean, 2)) / 1.0;
+        final double expectedVimpStandardError = Math.sqrt(expectedVimpVar / 2.0);
+        final double expectedZScore = expectedVimpMean / expectedVimpStandardError;
+
+        final double actualZScore = calculator.calculateVariableImportanceZScore(covariate, Optional.of(new Random(123)));
+
+        assertEquals(expectedZScore, actualZScore, 0.000001, "Z scores must match");
+    }
+
+
+
+    @Test
+    public void testVariableImportanceOnYOOB(){
+        // y is the NumericCovariate
+
+        final VariableImportanceCalculator<Double, Double> calculator = new VariableImportanceCalculator<>(
+                new RegressionErrorCalculator(),
+                this.forest.getTrees(),
+                this.rowList,
+                true
+        );
+
+        final Covariate covariate = this.covariates.get(1);
+
+        double importance[] = calculator.calculateVariableImportanceRaw(covariate, Optional.of(new Random(123)));
+
+        final double[] expectedBaselineError = {0.0, 4.0}; // first tree is accurate, second tree is not
+
+        // [5, 6, 8, 7]
+        // Actual:     [F, T, F, T]
+        // Seen:       [F, T, T, F]
+        // Difference: [=, =, +, -]x10
+        final List<Double> permutedPredictionsTree1 = Utils.easyList(
+                0., 10., 110., 100.
+        );
+
+        // [3, 4, 1, 2]
+        // Actual:     [F, T, F, T]
+        // Seen:       [F, T, F, T]
+        // Difference: [=, =, =, =]x10 no change
+        final List<Double> permutedPredictionsTree2 = Utils.easyList(
+                2., 12., 102., 112.
+        );
+
+        final List<Double> observedValues = this.rowList.stream().map(Row::getResponse).collect(Collectors.toList());
+        final List<Double> tree1OOBValues = observedValues.subList(4, 8);
+        final List<Double> tree2OOBValues = observedValues.subList(0, 4);
+
+        final double[] expectedError = new double[2];
+
+        expectedError[0] = new RegressionErrorCalculator().averageError(tree1OOBValues, permutedPredictionsTree1);
+        expectedError[1] = new RegressionErrorCalculator().averageError(tree2OOBValues, permutedPredictionsTree2);
+
+        final double[] expectedVimp = difference(expectedError, expectedBaselineError);
+
+        assertDoubleEquals(expectedVimp, importance);
+
+        final double expectedVimpMean = (expectedVimp[0] + expectedVimp[1]) / 2.0;
+        final double expectedVimpVar = (Math.pow(expectedVimp[0] - expectedVimpMean, 2) + Math.pow(expectedVimp[1] - expectedVimpMean, 2)) / 1.0;
+        final double expectedVimpStandardError = Math.sqrt(expectedVimpVar / 2.0);
+        final double expectedZScore = expectedVimpMean / expectedVimpStandardError;
+
+        final double actualZScore = calculator.calculateVariableImportanceZScore(covariate, Optional.of(new Random(123)));
+
+        assertEquals(expectedZScore, actualZScore, 0.000001, "Z scores must match");
+
+
+    }
+
+
+
+    @Test
+    public void testVariableImportanceOnZNoOOB(){
+        // z is the useless FactorCovariate
+
+        final VariableImportanceCalculator<Double, Double> calculator = new VariableImportanceCalculator<>(
+                new RegressionErrorCalculator(),
+                this.forest.getTrees(),
+                this.rowList,
+                false
+        );
+
+        final double[] importance = calculator.calculateVariableImportanceRaw(this.covariates.get(2), Optional.of(new Random(123)));
+        final double[] expectedImportance = {0.0, 0.0};
+
+
+        // FactorImportance did nothing; so permuting it will make no difference to baseline error
+        assertDoubleEquals(expectedImportance, importance);
+    }
+
+    @Test
+    public void testVariableImportanceOnZOOB(){
+        // z is the useless FactorCovariate
+
+        final VariableImportanceCalculator<Double, Double> calculator = new VariableImportanceCalculator<>(
+                new RegressionErrorCalculator(),
+                this.forest.getTrees(),
+                this.rowList,
+                true
+        );
+
+        final double[] importance = calculator.calculateVariableImportanceRaw(this.covariates.get(2), Optional.of(new Random(123)));
+        final double[] expectedImportance = {0.0, 0.0};
+
+
+        // FactorImportance did nothing; so permuting it will make no difference to baseline error
+        assertDoubleEquals(expectedImportance, importance);
+    }
+
+
+
+}

src/test/java/ca/joeltherrien/randomforest/utils/RightContinuousStepFunctionIntegrationTest.java

