Build permutation table using fisher-yates

Author: jwagner

simplex-noise.js

@@ -391,13 +391,15 @@ SimplexNoise.prototype = {
 
 function buildPermutationTable(random) {
   var i;
-  var j = [];
-  for (i = 0; i < 256; i++) {
-    j.push(i);
-  }
   var p = new Uint8Array(256);
   for (i = 0; i < 256; i++) {
-    p[i] = j.splice(~~(random() * (256 - i)), 1);
+    p[i] = i;
+  }
+  for (i = 0; i < 255; i++) {
+    var r = i + 1 + ~~(random() * (255 - i));
+    var aux = p[i];
+    p[i] = p[r];
+    p[r] = aux;
   }
   return p;
 }

test/simplex-noise-test.js

@@ -8,12 +8,13 @@ describe('SimplexNoise', function() {
   }
 
   describe('buildPermutationTable', function() {
-    var table = SimplexNoise._buildPermutationTable(getRandom());
-    var aTable = Array.prototype.slice.call(table);
-    assert.lengthOf(aTable, 256);
-    for (var i = 0; i < aTable.length; i++) {
-      assert.include(aTable, i);
-    }
+    it('contains all indices exactly once', function() {
+      var table = SimplexNoise._buildPermutationTable(getRandom());
+      var aTable = Array.prototype.slice.call(table);
+      for (var i = 0; i < aTable.length; i++) {
+        assert.include(aTable, i);
+      }
+    });
   });
 
   describe('constructor', function() {