import reduce from './reduce.js'
import wrapAsync from './internal/wrapAsync.js'
import { promiseCallback, PROMISE_SYMBOL } from './internal/promiseCallback.js'

/**
 * Version of the compose function that is more natural to read. Each function
 * consumes the return value of the previous function. It is the equivalent of
 * [compose]{@link module:ControlFlow.compose} with the arguments reversed.
 *
 * Each function is executed with the `this` binding of the composed function.
 *
 * @name seq
 * @static
 * @memberOf module:ControlFlow
 * @method
 * @see [async.compose]{@link module:ControlFlow.compose}
 * @category Control Flow
 * @param {...AsyncFunction} functions - the asynchronous functions to compose
 * @returns {Function} a function that composes the `functions` in order
 * @example
 *
 * // Requires lodash (or underscore), express3 and dresende's orm2.
 * // Part of an app, that fetches cats of the logged user.
 * // This example uses `seq` function to avoid overnesting and error
 * // handling clutter.
 * app.get('/cats', function(request, response) {
 *     var User = request.models.User;
 *     async.seq(
 *         User.get.bind(User),  // 'User.get' has signature (id, callback(err, data))
 *         function(user, fn) {
 *             user.getCats(fn);      // 'getCats' has signature (callback(err, data))
 *         }
 *     )(req.session.user_id, function (err, cats) {
 *         if (err) {
 *             console.error(err);
 *             response.json({ status: 'error', message: err.message });
 *         } else {
 *             response.json({ status: 'ok', message: 'Cats found', data: cats });
 *         }
 *     });
 * });
 */
export default function seq(...functions) {
    var _functions = functions.map(wrapAsync);
    return function (...args) {
        var that = this;

        var cb = args[args.length - 1];
        if (typeof cb == 'function') {
            args.pop();
        } else {
            cb = promiseCallback();
        }

        reduce(_functions, args, (newargs, fn, iterCb) => {
            fn.apply(that, newargs.concat((err, ...nextargs) => {
                iterCb(err, nextargs);
            }));
        },
        (err, results) => cb(err, ...results));

        return cb[PROMISE_SYMBOL]
    };
}