diff -r 000000000000 -r 6474c204b198 testing/mochitest/chunkifyTests.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/testing/mochitest/chunkifyTests.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,81 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+function chunkifyTests(tests, totalChunks, thisChunk, chunkByDir, logger) {
+  var total_chunks = parseInt(totalChunks);
+  // this_chunk is in the range [1,total_chunks]
+  var this_chunk = parseInt(thisChunk);
+  var returnTests;
+
+  // We want to split the tests up into chunks according to which directory
+  // they're in
+  if (chunkByDir) {
+    chunkByDir = parseInt(chunkByDir);
+    var tests_by_dir = {};
+    var test_dirs = []
+    for (var i = 0; i < tests.length; ++i) {
+      var test_path = tests[i];
+      if (test_path[0] == '/') {
+        test_path = test_path.substr(1);
+      }
+      // mochitest-chrome and mochitest-browser-chrome pass an array of chrome://
+      // URIs
+      var protocolRegexp = /^[a-zA-Z]+:\/\//;
+      if (protocolRegexp.test(test_path)) {
+        test_path = test_path.replace(protocolRegexp, "");
+      }
+      var dir = test_path.split("/");
+      // We want the first chunkByDir+1 components, or everything but the
+      // last component, whichever is less.
+      // we add 1 to chunkByDir since 'tests' is always part of the path, and
+      // want to ignore the last component since it's the test filename.
+      dir = dir.slice(0, Math.min(chunkByDir+1, dir.length-1));
+      // reconstruct a directory name
+      dir = dir.join("/");
+      if (!(dir in tests_by_dir)) {
+        tests_by_dir[dir] = [tests[i]];
+        test_dirs.push(dir);
+      } else {
+        tests_by_dir[dir].push(tests[i]);
+      }
+    }
+    var tests_per_chunk = test_dirs.length / total_chunks;
+    var start = Math.round((this_chunk-1) * tests_per_chunk);
+    var end = Math.round(this_chunk * tests_per_chunk);
+    returnTests = [];
+    var dirs = []
+    for (var i = start; i < end; ++i) {
+      var dir = test_dirs[i];
+      dirs.push(dir);
+      returnTests = returnTests.concat(tests_by_dir[dir]);
+    }
+    if (logger)
+      logger.log("Running tests in " + dirs.join(", "));
+  } else {
+    var tests_per_chunk = tests.length / total_chunks;
+    var start = Math.round((this_chunk-1) * tests_per_chunk);
+    var end = Math.round(this_chunk * tests_per_chunk);
+    returnTests = tests.slice(start, end);
+    if (logger)
+      logger.log("Running tests " + (start+1) + "-" + end + "/" + tests.length);
+  }
+
+  return returnTests;
+}
+
+function skipTests(tests, startTestPattern, endTestPattern) {
+  var startIndex = 0, endIndex = tests.length - 1;
+  for (var i = 0; i < tests.length; ++i) {
+    var test_path = tests[i];
+    if (startTestPattern && test_path.endsWith(startTestPattern)) {
+      startIndex = i;
+    }
+
+    if (endTestPattern && test_path.endsWith(endTestPattern)) {
+      endIndex = i;
+    }
+  }
+
+  return tests.slice(startIndex, endIndex + 1);
+}