Simplify execution by removing nextBlock

Everything is managed by the stack, including what the execute() function does.

src/engine/execute.js

@@ -6,40 +6,43 @@ var YieldTimers = require('../util/yieldtimers.js');
  */
 var DEBUG_BLOCK_CALLS = true;
 
-var execute = function (sequencer, thread, blockId) {
+var execute = function (sequencer, thread) {
     var runtime = sequencer.runtime;
 
+    // Current block to execute is the one on the top of the stack.
+    var currentBlockId = thread.peekStack();
+    var currentStackFrame = thread.peekStackFrame();
+
     // Save the yield timer ID, in case a primitive makes a new one
     // @todo hack - perhaps patch this to allow more than one timer per
     // primitive, for example...
     var oldYieldTimerId = YieldTimers.timerId;
 
-    var opcode = runtime.blocks.getOpcode(blockId);
-
-    // Push the current block to the stack
-    thread.pushStack(blockId);
-    var currentStackFrame = thread.getLastStackFrame();
+    var opcode = runtime.blocks.getOpcode(currentBlockId);
 
     // Generate values for arguments (inputs).
     var argValues = {};
 
     // Add all fields on this block to the argValues.
-    var fields = runtime.blocks.getFields(blockId);
+    var fields = runtime.blocks.getFields(currentBlockId);
     for (var fieldName in fields) {
         argValues[fieldName] = fields[fieldName].value;
     }
 
     // Recursively evaluate input blocks.
-    var inputs = runtime.blocks.getInputs(blockId);
+    var inputs = runtime.blocks.getInputs(currentBlockId);
     for (var inputName in inputs) {
         var input = inputs[inputName];
         var inputBlockId = input.block;
-        var result = execute(sequencer, thread, inputBlockId);
+        // Push to the stack to evaluate this input.
+        thread.pushStack(inputBlockId);
+        var result = execute(sequencer, thread);
+        thread.popStack();
         argValues[input.name] = result;
     }
 
     if (!opcode) {
-        console.warn('Could not get opcode for block: ' + blockId);
+        console.warn('Could not get opcode for block: ' + currentBlockId);
         console.groupEnd();
         return;
     }
@@ -62,12 +65,12 @@ var execute = function (sequencer, thread, blockId) {
         primitiveReturnValue = blockFunction(argValues, {
             yield: thread.yield.bind(thread),
             done: function() {
-                sequencer.proceedThread(thread, blockId);
+                sequencer.proceedThread(thread);
             },
             timeout: YieldTimers.timeout,
             stackFrame: currentStackFrame,
             startSubstack: function () {
-                sequencer.stepToSubstack(thread, blockId);
+                sequencer.stepToSubstack(thread);
             }
         });
     }
@@ -86,8 +89,6 @@ var execute = function (sequencer, thread, blockId) {
             console.log('returned: ', primitiveReturnValue);
             console.groupEnd();
         }
-        // Pop the stack and stack frame
-        thread.popStack();
         return primitiveReturnValue;
     }
 };

src/engine/runtime.js

@@ -122,6 +122,7 @@ Runtime.prototype.getOpcodeFunction = function (opcode) {
 Runtime.prototype._pushThread = function (id) {
     this.emit(Runtime.STACK_GLOW_ON, id);
     var thread = new Thread(id);
+    thread.pushStack(id);
     this.threads.push(thread);
 };
 

