semaphore.js

semaphore.js

class Semaphore {

	constructor(size, opt_sab) {
		this._sab = opt_sab || new SharedArrayBuffer(4);
		this._sema = new Int32Array(this._sab);
		this._size = size;
	}

	static connect(sema) {
		return new Semaphore(sema._size, sema._sab);
	}

	// acquire does not check for overflow.
	acquire() {
		let count = Atomics.load(this._sema,0);
		for(;;) {
			if (count + 1 > this._size) {
				Atomics.wait(this._sema, 0, count);
				count = Atomics.load(this._sema,0);
				continue;
			}
			let old = Atomics.compareExchange(this._sema, 0, count, count+1)
			if (old == count){
				return;
			}
			count = old;
		}
	}

	release() {
		if (Atomics.sub(this._sema, 0, 1) < 0){
			throw new Error('Semaphore is in inconsistent state: negative count.');
		}
		Atomics.notify(this._sema, 0);
	}
}