Add basics for the sequencer and threads

src/engine/runtime.js

@@ -3,7 +3,7 @@ var Sequencer = require('./sequencer');
 var util = require('util');
 
 /**
- * Manages blocks, stacks, threads, and the sequencer.
+ * Manages blocks, stacks, and the sequencer.
  */
 function Runtime () {
     // Bind event emitter
@@ -14,8 +14,6 @@ function Runtime () {
     this.blocks = {};
     /** @type {Array.<String>} */
     this.stacks = [];
-    /** @type {Array.<Thread>} */
-    this.threads = [];
 
     /** @type {!Sequencer} */
     this.sequencer = new Sequencer();

src/engine/sequencer.js

@@ -1,5 +1,59 @@
+var Timer = require('../util/Timer');
+
 function Sequencer () {
-    // @todo
+    /**
+     * A list of threads that are currently running in the VM.
+     * Threads are added when execution starts and pruned when execution ends.
+     * @type {Array.<Thread>}
+     */
+    this.threads = [];
+
+    /**
+     * A utility timer for timing thread sequencing.
+     * @type {!Timer}
+     */
+    this.timer = new Timer();
 }
 
+/**
+ * The sequencer does as much work as it can within WORK_TIME milliseconds,
+ * then yields. This is essentially a rate-limiter for blocks.
+ * In Scratch 2.0, this is set to 75% of the target stage frame-rate (30fps).
+ * @const {!number}
+ */
+Sequencer.WORK_TIME = 1000 / 60;
+
+/**
+ * Step through all threads in `this.threads`, running them in order.
+ */
+Sequencer.prototype.stepThreads = function () {
+    // Start counting toward WORK_TIME
+    this.timer.start();
+    // While there are still threads to run and we are within WORK_TIME,
+    // continue executing threads.
+    while (this.threads.length > 0 &&
+           this.timer.timeElapsed() < Sequencer.WORK_TIME) {
+        // New threads at the end of the iteration.
+        var newThreads = [];
+        // Attempt to run each thread one time
+        for (var i = 0; i < this.threads.length; i++) {
+            var activeThread = this.threads[i];
+            this.stepThread(activeThread);
+            if (activeThread.nextBlock !== null) {
+                newThreads.push(activeThread);
+            }
+        }
+        // Effectively filters out threads that have stopped.
+        this.threads = newThreads;
+    }
+};
+
+/**
+ * Step the requested thread
+ * @param {!Thread} thread Thread object to step
+ */
+Sequencer.protoype.stepThread = function (thread) {
+    // @todo
+};
+
 module.exports = Sequencer;

src/engine/thread.js

@@ -1,5 +1,15 @@
 function Thread () {
-    // @todo
+    /**
+     * Next block that the thread will execute.
+     * @type {string}
+     */
+    this.nextBlock = null;
+    /**
+     * Stack for the thread. When the sequencer enters a control structure,
+     * the block is pushed onto the stack so we know where to exit.
+     * @type {Array.<string>}
+     */
+    this.stack = [];
 }
 
 module.exports = Thread;

src/util/timer.js

@@ -13,8 +13,8 @@ Timer.prototype.start = function () {
     this.startTime = this.time();
 };
 
-Timer.prototype.stop = function () {
-    return this.startTime - this.time();
+Timer.prototype.timeElapsed = function () {
+    return this.time() - this.startTime;
 };
 
 module.exports = Timer;