Added float-based weighted random selection function.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3851 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/util/RandomUtil.java

@@ -76,7 +76,7 @@ public class RandomUtil
      * Pick a random index from the array, weighted by the value of the
      * corresponding array element.
      *
-     * @param weights an array of positive integers.
+     * @param weights an array of non-negative integers.
      * @return an index into the array, or -1 if the sum of the weights
      * is less than 1. 
      *
@@ -105,6 +105,46 @@ public class RandomUtil
         return 0;
     }
 
+    /**
+     * Pick a random index from the array, weighted by the value of the
+     * corresponding array element.
+     *
+     * @param weights an array of non-negative floats.
+     * @return an index into the array, or -1 if the sum of the weights
+     * is less than or equal to 0.0 or any individual element is negative. 
+     *
+     * For example, passing in {0.2, 0.0, 0.6, 0.8} will return
+     * <table><tr><td>0</td><td>1/8th of the time</td></tr>
+     * <tr><td>1</td><td>never</td></tr>
+     * <tr><td>2</td><td>3/8th of the time</td></tr>
+     * <tr><td>3</td><td>half of the time</td></tr></table>
+     */
+    public static int getWeightedIndex (float[] weights)
+    {
+        float sum = 0.0f;
+        for (int ii = 0; ii < weights.length; ii++) {
+            if (weights[ii] < 0.0f) {
+                return -1;
+            }
+            sum += weights[ii];
+        }
+
+        if (sum <= 0.0) {
+            return -1;
+        }
+        float pick = getFloat(sum);
+        for (int ii=0, nn=weights.length; ii < nn; ii++) {
+            pick -= weights[ii];
+            if (pick < 0.0) {
+                return ii;
+            }
+        }
+
+        // Impossible!
+        Log.logStackTrace(new Throwable());
+        return 0;
+    }
+
     /**
      * Picks a random object from the supplied array of values. Even
      * weight is given to all elements of the array.