/*
 *  runs callbacks in parallel and returns once all callbacks are executed
 *  accept arguments as array f function or object of a function
 *  should be called in form of array of object of following function
 *   const tasks = [function yourName(callback) {
 *           your_function(a, b, c, (data) => {
 *               callback(null, data);
 *           })
 *   },]
 *   You are able to access above  data as
 *    executeParallel(tasks, (err, results) => {
 *       const data = results.yourName; // your function name
 *     })
 */
export const executeParallel = function (tasks, cb) {
    let results;
    let pending;
    let keys;
    let isSync = true;

    if (Array.isArray(tasks)) {
        results = {};
        pending = tasks.length;
    } else {
        keys = Object.keys(tasks);
        results = {};
        pending = keys.length;
    }

    function done (err) {
        function end () {
            if (cb) cb(err, results);
            cb = null;
        }
        if (isSync) process.nextTick(end);
        else end();
    }

    function each (i, err, result, name) {
        results[name] = result;
        if (--pending === 0 || err) {
            done(err);
        }
    }

    if (!pending) {
        // empty
        done(null);
    } else if (keys) {
        // object
        keys.forEach(function (key) {
            tasks[key](function (err, result) { each(key, err, result) });
        })
    } else {
        // array
        tasks.forEach(function (task, i) {
            task(function (err, result) { each(i, err, result, task.name) });
        })
    }
    isSync = false;
}