HORIZON PROJECT

module.exports = Pend;

function Pend() {

  this.pending = 0;

  this.max = Infinity;

  this.listeners = [];

  this.waiting = [];

  this.error = null;

}

Pend.prototype.go = function(fn) {

  if (this.pending < this.max) {

    pendGo(this, fn);

  } else {

    this.waiting.push(fn);

  }

};

Pend.prototype.wait = function(cb) {

  if (this.pending === 0) {

    cb(this.error);

  } else {

    this.listeners.push(cb);

  }

};

Pend.prototype.hold = function() {

  return pendHold(this);

};

function pendHold(self) {

  self.pending += 1;

  var called = false;

  return onCb;

  function onCb(err) {

    if (called) throw new Error("callback called twice");

    called = true;

    self.error = self.error || err;

    self.pending -= 1;

    if (self.waiting.length > 0 && self.pending < self.max) {

      pendGo(self, self.waiting.shift());

    } else if (self.pending === 0) {

      var listeners = self.listeners;

      self.listeners = [];

      listeners.forEach(cbListener);

    }

  }

  function cbListener(listener) {

    listener(self.error);

  }

}

function pendGo(self, fn) {

  fn(pendHold(self));

}