RandomUtil.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@4983 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/util/RandomUtil.as

@@ -0,0 +1,40 @@
+//
+// $Id$
+
+package com.threerings.util {
+
+/**
+ * Random Random utilities.
+ */
+public class RandomUtil
+{
+    /**
+     * Return the picked index from a weighted list.
+     *
+     * @param weights an Array containing Numbers, ints, or uints. All values should be
+     * non-negative.
+     * @param a random function that returns (0 <= value < 1), if null then Math.random() will
+     * be used.
+     */
+    public static function getWeightedIndex (weights :Array, randomFn :Function = null) :int
+    {
+        var sum :Number = 0;
+        for each (var n :Number in weights) {
+            sum += n;
+        }
+        if (sum < 0) {
+            return -1;
+        }
+        var pick :Number = ((randomFn == null) ? Math.random() : randomFn()) * sum;
+        for (var ii :int = 0; ii < weights.length; ii++) {
+            pick -= Number(weights[ii]);
+            if (pick < 0) {
+                return ii;
+            }
+        }
+
+        // since we're dealing with floats, it's possible that a rounding error left us here
+        return 0; // TODO: largest weighted? Re-pick?
+    }
+}
+}