Meter the virtual machine

document.js

@@ -1,5 +1,5 @@
 export default class Document {
-  constructor(element, createPage) {
+  constructor(element, createPage, meterFaultButton) {
     const self = this;
     this.document = element.ownerDocument;
     this.parent = element;
@@ -20,6 +20,8 @@ export default class Document {
       self.answer(event.target.number);
     };
     this.createPage = createPage || this.createPage;
+    this.meterFaultButton = meterFaultButton;
+
     Object.seal(this);
   }
 
@@ -144,6 +146,12 @@ export default class Document {
     }
   }
 
+  meterFault() {
+    if (this.meterFaultButton) {
+      this.body.appendChild(this.meterFaultButton);
+    }
+  }
+
   ask(_cue) {}
 
   answer(text) {

engine.js

@@ -34,6 +34,8 @@ export default class Engine {
     this.story = args.story;
     this.labels = Object.keys(this.story);
     this.handler = args.handler;
+    this.meter = 0;
+    this.limit = 10e3; // bottles.kni, for example, runs long
     this.options = [];
     this.keywords = {};
     this.noOption = null;
@@ -47,6 +49,7 @@ export default class Engine {
     this.dialog.engine = this;
     this.randomer = args.randomer || Math;
     this.waypoint = this.capture();
+    this.answerOnClearMeterFault = [];
     Object.seal(this);
   }
 
@@ -63,11 +66,15 @@ export default class Engine {
     this.resume();
   }
 
+  /**
+   * Runs the event loop until it yields.
+   */
   continue() {
-    let _continue;
-    do {
+    this.meter = 0;
+    for (;;) {
       if (this.debug) {
         console.log(`${this.label} ${this.instruction.type} ${describe(this.instruction)}`);
+        console.log(this.top);
       }
       if (this.instruction == null) {
         // TODO user error for non-console interaction.
@@ -79,8 +86,31 @@ export default class Engine {
         console.error(`Unexpected instruction type: ${this.instruction.type}`, this.instruction);
         this.resume();
       }
-      _continue = this[`$${this.instruction.type}`](this.instruction);
-    } while (_continue);
+      const proceed = this[`$${this.instruction.type}`](this.instruction);
+      if (!proceed) {
+        return;
+      }
+      this.meter += 1;
+      if (this.meter >= this.limit) {
+        this.display();
+        this.dialog.meterFault();
+        return;
+      }
+    }
+  }
+
+  clearMeterFault() {
+    if (this.meter >= this.limit) {
+      this.render.clear();
+      this.continue();
+
+      // flush answers posted while faulted
+      const count = this.answerOnClearMeterFault.length;
+      const answers = this.answerOnClearMeterFault.splice(0, count);
+      for (const answer of answers) {
+        this.answer(answer);
+      }
+    }
   }
 
   goto(label) {
@@ -170,6 +200,10 @@ export default class Engine {
   }
 
   answer(text) {
+    if (this.meter >= this.limit) {
+      this.answerOnClearMeterFault.push(text);
+      return;
+    }
     if (this.handler && this.handler.answer) {
       this.handler.answer(text, this);
     }
@@ -256,6 +290,9 @@ export default class Engine {
     ];
   }
 
+  /**
+   * Resumes from a snapshot.
+   */
   resume(snapshot) {
     this.render.clear();
     this.flush();

entry.js

@@ -38,7 +38,13 @@ const handler = {
   },
 };
 
-const doc = new Document(document.body);
+const meterFaultButton = document.createElement('a');
+meterFaultButton.innerText = 'Continue…';
+meterFaultButton.addEventListener('click', () => {
+  engine.clearMeterFault();
+});
+
+const doc = new Document(document.body, null, meterFaultButton);
 
 const engine = new Engine({
   story: story,

readline.js

@@ -18,6 +18,16 @@ export default class Readline {
     Object.seal(this);
   }
 
+  meterFault() {
+    this.readline.question(`Enter any command to continue... `, (answer) => {
+      if (answer === 'quit') {
+        this.close();
+      } else {
+        this.engine.clearMeterFault();
+      }
+    });
+  }
+
   ask(cue) {
     this.readline.question(`${cue || ''}> `, this.boundAnswer);
   }

verify.js

@@ -74,7 +74,7 @@ const verify = (kni, trans, handler, kniscript) => {
 
   const writer = new StringWriter();
   const render = new Console(writer);
-  const readline = new FakeReadline(writer, answers);
+  const readline = new FakeReadline(writer, answers, kniscript);
   const engine = new Engine({
     story: states,
     start: 'start',
@@ -98,9 +98,10 @@ const verify = (kni, trans, handler, kniscript) => {
 export default verify;
 
 class FakeReadline {
-  constructor(writer, answers) {
+  constructor(writer, answers, kniscript) {
     this.writer = writer;
     this.answers = answers;
+    this.kniscript = kniscript;
     this.engine = null;
     this.history = [];
     Object.seal(this);
@@ -129,6 +130,10 @@ class FakeReadline {
   }
 
   close() {}
+
+  meterFault() {
+    throw new Error(`meter fault in ${this.kniscript}`);
+  }
 }
 
 class StringWriter {