class Queue {

    constructor (id) {
        this.id = id;
        this.weight = 0;
        this.isClosed = false;
        this._p = Promise.resolve();
    }

    add (weight, task) {
        this.weight += weight;
        this._p = this._p
            .then(() => {
                if (this.isClosed) return;
                log(`Queue #${this.id}: task is started`);
                return task();
            })
            .catch(log)
            .then(() => {
                this.weight -= weight;
                if (this.isClosed) return;
                log(`Queue #${this.id}: task is finished`);
            });
    }

    close () {
        this.isClosed = true;
    }
}