src/engine/sequencer.js

@@ -49,17 +49,18 @@ Sequencer.prototype.stepThreads = function (threads) {
             var activeThread = threads[i];
             if (activeThread.status === Thread.STATUS_RUNNING) {
                 // Normal-mode thread: step.
-                this.stepThread(activeThread);
+                this.startThread(activeThread);
             } else if (activeThread.status === Thread.STATUS_YIELD) {
                 // Yield-mode thread: check if the time has passed.
-                YieldTimers.resolve(activeThread.yieldTimerId);
-                numYieldingThreads++;
+                if (!YieldTimers.resolve(activeThread.yieldTimerId)) {
+                    numYieldingThreads++;
+                }
             } else if (activeThread.status === Thread.STATUS_DONE) {
                 // Moved to a done state - finish up
                 activeThread.status = Thread.STATUS_RUNNING;
                 // @todo Deal with the return value
             }
-            if (activeThread.nextBlock === null &&
+            if (activeThread.stack.length === 0 &&
                 activeThread.status === Thread.STATUS_DONE) {
                 // Finished with this thread - tell runtime to clean it up.
                 inactiveThreads.push(activeThread);
@@ -78,21 +79,27 @@ Sequencer.prototype.stepThreads = function (threads) {
  * Step the requested thread
  * @param {!Thread} thread Thread object to step
  */
-Sequencer.prototype.stepThread = function (thread) {
-    // Save the current block and set the nextBlock.
-    // If the primitive would like to do control flow,
-    // it can overwrite nextBlock.
-    var currentBlockId = thread.nextBlock;
+Sequencer.prototype.startThread = function (thread) {
+    var currentBlockId = thread.peekStack();
     if (!currentBlockId || !this.runtime.blocks.getBlock(currentBlockId)) {
         thread.status = Thread.STATUS_DONE;
         return;
     }
     // Start showing run feedback in the editor.
     this.runtime.glowBlock(currentBlockId, true);
-    // Execute the block
-    execute(this, thread, currentBlockId, false);
-    // If the block executed without yielding, move to done.
-    if (thread.status === Thread.STATUS_RUNNING && !thread.switchedStack) {
+
+    // Push the current block to the stack, if executing for the first time.
+    if (thread.peekStack() != currentBlockId) {
+        thread.pushStack(currentBlockId);
+    }
+
+    // Execute the current block
+    execute(this, thread);
+
+    // If the block executed without yielding and without doing control flow,
+    // move to done.
+    if (thread.status === Thread.STATUS_RUNNING &&
+        thread.peekStack() === currentBlockId) {
         this.proceedThread(thread, currentBlockId);
     }
 };
@@ -102,15 +109,16 @@ Sequencer.prototype.stepThread = function (thread) {
  * @param {!Thread} thread Thread object to step to substack.
  * @param {string} currentBlockId Block which owns a substack to step to.
  */
-Sequencer.prototype.stepToSubstack = function (thread, currentBlockId) {
-    // Set nextBlock to the start of the substack
-    var substack = this.runtime.blocks.getSubstack(currentBlockId);
-    if (substack && substack.value) {
-        thread.nextBlock = substack.value;
+Sequencer.prototype.stepToSubstack = function (thread) {
+    var currentBlockId = thread.peekStack();
+    var substackId = this.runtime.blocks.getSubstack(currentBlockId);
+    if (substackId) {
+        // Push substack ID to the thread's stack.
+        thread.pushStack(substackId);
     } else {
-        thread.nextBlock = null;
+        // Push null, so we come back to the current block.
+        thread.pushStack(null);
     }
-    thread.switchedStack = true;
 };
 
 /**
@@ -118,17 +126,21 @@ Sequencer.prototype.stepToSubstack = function (thread, currentBlockId) {
  * @param {!Thread} thread Thread object to proceed.
  * @param {string} currentBlockId Block we are finished with.
  */
-Sequencer.prototype.proceedThread = function (thread, currentBlockId) {
-    // Stop showing run feedback in the editor.
+Sequencer.prototype.proceedThread = function (thread) {
+    var currentBlockId = thread.peekStack();
+    // Mark the status as done and proceed to the next block.
     this.runtime.glowBlock(currentBlockId, false);
-    // Mark the thread as done and proceed to the next block.
     thread.status = Thread.STATUS_DONE;
-    // Refresh nextBlock in case it has changed during a yield.
-    thread.nextBlock = this.runtime.blocks.getNextBlock(currentBlockId);
-    // If none is available, attempt to pop from the stack.
-    // First attempt to pop from the stack
-    if (!thread.nextBlock && thread.stack.length > 0) {
-        thread.nextBlock = thread.popStack();
+    // Pop from the stack - finished this level of execution.
+    thread.popStack();
+    // Push next connected block, if there is one.
+    var nextBlockId = this.runtime.blocks.getNextBlock(currentBlockId);
+    if (nextBlockId) {
+        thread.pushStack(nextBlockId);
+    }
+    // Pop from the stack until we have a next block.
+    while (thread.peekStack() === null && thread.stack.length > 0) {
+        thread.popStack();
     }
 };
 

src/engine/thread.js

@@ -9,11 +9,7 @@ function Thread (firstBlock) {
      * @type {!string}
      */
     this.topBlock = firstBlock;
-    /**
-     * ID of next block that the thread will execute, or null if none.
-     * @type {?string}
-     */
-    this.nextBlock = firstBlock;
+
     /**
      * Stack for the thread. When the sequencer enters a control structure,
      * the block is pushed onto the stack so we know where to exit.
@@ -38,12 +34,6 @@ function Thread (firstBlock) {
      * @type {number}
      */
     this.yieldTimerId = -1;
-
-    /**
-     * Whether the thread has switched stack in the course of execution.
-     * @type {boolean}
-     */
-    this.switchedStack = false;
 }
 
 /**
@@ -83,7 +73,7 @@ Thread.prototype.pushStack = function (blockId) {
 
 /**
  * Pop last block on the stack and its stack frame.
- * @returns {string} Block ID popped from the stack.
+ * @return {string} Block ID popped from the stack.
  */
 Thread.prototype.popStack = function () {
     this.stackFrames.pop();
@@ -91,10 +81,19 @@ Thread.prototype.popStack = function () {
 };
 
 /**
- * Get last stack frame.
+ * Get top stack item.
+ * @return {?string} Block ID on top of stack.
+ */
+Thread.prototype.peekStack = function () {
+    return this.stack[this.stack.length - 1];
+};
+
+
+/**
+ * Get top stack frame.
  * @return {?Object} Last stack frame stored on this thread.
  */
-Thread.prototype.getLastStackFrame = function () {
+Thread.prototype.peekStackFrame = function () {
     return this.stackFrames[this.stackFrames.length - 1];
 };