/**
 * @author Deepkolos / https://github.com/deepkolos
 */

export class WorkerPool {

	constructor ( pool = 4 ) {

		this.pool = pool;
		this.queue = [];
		this.workers = [];
		this.workersResolve = [];
		this.workerStatus = 0;

	}

	_initWorker ( workerId ) {

		if ( !this.workers[ workerId ] ) {

			const worker = this.workerCreator();
			worker.addEventListener( 'message', this._onMessage.bind( this, workerId ) );
			this.workers[ workerId ] = worker;

		}

	}

	_getIdleWorker () {

		for ( let i = 0 ; i < this.pool ; i ++ ) 
			if ( ! ( this.workerStatus & ( 1 << i ) ) ) return i;

		return -1;

	}

	_onMessage( workerId, msg ) {

		const resolve = this.workersResolve[ workerId ];
		resolve && resolve( msg );

		if ( this.queue.length ) {

			const { resolve, msg, transfer } = this.queue.shift();
			this.workersResolve[ workerId ] = resolve;
			this.workers[ workerId ].postMessage( msg, transfer );

		} else {

			this.workerStatus ^= 1 << workerId;

		}

	}

	setWorkerCreator ( workerCreator ) {

		this.workerCreator = workerCreator;

	}

	setWorkerLimit ( pool ) {

		this.pool = pool;

	}

	postMessage ( msg, transfer ) {

		return new Promise( ( resolve ) => {

			const workerId = this._getIdleWorker();

			if ( workerId !== -1 ) {

				this._initWorker( workerId );
				this.workerStatus |= 1 << workerId;
				this.workersResolve[ workerId ] = resolve;
				this.workers[ workerId ].postMessage( msg, transfer );

			} else {

				this.queue.push( { resolve, msg, transfer } );

			}

		} );

	}

	dispose () {

		this.workers.forEach( ( worker ) => worker.terminate() );
		this.workersResolve.length = 0;
		this.workers.length = 0;
		this.queue.length = 0;
		this.workerStatus = 0;

	}

}