#Introduction

Extremely fast double-ended queue implementation. Double-ended queue can also be used as a:

• Stack
• Queue

The implementation is GC and CPU cache friendly circular buffer. It will run circles around any "linked list" implementation.

Every queue operation is done in constant O(1) - including random access from .get().

#Topics

• Quick start
• Why not use an Array?
• Using double-ended queue as a normal queue
• API reference and examples
• Performance

#Quick start

npm install double-ended-queue

var Deque = require("double-ended-queue");

var deque = new Deque([1,2,3,4]);
deque.shift(); //1
deque.pop(); //4

#Why not use an Array?

Arrays take linear O(N) time to do shift and unshift operations. That means in theory that an array with 1000 items is 1000x slower to do those operations than a deque with 1000 items. 10000x slower with 10000 items and so on.

V8 implements a trick for small arrays where these operations are done in constant time, however even with this trick deque is still 4x faster.

But arrays use "native" methods, they must be faster!

In V8, there is almost no advantage for a method to be a built-in. In fact many times built-ins are at a severe disadvantage of having to implement far more complex semantics than is actually needed in practice. For example, sparse array handling punishes almost every built-in array method even though nobody uses sparse arrays as is evidenced by the popularity of the underscore library which doesn't handle sparse arrays in the same way across different browsers.

#Using double-ended queue as a normal queue

Queue is a more commonly needed data structure however a separate implementation does not provide any advantage in terms of performance. Aliases are provided specifically for the queue use-case. You may use .enqueue(items...) to enqueue item(s) and .dequeue() to dequeue an item.

#API

• new Deque()
• new Deque(Array items)
• new Deque(int capacity)
• push(dynamic items...)
• unshift(dynamic items...)
• pop()
• shift()
• toArray()
• peekBack()
• peekFront()
• get(int index)
• isEmpty()
• clear()

#####new Deque() -> Deque

Creates an empty double-ended queue with initial capacity of 16. If you know the optimal size before-hand, use new Deque(int capacity).

var deque = new Deque();
deque.push(1, 2, 3);
deque.shift(); //1
deque.pop(); //3

---

#####new Deque(Array items) -> Deque

Creates a double-ended queue from items.

var deque = new Deque([1,2,3,4]);
deque.shift(); //1
deque.pop(); //4

---

#####new Deque(int capacity) -> Deque

Creates an empty double-ended queue with the given capacity. Capacity should be the maximum amount of items the queue will hold at a given time.

The reason to give an initial capacity is to avoid potentially expensive resizing operations at runtime.

var deque = new Deque(100);
deque.push(1, 2, 3);
deque.shift(); //1
deque.pop(); //3

---

#####push(dynamic items...) -> int

Push items to the back of this queue. Returns the amount of items currently in the queue after the operation.

var deque = new Deque();
deque.push(1);
deque.pop(); //1
deque.push(1, 2, 3);
deque.shift(); //1
deque.shift(); //2
deque.shift(); //3

Aliases: enqueue, insertBack

---

#####unshift(dynamic items...) -> int

Unshift items to the front of this queue. Returns the amount of items currently in the queue after the operation.

var deque = new Deque([2,3]);
deque.unshift(1);
deque.toString(); //"1,2,3"
deque.unshift(-2, -1, 0);
deque.toString(); //"-2,-1,0,1,2,3"

Aliases: insertFront

---

#####pop() -> dynamic

Pop off the item at the back of this queue.

Note: The item will be removed from the queue. If you simply want to see what's at the back of the queue use peekBack() or .get(-1).

If the queue is empty, undefined is returned. If you need to differentiate between undefined values in the queue and pop() return value - check the queue .length before popping.

var deque = new Deque([1,2,3]);
deque.pop(); //3
deque.pop(); //2
deque.pop(); //1
deque.pop(); //undefined

Aliases: removeBack

---

#####shift() -> dynamic

Shifts off the item at the front of this queue.

Note: The item will be removed from the queue. If you simply want to see what's at the front of the queue use peekFront() or .get(0).

If the queue is empty, undefined is returned. If you need to differentiate between undefined values in the queue and shift() return value - check the queue .length before shifting.

var deque = new Deque([1,2,3]);
deque.shift(); //1
deque.shift(); //2
deque.shift(); //3
deque.shift(); //undefined

Aliases: removeFront, dequeue

---

#####toArray() -> Array

Returns the items in the queue as an array. Starting from the item in the front of the queue and ending to the item at the back of the queue.

var deque = new Deque([1,2,3]);
deque.push(4);
deque.unshift(0);
deque.toArray(); //[0,1,2,3,4]

Aliases: toJSON

---

#####peekBack() -> dynamic

Returns the item that is at the back of this queue without removing it.

If the queue is empty, undefined is returned.

var deque = new Deque([1,2,3]);
deque.push(4);
deque.peekBack(); //4

---

#####peekFront() -> dynamic

Returns the item that is at the front of this queue without removing it.

If the queue is empty, undefined is returned.

var deque = new Deque([1,2,3]);
deque.push(4);
deque.peekFront(); //1

---

#####get(int index) -> dynamic

Returns the item that is at the given index of this queue without removing it.

The index is zero-based, so .get(0) will return the item that is at the front, .get(1) will return the item that comes after and so on.

The index can be negative to read items at the back of the queue. .get(-1) returns the item that is at the back of the queue, .get(-2) will return the item that comes before and so on.

Returns undefined if index is not a valid index into the queue.

var deque = new Deque([1,2,3]);
deque.get(0); //1
deque.get(1); //2
deque.get(2); //3

deque.get(-1); //3
deque.get(-2); //2
deque.get(-3); //1

Note: Even though indexed accessor (e.g. queue[0]) could appear to return a correct value sometimes, this is completely unreliable. The numeric slots of the deque object are internally used as an optimization and have no meaningful order or meaning to outside. Always use .get().

Note: The implementation has O(1) random access using .get().

---

#####isEmpty() -> boolean

Return true if this queue is empty, false otherwise.

var deque = new Deque();
deque.isEmpty(); //true
deque.push(1);
deque.isEmpty(); //false

---

#####clear() -> void

Remove all items from this queue. Does not change the queue's capacity.

var deque = new Deque([1,2,3]);
deque.toString(); //"1,2,3"
deque.clear();
deque.toString(); //""

---

#Performance

Clone the repo and npm install. Then run the bench script.

##1000 items in the queue

double-ended-queue x 15,532,714 ops/sec ±0.19% (96 runs sampled)
built-in array x 6,501,398 ops/sec ±0.87% (95 runs sampled)
node-deque x 2,938,068 ops/sec ±3.50% (68 runs sampled)

##2 million items in the queue

double-ended-queue x 14,425,547 ops/sec ±0.17% (94 runs sampled)
node-deque x 2,815,628 ops/sec ±10.56% (76 runs sampled)
built-in array x 19.23 ops/sec ±0.35% (51 runs sampled)

Noteworthy is just how bad the degradation can be for built-in array when V8 cannot use the trick.