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* AsyncQueue - A TypeScript implementation of an async producer-consumer queue with backpressure control
* Uses circular buffer for O(1) enqueue/dequeue operations
*
* Developed by AI Hive® at O2.services
* https://o2.services
*
* Copyright (c) 2024 AI Hive® at O2.services
* Licensed under the MIT License
*/
/**
* Promise resolver function type
*/
type PromiseResolver = () => void;
/**
* AsyncQueue provides a thread-safe producer-consumer queue with backpressure control,
* similar to .NET's Channel<T> or Go channels.
* Uses a circular buffer for optimal performance.
*
* @template T The type of items in the queue
*/
export class AsyncQueue<T = any> {
private readonly maxSize: number;
private readonly buffer: (T | undefined)[];
private head = 0; // Index where we dequeue from
private tail = 0; // Index where we enqueue to
private count = 0; // Number of items in queue
// Waiting queues with reserved capacity - never shrink, only grow
private waitingConsumers: (PromiseResolver | undefined)[] = [];
private waitingConsumersCount = 0;
private waitingProducers: (PromiseResolver | undefined)[] = [];
private waitingProducersCount = 0;
private readonly INITIAL_WAITING_CAPACITY = 16;
private closed = false;
/**
* Creates a new AsyncQueue instance
* @param maxSize Maximum number of items the queue can hold before producers block (default: 1)
*/
constructor(maxSize = 1) {
if (maxSize < 1) {
throw new Error('maxSize must be at least 1');
}
// Use power of 2 for faster modulo operation with bitwise AND
// Round up to nearest power of 2
this.maxSize = maxSize;
const bufferSize = 1 << (32 - Math.clz32(maxSize - 1));
this.buffer = new Array(bufferSize);
// Pre-allocate initial capacity for waiting queues
this.waitingConsumers.length = this.INITIAL_WAITING_CAPACITY;
this.waitingProducers.length = this.INITIAL_WAITING_CAPACITY;
}
/**
* Adds an item to the circular buffer
*/
private addToBuffer(item: T): void {
this.buffer[this.tail] = item;
this.tail = (this.tail + 1) & (this.buffer.length - 1);
this.count++;
}
/**
* Removes an item from the circular buffer
*/
private removeFromBuffer(): T | undefined {
Iif (this.count === 0) return undefined;
const item = this.buffer[this.head];
this.buffer[this.head] = undefined; // Help GC
this.head = (this.head + 1) & (this.buffer.length - 1);
this.count--;
return item;
}
/**
* Pushes a resolver onto a waiting queue with capacity management
*/
private pushWaiter(queue: (PromiseResolver | undefined)[], count: number, resolver: PromiseResolver): number {
// Grow capacity if needed (double the size)
if (count >= queue.length) {
queue.length = queue.length * 2;
}
queue[count] = resolver;
return count + 1;
}
/**
* Pops a resolver from a waiting queue (LIFO)
*/
private popWaiter(queue: (PromiseResolver | undefined)[], count: number): PromiseResolver | undefined {
Iif (count === 0) return undefined;
const resolver = queue[count - 1];
queue[count - 1] = undefined; // Help GC
return resolver;
}
/**
* Adds an item to the queue. Blocks if the queue is full.
* @param item The item to add to the queue
* @returns A promise that resolves when the item has been added
* @throws Error if the queue has been closed
*/
async enqueue(item: T): Promise<void> {
// Prevent new items after close() to ensure clean shutdown
if (this.closed) {
throw new Error('Queue is closed');
}
// BLOCKING MECHANISM: Wait if queue is at capacity
// This implements backpressure - fast producers slow down to match consumers
while (this.count >= this.maxSize && !this.closed) {
// Create unresolved Promise, store only the resolve function
// This suspends the producer until a consumer makes space
await new Promise<void>(resolve => {
this.waitingProducersCount = this.pushWaiter(this.waitingProducers, this.waitingProducersCount, resolve);
});
// Check again after waking - queue might have been closed while waiting
if (this.closed) {
throw new Error('Queue is closed');
}
}
// Add item to circular buffer (we now have space)
this.addToBuffer(item);
// WAKE MECHANISM: If any consumer is waiting for an item, wake ONE
// Uses LIFO (stack) for O(1) performance - order doesn't affect correctness
if (this.waitingConsumersCount > 0) {
const consumer = this.popWaiter(this.waitingConsumers, this.waitingConsumersCount);
this.waitingConsumersCount--;
consumer?.(); // Calling resolve() wakes the awaiting consumer
}
}
/**
* Removes and returns the oldest item from the queue. Blocks if the queue is empty.
* @returns A promise that resolves to the item, or undefined if the queue is closed and empty
*/
async dequeue(): Promise<T | undefined> {
// BLOCKING MECHANISM: Wait if queue is empty
// Consumers block here until producers provide items or queue closes
while (this.count === 0 && !this.closed) {
// Create unresolved Promise, store only the resolve function
// This suspends the consumer until a producer adds an item
await new Promise<void>(resolve => {
this.waitingConsumersCount = this.pushWaiter(this.waitingConsumers, this.waitingConsumersCount, resolve);
});
}
// After waking/looping, check if we exited due to close (not an item)
// Return undefined to signal "end of stream" to consumers
if (this.count === 0 && this.closed) {
return undefined;
}
// Remove and get the oldest item from circular buffer (FIFO order)
const item = this.removeFromBuffer();
// WAKE MECHANISM: If any producer is waiting for space, wake ONE
// Uses LIFO (stack) for O(1) performance - order doesn't affect correctness
if (this.waitingProducersCount > 0) {
const producer = this.popWaiter(this.waitingProducers, this.waitingProducersCount);
this.waitingProducersCount--;
producer?.(); // Calling resolve() wakes the awaiting producer
}
return item;
}
/**
* Signals that no more items will be added to the queue.
