Change ResponseCombiner to be a Collector that's compatible

with Streams.

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

@@ -1,8 +1,9 @@
 package ca.joeltherrien.randomforest;
 
 import java.util.List;
+import java.util.stream.Collector;
 
-public interface ResponseCombiner<Y> {
+public interface ResponseCombiner<Y, K> extends Collector<Y, K, Y> {
 
     Y combine(List<Y> responses);
 

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

@@ -3,8 +3,19 @@ package ca.joeltherrien.randomforest.regression;
 import ca.joeltherrien.randomforest.ResponseCombiner;
 
 import java.util.List;
+import java.util.Set;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
 
-public class MeanResponseCombiner implements ResponseCombiner<Double> {
+/**
+ * This implementation of the collector isn't great... but good enough given that I'm not planning to fully support regression trees.
+ *
+ * (It's not great because you'll lose accuracy as you sum up the doubles, since dividing by n is the very last step.)
+ *
+ */
+public class MeanResponseCombiner implements ResponseCombiner<Double, MeanResponseCombiner.Container> {
 
     @Override
     public Double combine(List<Double> responses) {
@@ -13,4 +24,52 @@ public class MeanResponseCombiner implements ResponseCombiner<Double> {
         return responses.stream().mapToDouble(db -> db/size).sum();
 
     }
+
+    @Override
+    public Supplier<Container> supplier() {
+        return () -> new Container(0 ,0);
+    }
+
+    @Override
+    public BiConsumer<Container, Double> accumulator() {
+        return (container, number) -> {
+            container.number+=number;
+            container.n++;
+        };
+    }
+
+    @Override
+    public BinaryOperator<Container> combiner() {
+        return (c1, c2) -> {
+            c1.number += c2.number;
+            c1.n += c2.n;
+
+            return c1;
+        };
+    }
+
+    @Override
+    public Function<Container, Double> finisher() {
+        return (container) -> container.number/(double)container.n;
+    }
+
+    @Override
+    public Set<Characteristics> characteristics() {
+        return Set.of(Characteristics.UNORDERED);
+    }
+
+
+    public static class Container{
+
+        Container(double number, int n){
+            this.number = number;
+            this.n = n;
+        }
+
+        public Double number;
+        public int n;
+
+    }
+
+
 }

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

@@ -13,7 +13,7 @@ import java.util.stream.Collectors;
 @Builder
 public class TreeTrainer<Y> {
 
-    private final ResponseCombiner<Y> responseCombiner;
+    private final ResponseCombiner<Y, ?> responseCombiner;
     private final GroupDifferentiator<Y> groupDifferentiator;
 
     /**
@@ -43,10 +43,10 @@ public class TreeTrainer<Y> {
 
         }
         else{
-            return new TerminalNode<>(responseCombiner.combine(
+            return new TerminalNode<>(
                     data.stream()
-                            .map(row -> row.getResponse())
-                            .collect(Collectors.toList()))
+                        .map(row -> row.getResponse())
+                        .collect(responseCombiner)
 
             );
         }