Add 'throttler' for gen-thread.js

example.js

@@ -22,3 +22,13 @@ function* test(thread)
 }
 
 gen.run(test, null, function(result) { console.log(result); });
+
+function* test_throttle(thread)
+{
+    yield thread.throttle(5);
+    console.log('at most 5');
+    yield setTimeout(thread, 1000);
+}
+
+for (var i = 0; i < 15; i++)
+    gen.run(test_throttle);

index.js

@@ -1,17 +1,65 @@
 module.exports.run = runThread;
 module.exports.runParallel = runParallel;
 
+var q = [];
+var pending = [];
+
+function finishq()
+{
+    for (var i = 0; i < q.length; i++)
+    {
+        if (q[i]._done)
+        {
+            q.splice(i, 1);
+            i--;
+        }
+    }
+    while (pending.length > 0 && q.length < pending[0][1])
+    {
+        var t = pending.shift();
+        q.push(t[0]);
+        t[0]();
+    }
+}
+
+var tid = 0;
 function runThread(main, arg, done)
 {
     var thread = function()
     {
         // pass parameters as yield result
         var pass = Array.prototype.slice.call(arguments, 0);
-        var v = thread.gen.next(pass);
+        try
+        {
+            v = thread.gen.next(pass);
+        }
+        catch (e)
+        {
+            v = { done: 1 };
+        }
+        if (v.done)
+        {
+            thread._done = true;
+            finishq();
+        }
         if (v.done && done)
             done(v.value);
     };
+    thread.id = tid++;
     thread.gen = main(thread, arg);
+    thread.throttle = function(count)
+    {
+        finishq();
+        if (q.length < count)
+        {
+            q.push(thread);
+            setTimeout(thread, 1);
+        }
+        else
+        {
+            pending.push([ thread, count ]);
+        }
+    };
     thread();
 }