@@ -19,6 +19,9 @@ package ca.joeltherrien.randomforest.utils;
 import ca.joeltherrien.randomforest.TestUtils;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
 public class RightContinuousStepFunctionIntegrationTest {
 
     private RightContinuousStepFunction createTestFunction(){
@@ -75,5 +78,78 @@ public class RightContinuousStepFunctionIntegrationTest {
 
     }
 
+    @Test
+    public void testInvertedFromTo(){
+        final RightContinuousStepFunction function = createTestFunction();
+
+        final double area1 = function.integrate(0, 3.0);
+        final double area2 = function.integrate(3.0, 0.0);
+
+        assertEquals(area1, -area2, 0.0000001);
+
+    }
+
+    @Test
+    public void testIntegratingUpToNan(){
+        // Idea here - you have a function that is valid up to point x where it becomes NaN
+        // You should be able to integrate *up to* point x and not get an NaN
+
+        final RightContinuousStepFunction function1 = new RightContinuousStepFunction(
+                new double[]{1.0, 2.0, 3.0, 4.0},
+                new double[]{1.0, 1.0, 1.0, Double.NaN},
+                0.0);
+
+
+        final double area1 = function1.integrate(0.0, 4.0);
+        assertEquals(3.0, area1, 0.000000001);
+
+        final double nanArea1 = function1.integrate(0.0, 4.0001);
+        assertTrue(Double.isNaN(nanArea1));
+
+
+        // This tests integrating over the defaultY up to the NaN point
+        final RightContinuousStepFunction function2 = new RightContinuousStepFunction(
+                new double[]{1.0, 2.0, 3.0, 4.0},
+                new double[]{Double.NaN, 1.0, 1.0, Double.NaN},
+                1.0);
+
+
+        final double area2 = function2.integrate(0.0, 1.0);
+        assertEquals(1.0, area2, 0.000000001);
+
+        final double nanArea2 = function2.integrate(0.0, 4.0);
+        assertTrue(Double.isNaN(nanArea2));
+
+
+        // This tests integrating between two NaN points. Note that of course for RightContinuousValues carry the previous
+        // value until the next x point, so this is just making sure the code works if the x-value we pass over is NaN
+        final RightContinuousStepFunction function3 = new RightContinuousStepFunction(
+                new double[]{1.0, 2.0, 3.0, 4.0},
+                new double[]{Double.NaN, 1.0, 1.0, Double.NaN},
+                0.0);
+
+
+        final double area3 = function3.integrate(2.0, 4.0);
+        assertEquals(2.0, area3, 0.000000001);
+
+        final double nanArea3 = function3.integrate(0.0, 4.0001);
+        assertTrue(Double.isNaN(nanArea3));
+
+    }
+
+
+    @Test
+    public void testIntegratingEmptyFunction(){
+        // A function might have no points, but we'll still need to integrate it.
+
+        final RightContinuousStepFunction function = new RightContinuousStepFunction(
+                new double[]{}, new double[]{}, 1.0
+        );
+
+        final double area = function.integrate(1.0 ,3.0);
+        assertEquals(2.0, area, 0.000001);
+
+    }
+
 
 }

src/test/java/ca/joeltherrien/randomforest/utils/RightContinuousStepFunctionOperatorTests.java

