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'use strict';

function Queue(options) {

if (!(this instanceof Queue)) {

return new Queue(options);

}

options = options || {};

this.concurrency = options.concurrency || Infinity;

this.pending = 0;

this.jobs = [];

this.cbs = [];

this._done = done.bind(this);

}

var arrayAddMethods = [

'push',

'unshift',

'splice'

];

arrayAddMethods.forEach(function(method) {

Queue.prototype[method] = function() {

var methodResult = Array.prototype[method].apply(this.jobs, arguments);

this._run();

return methodResult;

};

});

Object.defineProperty(Queue.prototype, 'length', {

get: function() {

return this.pending + this.jobs.length;

}

});

Queue.prototype._run = function() {

if (this.pending === this.concurrency) {

return;

}

if (this.jobs.length) {

var job = this.jobs.shift();

this.pending++;

job(this._done);

this._run();

}

if (this.pending === 0) {

while (this.cbs.length !== 0) {

var cb = this.cbs.pop();

process.nextTick(cb);

}

}

};

Queue.prototype.onDone = function(cb) {

if (typeof cb === 'function') {

this.cbs.push(cb);

this._run();

}

};

function done() {

this.pending--;

this._run();

}

module.exports = Queue;