Initial token bucket implementation (#1314)

lib/internal/TokenBucket.js

@@ -0,0 +1,38 @@
+import DLL from './DoublyLinkedList';
+
+/**
+ * An internal implementation of [Token Bucket](https://en.wikipedia.org/wiki/Token_bucket)
+ * for rate-limiting/traffic shaping. Our token bucket starts with a slight twist from the
+ * conventional token bucket, in which it starts with bucketSize tokens already available.
+ *
+ * @param {Number} bucketSize - the maximum number of items (inclusive) which can be queued in
+ * a interval of time.
+ * @param {Number} interval - the period in miliseconds to stop tracking a sent item
+ */
+export function TokenBucket(bucketSize, interval) {
+    this.bucketSize = bucketSize;
+    this.interval = interval;
+    this.queue = new DLL();
+    this.queued = 0; // Number of items sent + size of queue
+}
+
+// Enqueue an operation to be executed when the rate limit is not exceeded.
+TokenBucket.prototype.enqueue = function(operation) {
+    this.queued++;
+    if (this.queued <= this.bucketSize) {
+        operation();
+    } else {
+        this.queue.push(operation);
+    }
+
+    // after interval, decrement the queued count and call a queued operation (if bucket is full)
+    setTimeout(onIntervalComplete, this.interval, this);
+}
+
+function onIntervalComplete(bucket) {
+    bucket.queued--;
+    if (bucket.queue.length > 0) {
+        // call first queued operation
+        (bucket.queue.shift())();
+    }
+}