From 98cb97a1f1bfee2e54994f43be18bbd8e1add5ae Mon Sep 17 00:00:00 2001
From: Joel Therrien <joel@joeltherrien.ca>
Date: Tue, 11 Sep 2018 17:12:27 -0700
Subject: [PATCH] Improve performance by integrating binary search into
 MathFunction

---
 .../randomforest/utils/MathFunction.java      | 72 ++++++++++++++-----
 1 file changed, 54 insertions(+), 18 deletions(-)

diff --git a/src/main/java/ca/joeltherrien/randomforest/utils/MathFunction.java b/src/main/java/ca/joeltherrien/randomforest/utils/MathFunction.java
index 3425787..61d4509 100644
--- a/src/main/java/ca/joeltherrien/randomforest/utils/MathFunction.java
+++ b/src/main/java/ca/joeltherrien/randomforest/utils/MathFunction.java
@@ -4,9 +4,7 @@ import lombok.Getter;
 
 import java.io.Serializable;
 import java.util.Collections;
-import java.util.Comparator;
 import java.util.List;
-import java.util.Optional;
 
 /**
  * Represents a function represented by discrete points. We assume that the function is a stepwise continuous function,
@@ -35,29 +33,67 @@ public class MathFunction implements Serializable {
     }
 
     public Point evaluate(double time){
-        Point point = defaultValue;
-
-        for(final Point currentPoint: points){
-            if(currentPoint.getTime() > time){
-                break;
-            }
-            point = currentPoint;
+        int index = binarySearch(points, time);
+        if(index < 0){
+            return defaultValue;
+        }
+        else{
+            return points.get(index);
         }
-
-        return point;
     }
 
     public Point evaluatePrevious(double time){
-        Point point = defaultValue;
 
-        for(final Point currentPoint: points){
-            if(currentPoint.getTime() >= time){
-                break;
-            }
-            point = currentPoint;
+        int index = binarySearch(points, time) - 1;
+        if(index < 0){
+            return defaultValue;
+        }
+        else{
+            return points.get(index);
         }
 
-        return point;
+
+    }
+
+    /**
+     * Returns the index of the largest (in terms of time) Point that is <= the provided time value.
+     *
+     * @param points
+     * @param time
+     * @return The index of the largest Point who's time is <= the time parameter.
+     */
+    private static int binarySearch(List<Point> points, double time){
+        final int pointSize = points.size();
+
+        if(pointSize == 0 || points.get(pointSize-1).getTime() <= time){
+            // we're already too far
+            return pointSize - 1;
+        }
+
+        if(pointSize < 200){
+            for(int i = 0; i < pointSize; i++){
+                if(points.get(i).getTime() > time){
+                    return i - 1;
+                }
+            }
+        }
+
+         // else
+
+
+        final int middle = pointSize / 2;
+        final double middleTime = points.get(middle).getTime();
+        if(middleTime < time){
+            // go right
+            return binarySearch(points.subList(middle, pointSize), time) + middle;
+        }
+        else if(middleTime > time){
+            // go left
+            return binarySearch(points.subList(0, middle), time);
+        }
+        else{ // middleTime == time
+            return middle;
+        }
     }
 
     @Override