@@ -0,0 +1,180 @@
+package ca.joeltherrien.randomforest.utils;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.function.DoubleBinaryOperator;
+import java.util.function.DoubleUnaryOperator;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class RightContinuousStepFunctionOperatorTests {
+
+
+    // Idea - small and middle slightly overlap; middle and large slightly overlap.
+    // small and large never overlap (i.e. small's x values always occur before large's)
+    private final RightContinuousStepFunction smallNumbers;
+    private final RightContinuousStepFunction middleNumbers;
+    private final RightContinuousStepFunction largeNumbers;
+
+    private final double delta = 0.0000000001;
+
+    public RightContinuousStepFunctionOperatorTests(){
+        smallNumbers = RightContinuousStepFunction.constructFromPoints(Utils.easyList(
+                new Point(1.0, 1.0),
+                new Point(2.0, 3.0),
+                new Point(3.0, 2.0),
+                new Point(4.0, 1.0)
+        ), 0.0);
+
+        middleNumbers = RightContinuousStepFunction.constructFromPoints(Utils.easyList(
+                new Point(3.5, 4.0),
+                new Point(4.0, 3.0),
+                new Point(5.0, 2.0),
+                new Point(6.0, 1.0)
+        ), 5.0);
+
+        largeNumbers = RightContinuousStepFunction.constructFromPoints(Utils.easyList(
+                new Point(5.0, 5.0),
+                new Point(6.0, 6.0),
+                new Point(7.0, 3.0),
+                new Point(8.0, 2.0)
+        ), 3.0);
+
+    }
+
+    @Test
+    public void testDifferenceNoOverlapLargeMinusSmall(){
+        DoubleBinaryOperator operator = (a, b) -> a - b;
+
+        final RightContinuousStepFunction largeSmallDifference = RightContinuousStepFunction.biOperation(
+                largeNumbers,
+                smallNumbers,
+                operator);
+
+        assertEquals(8, largeSmallDifference.getX().length);
+        assertEquals(8, largeSmallDifference.getY().length);
+
+        final double[] offsetTimes = {-0.1, 0.0, 0.1};
+
+        for(int time = 1; time <= 9; time++){
+            for(double offsetTime : offsetTimes){
+                final double timeToEvaluateAt = (double) time + offsetTime;
+
+                final double largeFunEvaluation = largeNumbers.evaluate(timeToEvaluateAt);
+                final double smallFunEvaluation = smallNumbers.evaluate(timeToEvaluateAt);
+                final double expectedDifference = operator.applyAsDouble(largeFunEvaluation, smallFunEvaluation);
+
+                final double actualEvaluation = largeSmallDifference.evaluate(timeToEvaluateAt);
+
+                assertEquals(expectedDifference, actualEvaluation, delta);
+            }
+        }
+    }
+
+    @Test
+    public void testDifferenceNoOverlapSmallMinusLarge(){
+        DoubleBinaryOperator operator = (a, b) -> a - b;
+
+        final RightContinuousStepFunction smallLargeDifference = RightContinuousStepFunction.biOperation(
+                smallNumbers,
+                largeNumbers,
+                operator);
+
+        assertEquals(8, smallLargeDifference.getX().length);
+        assertEquals(8, smallLargeDifference.getY().length);
+
+        final double[] offsetTimes = {-0.1, 0.0, 0.1};
+
+        for(int time = 1; time <= 9; time++){
+            for(double offsetTime : offsetTimes){
+                final double timeToEvaluateAt = (double) time + offsetTime;
+
+                final double smallFunEvaluation = smallNumbers.evaluate(timeToEvaluateAt);
+                final double largeFunEvaluation = largeNumbers.evaluate(timeToEvaluateAt);
+                final double expectedDifference = operator.applyAsDouble(smallFunEvaluation, largeFunEvaluation);
+
+                final double actualEvaluation = smallLargeDifference.evaluate(timeToEvaluateAt);
+
+                assertEquals(expectedDifference, actualEvaluation, delta);
+            }
+        }
+    }
+
+    @Test
+    public void testDifferenceSomeOverlapLargeMinusMiddle(){
+        DoubleBinaryOperator operator = (a, b) -> a - b;
+
+        final RightContinuousStepFunction combinedFunction = RightContinuousStepFunction.biOperation(
+                largeNumbers,
+                middleNumbers,
+                operator);
+
+        assertEquals(6, combinedFunction.getX().length);
+        assertEquals(6, combinedFunction.getY().length);
+
+        final double[] offsetTimes = {-0.1, 0.0, 0.1};
+
+        for(int time = 1; time <= 9; time++){
+            for(double offsetTime : offsetTimes){
+                final double timeToEvaluateAt = (double) time + offsetTime;
+
+                final double middleFunEvaluation = middleNumbers.evaluate(timeToEvaluateAt);
+                final double largeFunEvaluation = largeNumbers.evaluate(timeToEvaluateAt);
+                final double expectedDifference = operator.applyAsDouble(largeFunEvaluation, middleFunEvaluation);
+
+                final double actualEvaluation = combinedFunction.evaluate(timeToEvaluateAt);
+
+                assertEquals(expectedDifference, actualEvaluation, delta);
+            }
+        }
+    }
+
+    @Test
+    public void testDifferenceCompleteOverlap(){
+        DoubleBinaryOperator operator = (a, b) -> a - b;
+
+        final RightContinuousStepFunction combinedFunction = RightContinuousStepFunction.biOperation(
+                middleNumbers,
+                middleNumbers,
+                operator);
+
+        assertEquals(4, combinedFunction.getX().length);
+        assertEquals(4, combinedFunction.getY().length);
+
+        final double[] offsetTimes = {-0.1, 0.0, 0.1};
+
+        for(int time = 1; time <= 9; time++){
+            for(double offsetTime : offsetTimes){
+                final double timeToEvaluateAt = (double) time + offsetTime;
+
+                final double actualEvaluation = combinedFunction.evaluate(timeToEvaluateAt);
+
+                assertEquals(0.0, actualEvaluation, delta);
+            }
+        }
+    }
+
+    @Test
+    public void testPowerFunction(){
+        final DoubleUnaryOperator operator = d -> d*d;
+
+        final RightContinuousStepFunction squaredFunction = smallNumbers.unaryOperation(operator);
+
+        assertEquals(4, squaredFunction.getX().length);
+        assertEquals(4, squaredFunction.getY().length);
+
+        final double[] offsetTimes = {-0.1, 0.0, 0.1};
+
+        for(int time = 1; time <= 9; time++){
+            for(double offsetTime : offsetTimes){
+                final double timeToEvaluateAt = (double) time + offsetTime;
+
+                final double expectedEvaluation = operator.applyAsDouble(smallNumbers.evaluate(timeToEvaluateAt));
+                final double actualEvaluation = squaredFunction.evaluate(timeToEvaluateAt);
+
+                assertEquals(expectedEvaluation, actualEvaluation, delta);
+            }
+        }
+    }
+
+}

