'use strict';

/**
 * @example
 *
 * var Q = Q(function(o, done) {
 *
 * }, max_queue_item)
 *
 * q.enqueue(o, function(r) {
 *
 * }, ctx)
 *
 *
 */

function Q(fn, _max_size, when_empty) {
  const ctx = this;
  const max_size = _max_size ? _max_size : Infinity;
  let e = null;
  let count = 0;

  const ret = {
    enqueue,
  };

  function QItem(o, fn, ctx) {
    return {
      fn,
      ctx,
      data: o,
      next: null,
    };
  }

  function enqueue(o, fn, ctx) {
    if (count >= max_size) {
      throw new Error('queue fulled');
    }
    count++;
    const item = QItem(o, fn, ctx);
    if (e == null) {
      e = item;
      process.nextTick(next.bind(ctx, item));
    } else {
      e.next = item;
      e = e.next;
    }

    return ret;
  }

  function next(h) {
    if (h == null) {
      e = null;
      if (when_empty) when_empty();

      return;
    }
    count--;
    fn(h.data, (...args) => {
      h.fn.apply(h.ctx || ctx, args);
    }).then(next.bind(ctx, h.next), next.bind(ctx, h.next));
  }

  return ret;
}

module.exports = Q;