added getWeightedIndex().

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@2298 542714f4-19e9-0310-aa3c-eee0fc999fb1

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

@@ -1,8 +1,10 @@
 //
-// $Id: RandomUtil.java,v 1.5 2002/04/15 18:18:20 mdb Exp $
+// $Id: RandomUtil.java,v 1.6 2003/03/05 02:50:28 ray Exp $
 
 package com.threerings.util;
 
+import com.samskivert.util.IntListUtil;
+
 import java.util.Random;
 
 /**
@@ -36,4 +38,31 @@ public class RandomUtil
     {
         return rand.nextFloat() * high;
     }
+
+    /**
+     * Pick a random index from the array, weighted by the value of the
+     * corresponding array element.
+     *
+     * @param weights an array of positive integers.
+     *
+     * For example, passing in {1, 0, 3, 4} 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 (int[] weights)
+    {
+        int pick = getInt(IntListUtil.sum(weights));
+        for (int ii=0, nn=weights.length; ii < nn; ii++) {
+            pick -= weights[ii];
+            if (pick < 0) {
+                return ii;
+            }
+        }
+
+        // Impossible!
+        Log.logStackTrace(new Throwable());
+        return 0;
+    }
 }