From 4deee071b2a2d34fee3a6bb97aefe549b847b35d Mon Sep 17 00:00:00 2001
From: Tim Mickel <tim.mickel@gmail.com>
Date: Thu, 10 Nov 2016 15:05:49 -0500
Subject: [PATCH] Don't copy threads in step; stable restartThread (#331)

---
 src/engine/runtime.js   | 21 ++++++++++++++++++++-
 src/engine/sequencer.js |  6 ++----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 4fe871a6..36722a45 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -329,6 +329,24 @@ Runtime.prototype._removeThread = function (thread) {
     }
 };
 
+/**
+ * Restart a thread in place, maintaining its position in the list of threads.
+ * This is used by `startHats` to and is necessary to ensure 2.0-like execution order.
+ * Test project: https://scratch.mit.edu/projects/130183108/
+ * @param {!Thread} thread Thread object to restart.
+ */
+Runtime.prototype._restartThread = function (thread) {
+    var newThread = new Thread(thread.topBlock);
+    newThread.target = thread.target;
+    newThread.pushStack(thread.topBlock);
+    var i = this.threads.indexOf(thread);
+    if (i > -1) {
+        this.threads[i] = newThread;
+    } else {
+        this.threads.push(thread);
+    }
+};
+
 /**
  * Return whether a thread is currently active/running.
  * @param {?Thread} thread Thread object to check.
@@ -422,7 +440,8 @@ Runtime.prototype.startHats = function (requestedHatOpcode,
             for (var i = 0; i < instance.threads.length; i++) {
                 if (instance.threads[i].topBlock === topBlockId &&
                     instance.threads[i].target === target) {
-                    instance._removeThread(instance.threads[i]);
+                    instance._restartThread(instance.threads[i]);
+                    return;
                 }
             }
         } else {
diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index c777949b..8c732ca9 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -45,11 +45,9 @@ Sequencer.prototype.stepThreads = function () {
            this.timer.timeElapsed() < WORK_TIME &&
            (this.runtime.turboMode || !this.runtime.redrawRequested)) {
         numActiveThreads = 0;
-        // Inline copy of the threads, updated on each step.
-        var threadsCopy = this.runtime.threads.slice();
         // Attempt to run each thread one time.
-        for (var i = 0; i < threadsCopy.length; i++) {
-            var activeThread = threadsCopy[i];
+        for (var i = 0; i < this.runtime.threads.length; i++) {
+            var activeThread = this.runtime.threads[i];
             if (activeThread.stack.length === 0 ||
                 activeThread.status === Thread.STATUS_DONE) {
                 // Finished with this thread.