/* This Source Code Form is subject to the terms of the Mozilla Public

 * License, v. 2.0. If a copy of the MPL was not distributed with this file,

 * You can obtain one at http://mozilla.org/MPL/2.0/. */

/*

 * This is an approximate implementation of ES7's async-await pattern.

 * see: https://github.com/tc39/ecmascript-asyncawait

 *

 * It allows for simple creation of async function and "tasks".

 *

 * For example:

 *

 *  var myThinger = {

 *    doAsynThing: async(function*(url){

 *      var result = yield fetch(url);

 *      return process(result);

 *    });

 * }

 *

 * And Task-like things can be created as follows:

 *

 * var myTask = async(function*{

 *   var result = yield fetch(url);

 *   return result;

 * });

 * //returns a promise

 *

 * myTask().then(doSomethingElse);

 *

 */

(function(exports) {

  "use strict";

  function async(func, self) {

    return function asyncFunction() {

      const functionArgs = Array.from(arguments);

      return new Promise(function(resolve, reject) {

        var gen;

        if (typeof func !== "function") {

          reject(new TypeError("Expected a Function."));

        }

        //not a generator, wrap it.

        if (func.constructor.name !== "GeneratorFunction") {

          gen = (function*() {

            return func.apply(self, functionArgs);

          }());

        } else {

          gen = func.apply(self, functionArgs);

        }

        try {

          step(gen.next(undefined));

        } catch (err) {

          reject(err);

        }

        function step({value, done}) {

          if (done) {

            return resolve(value);

          }

          if (value instanceof Promise) {

            return value.then(

              result => step(gen.next(result)),

              error => {

                try {

                  step(gen.throw(error));

                } catch (err) {

                  throw err;

                }

              }

            ).catch(err => reject(err));

          }

          step(gen.next(value));

        }

      });

    };

  }

  exports.async = async;

}(this || self));