1 files changed, 117 insertions, 0 deletions
diff --git a/lib/node/fixed-queue.js b/lib/node/fixed-queue.js
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+/* eslint-disable */
+
+'use strict'
+
+// Extracted from node/lib/internal/fixed_queue.js
+
+// Currently optimal queue size, tested on V8 6.0 - 6.6. Must be power of two.
+const kSize = 2048;
+const kMask = kSize - 1;
+
+// The FixedQueue is implemented as a singly-linked list of fixed-size
+// circular buffers. It looks something like this:
+//
+//  head                                                       tail
+//    |                                                          |
+//    v                                                          v
+// +-----------+ <-----\       +-----------+ <------\         +-----------+
+// |  [null]   |        \----- |   next    |         \------- |   next    |
+// +-----------+               +-----------+                  +-----------+
+// |   item    | <-- bottom    |   item    | <-- bottom       |  [empty]  |
+// |   item    |               |   item    |                  |  [empty]  |
+// |   item    |               |   item    |                  |  [empty]  |
+// |   item    |               |   item    |                  |  [empty]  |
+// |   item    |               |   item    |       bottom --> |   item    |
+// |   item    |               |   item    |                  |   item    |
+// |    ...    |               |    ...    |                  |    ...    |
+// |   item    |               |   item    |                  |   item    |
+// |   item    |               |   item    |                  |   item    |
+// |  [empty]  | <-- top       |   item    |                  |   item    |
+// |  [empty]  |               |   item    |                  |   item    |
+// |  [empty]  |               |  [empty]  | <-- top  top --> |  [empty]  |
+// +-----------+               +-----------+                  +-----------+
+//
+// Or, if there is only one circular buffer, it looks something
+// like either of these:
+//
+//  head   tail                                 head   tail
+//    |     |                                     |     |
+//    v     v                                     v     v
+// +-----------+                               +-----------+
+// |  [null]   |                               |  [null]   |
+// +-----------+                               +-----------+
+// |  [empty]  |                               |   item    |
+// |  [empty]  |                               |   item    |
+// |   item    | <-- bottom            top --> |  [empty]  |
+// |   item    |                               |  [empty]  |
+// |  [empty]  | <-- top            bottom --> |   item    |
+// |  [empty]  |                               |   item    |
+// +-----------+                               +-----------+
+//
+// Adding a value means moving `top` forward by one, removing means
+// moving `bottom` forward by one. After reaching the end, the queue
+// wraps around.
+//
+// When `top === bottom` the current queue is empty and when
+// `top + 1 === bottom` it's full. This wastes a single space of storage
+// but allows much quicker checks.
+
+class FixedCircularBuffer {
+  constructor() {
+    this.bottom = 0;
+    this.top = 0;
+    this.list = new Array(kSize);
+    this.next = null;
+  }
+
+  isEmpty() {
+    return this.top === this.bottom;
+  }
+
+  isFull() {
+    return ((this.top + 1) & kMask) === this.bottom;
+  }
+
+  push(data) {
+    this.list[this.top] = data;
+    this.top = (this.top + 1) & kMask;
+  }
+
+  shift() {
+    const nextItem = this.list[this.bottom];
+    if (nextItem === undefined)
+      return null;
+    this.list[this.bottom] = undefined;
+    this.bottom = (this.bottom + 1) & kMask;
+    return nextItem;
+  }
+}
+
+module.exports = class FixedQueue {
+  constructor() {
+    this.head = this.tail = new FixedCircularBuffer();
+  }
+
+  isEmpty() {
+    return this.head.isEmpty();
+  }
+
+  push(data) {
+    if (this.head.isFull()) {
+      // Head is full: Creates a new queue, sets the old queue's `.next` to it,
+      // and sets it as the new main queue.
+      this.head = this.head.next = new FixedCircularBuffer();
+    }
+    this.head.push(data);
+  }
+
+  shift() {
+    const tail = this.tail;
+    const next = tail.shift();
+    if (tail.isEmpty() && tail.next !== null) {
+      // If there is another queue, it forms the new tail.
+      this.tail = tail.next;
+    }
+    return next;
+  }
+};