From 4e24a3f3807bd30dca6785172408b66ba4bf2c61 Mon Sep 17 00:00:00 2001
From: "Michael \"Z\" Goddard" <mzgoddard@gmail.com>
Date: Fri, 3 Nov 2017 11:52:42 -0400
Subject: [PATCH 1/2] Constant time check if thread is in doneThreads

Remove indexOf tests for thread existence in doneThreads. Maintain a
list of null and thread objects mirroring the index position of threads
that are done in runtime.threads. Filter out null values after
filtering out done threads from runtime.threads.

This has a small side effect that threads that normally became DONE and
were added to doneThreads before being removed or restarted will not be
in doneThreads in this version. Restarted threads (_restartThread) do
not appear in this version but new copies will be in Runtime
this.threads supporting glow and monitor updates. Removed threads
(_removeThread) do not appear in this version if they are removed after
they were seen as DONE by the prior version. Threads removed before
they are seen as DONE do not appear in doneThreads in the prior or this
version.

Threads that are removed before normally becoming DONE do not appear in
doneThreads in either case. Threads that are restarted before the loop
checks if the thread is done do not appear in doneThreads in either
case.
---
 src/engine/sequencer.js | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index 0a126d67e..9664290c4 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -38,7 +38,7 @@ class Sequencer {
         let numActiveThreads = Infinity;
         // Whether `stepThreads` has run through a full single tick.
         let ranFirstTick = false;
-        const doneThreads = [];
+        const doneThreads = this.runtime.threads.map(() => null);
         // Conditions for continuing to stepping threads:
         // 1. We must have threads in the list, and some must be active.
         // 2. Time elapsed must be less than WORK_TIME.
@@ -54,11 +54,13 @@ class Sequencer {
                 if (activeThread.stack.length === 0 ||
                     activeThread.status === Thread.STATUS_DONE) {
                     // Finished with this thread.
-                    if (doneThreads.indexOf(activeThread) < 0) {
-                        doneThreads.push(activeThread);
-                    }
+                    doneThreads[i] = activeThread;
                     continue;
                 }
+                // A thread was removed, added or this thread was restarted.
+                if (doneThreads[i] !== null) {
+                    doneThreads[i] = null;
+                }
                 if (activeThread.status === Thread.STATUS_YIELD_TICK &&
                     !ranFirstTick) {
                     // Clear single-tick yield from the last call of `stepThreads`.
@@ -82,13 +84,13 @@ class Sequencer {
             ranFirstTick = true;
         }
         // Filter inactive threads from `this.runtime.threads`.
-        this.runtime.threads = this.runtime.threads.filter(thread => {
-            if (doneThreads.indexOf(thread) > -1) {
+        this.runtime.threads = this.runtime.threads.filter((thread, i) => {
+            if (doneThreads[i] === null) {
                 return false;
             }
             return true;
         });
-        return doneThreads;
+        return doneThreads.filter(Boolean);
     }
 
     /**

From 6d82c0f115713ae729dfadc024ddf6d70e21b25f Mon Sep 17 00:00:00 2001
From: "Michael \"Z\" Goddard" <mzgoddard@gmail.com>
Date: Fri, 3 Nov 2017 16:24:12 -0400
Subject: [PATCH 2/2] In place filter threads at end of stepThreads

Skip a large array allocation by filtering done threads in place with a
for loop.
---
 src/engine/sequencer.js | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index 9664290c4..f4edc966c 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -84,13 +84,28 @@ class Sequencer {
             ranFirstTick = true;
         }
         // Filter inactive threads from `this.runtime.threads`.
-        this.runtime.threads = this.runtime.threads.filter((thread, i) => {
+        numActiveThreads = 0;
+        for (let i = 0; i < this.runtime.threads.length; i++) {
+            const thread = this.runtime.threads[i];
             if (doneThreads[i] === null) {
-                return false;
+                this.runtime.threads[numActiveThreads] = thread;
+                numActiveThreads++;
             }
-            return true;
-        });
-        return doneThreads.filter(Boolean);
+        }
+        this.runtime.threads.length = numActiveThreads;
+
+        // Filter undefined and null values from `doneThreads`.
+        let numDoneThreads = 0;
+        for (let i = 0; i < doneThreads.length; i++) {
+            const maybeThread = doneThreads[i];
+            if (maybeThread !== null) {
+                doneThreads[numDoneThreads] = maybeThread;
+                numDoneThreads++;
+            }
+        }
+        doneThreads.length = numDoneThreads;
+
+        return doneThreads;
     }
 
     /**