* Existing items can still be consumed.
*/
close(): void {
// Signal that no more items will be added
// Existing items can still be consumed
this.closed = true;
// Wake ALL waiting consumers - they'll return undefined
// This allows graceful shutdown where all consumers exit cleanly
for (let i = 0; i < this.waitingConsumersCount; i++) {
this.waitingConsumers[i]?.();
this.waitingConsumers[i] = undefined; // Help GC
}
this.waitingConsumersCount = 0;
// Wake ALL waiting producers - they'll throw an error
// This prevents deadlock where producers wait forever
for (let i = 0; i < this.waitingProducersCount; i++) {
this.waitingProducers[i]?.();
this.waitingProducers[i] = undefined; // Help GC
}
this.waitingProducersCount = 0;
}
/**
* Checks if the queue is closed AND empty
* @returns true if the queue is closed and has no remaining items
*/
get isClosed(): boolean {
// Queue is "fully closed" only when closed AND empty
// This allows consumers to drain remaining items after close()
return this.closed && this.count === 0;
}
/**
* Gets the current number of items in the queue
* @returns The number of items currently in the queue
*/
get size(): number {
return this.count;
}
/**
* Gets the maximum size of the queue
* @returns The maximum number of items the queue can hold
*/
get capacity(): number {
return this.maxSize;
}
/**
* Checks if the queue is at full capacity
* @returns true if the queue is full
*/
get isFull(): boolean {
return this.count >= this.maxSize;
}
/**
* Checks if the queue is empty
* @returns true if the queue has no items
*/
get isEmpty(): boolean {
return this.count === 0;
}
/**
* Gets the number of waiting consumers
* @returns The number of consumers waiting for items
*/
get waitingConsumerCount(): number {
return this.waitingConsumersCount;
}
/**
* Gets the number of waiting producers
* @returns The number of producers waiting for space
*/
get waitingProducerCount(): number {
return this.waitingProducersCount;
}
/**
* Returns an async iterator for consuming items from the queue.
* Allows the queue to be used with for-await-of loops.
* The iterator will complete when the queue is closed and empty.
*
* @example
* ```typescript
* const queue = new AsyncQueue<number>();
*
* // Producer
* setTimeout(async () => {
* for (let i = 0; i < 5; i++) {
* await queue.enqueue(i);
* }
* queue.close();
* }, 0);
*
* // Consumer using async iterator
* for await (const item of queue) {
* console.log(item); // 0, 1, 2, 3, 4
* }
* ```
*/
async *[Symbol.asyncIterator](): AsyncIterator<T> {
while (true) {
const item = await this.dequeue();
if (item === undefined) {
// dequeue returns undefined only when queue is closed and empty
break;
}
yield item;
}
}
/**
* Creates an async iterable that consumes items from the queue.
* This is an alternative way to get an async iterator.
*
* @returns An async iterable for consuming queue items
* @example
* ```typescript
* const queue = new AsyncQueue<string>();
* const iterator = queue.iterate();
*
* for await (const item of iterator) {
* console.log(item);
* }
* ```
*/
iterate(): AsyncIterable<T> {
return {
[Symbol.asyncIterator]: () => this[Symbol.asyncIterator]()
};
}
/**
* Converts the queue to an async generator.
* Useful for transformation pipelines.
*
* @returns An async generator that yields items from the queue
* @example
* ```typescript
* const queue = new AsyncQueue<number>();
* const generator = queue.toAsyncGenerator();
*
* // Transform items
* async function* double(source: AsyncGenerator<number>) {
* for await (const item of source) {
* yield item * 2;
* }
* }
*
* for await (const item of double(generator)) {
* console.log(item);
* }
* ```
*/
async *toAsyncGenerator(): AsyncGenerator<T> {
yield* this;
}
/**
* Drains all items from the queue into an array.
* Waits until the queue is closed and returns all items.
*
* @returns A promise that resolves to an array of all items
* @example
* ```typescript
* const queue = new AsyncQueue<number>();
*
* // Producer
* (async () => {
* for (let i = 0; i < 5; i++) {
* await queue.enqueue(i);
* }
* queue.close();
* })();
*
* const items = await queue.drain();
* console.log(items); // [0, 1, 2, 3, 4]
* ```
*/
async drain(): Promise<T[]> {
const items: T[] = [];
for await (const item of this) {
items.push(item);
}
return items;
}
/**
* Takes up to n items from the queue.
* Returns early if the queue is closed before n items are received.
*
* @param n The maximum number of items to take
* @returns A promise that resolves to an array of items
* @example
* ```typescript
* const queue = new AsyncQueue<number>();
*
* // Take first 3 items
* const items = await queue.take(3);
* ```
*/
async take(n: number): Promise<T[]> {
const items: T[] = [];
for (let i = 0; i < n && !this.isClosed; i++) {
const item = await this.dequeue();
if (item !== undefined) {
items.push(item);
} else {
break;
}
}
return items;
}
}
// Default export for CommonJS compatibility
export default AsyncQueue; |