HORIZON PROJECT

'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;