src/test/java/ca/joeltherrien/randomforest/workshop/TrainForest.java

@@ -43,7 +43,7 @@ public class TrainForest {
 
         final List<Covariate> covariateList = new ArrayList<>(p);
         for(int j =0; j < p; j++){
-            final NumericCovariate covariate = new NumericCovariate("x"+j, j);
+            final NumericCovariate covariate = new NumericCovariate("x"+j, j, false);
             covariateList.add(covariate);
         }
 

src/test/java/ca/joeltherrien/randomforest/workshop/TrainSingleTree.java

@@ -39,8 +39,8 @@ public class TrainSingleTree {
         final int n = 1000;
         final List<Row<Double>> trainingSet = new ArrayList<>(n);
 
-        final Covariate<Double> x1Covariate = new NumericCovariate("x1", 0);
+        final Covariate<Double> x1Covariate = new NumericCovariate("x1", 0, false);
-        final Covariate<Double> x2Covariate = new NumericCovariate("x2", 1);
+        final Covariate<Double> x2Covariate = new NumericCovariate("x2", 1, false);
 
         final List<Covariate.Value<Double>> x1List = DoubleStream
                 .generate(() -> random.nextDouble()*10.0)

src/test/java/ca/joeltherrien/randomforest/workshop/TrainSingleTreeFactor.java

@@ -41,9 +41,9 @@ public class TrainSingleTreeFactor {
         final int n = 10000;
         final List<Row<Double>> trainingSet = new ArrayList<>(n);
 
-        final Covariate<Double> x1Covariate = new NumericCovariate("x1", 0);
+        final Covariate<Double> x1Covariate = new NumericCovariate("x1", 0, false);
-        final Covariate<Double> x2Covariate = new NumericCovariate("x2", 1);
+        final Covariate<Double> x2Covariate = new NumericCovariate("x2", 1, false);
-        final FactorCovariate x3Covariate = new FactorCovariate("x3", 2, Utils.easyList("cat", "dog", "mouse"));
+        final FactorCovariate x3Covariate = new FactorCovariate("x3", 2, Utils.easyList("cat", "dog", "mouse"), false);
 
         final List<Covariate.Value<Double>> x1List = DoubleStream
                 .generate(() -> random.nextDouble()*10.0)