auto = function (tasks, concurrency, callback) {
    if (typeof concurrency === 'function') {
        // concurrency is optional, shift the args.
        callback = concurrency;
        concurrency = null;
    }
    callback = once(callback || noop);
    var keys$$1 = keys(tasks);
    var numTasks = keys$$1.length;
    if (!numTasks) {
        return callback(null);
    }
    if (!concurrency) {
        concurrency = numTasks;
    }

    var results = {};
    var runningTasks = 0;
    var hasError = false;

    var listeners = Object.create(null);

    var readyTasks = [];

    // for cycle detection:
    var readyToCheck = []; // tasks that have been identified as reachable
    // without the possibility of returning to an ancestor task
    var uncheckedDependencies = {};

    baseForOwn(tasks, function (task, key) {
        if (!isArray(task)) {
            // no dependencies
            enqueueTask(key, [task]);
            readyToCheck.push(key);
            return;
        }

        var dependencies = task.slice(0, task.length - 1);
        var remainingDependencies = dependencies.length;
        if (remainingDependencies === 0) {
            enqueueTask(key, task);
            readyToCheck.push(key);
            return;
        }
        uncheckedDependencies[key] = remainingDependencies;

        arrayEach(dependencies, function (dependencyName) {
            if (!tasks[dependencyName]) {
                throw new Error('async.auto task `' + key +
                    '` has a non-existent dependency `' +
                    dependencyName + '` in ' +
                    dependencies.join(', '));
            }
            addListener(dependencyName, function () {
                remainingDependencies--;
                if (remainingDependencies === 0) {
                    enqueueTask(key, task);
                }
            });
        });
    });

    checkForDeadlocks();
    processQueue();

    function enqueueTask(key, task) {
        readyTasks.push(function () {
            runTask(key, task);
        });
    }

    function processQueue() {
        if (readyTasks.length === 0 && runningTasks === 0) {
            return callback(null, results);
        }
        while(readyTasks.length && runningTasks < concurrency) {
            var run = readyTasks.shift();
            run();
        }

    }

    function addListener(taskName, fn) {
        var taskListeners = listeners[taskName];
        if (!taskListeners) {
            taskListeners = listeners[taskName] = [];
        }

        taskListeners.push(fn);
    }

    function taskComplete(taskName) {
        var taskListeners = listeners[taskName] || [];
        arrayEach(taskListeners, function (fn) {
            fn();
        });
        processQueue();
    }


    function runTask(key, task) {
        if (hasError) return;

        var taskCallback = onlyOnce(function(err, result) {
            runningTasks--;
            if (arguments.length > 2) {
                result = slice(arguments, 1);
            }
            if (err) {
                var safeResults = {};
                baseForOwn(results, function(val, rkey) {
                    safeResults[rkey] = val;
                });
                safeResults[key] = result;
                hasError = true;
                listeners = Object.create(null);

                callback(err, safeResults);
            } else {
                results[key] = result;
                taskComplete(key);
            }
        });

        runningTasks++;
        var taskFn = wrapAsync(task[task.length - 1]);
        if (task.length > 1) {
            taskFn(results, taskCallback);
        } else {
            taskFn(taskCallback);
        }
    }

    function checkForDeadlocks() {
        // Kahn's algorithm
        // https://en.wikipedia.org/wiki/Topological_sorting#Kahn.27s_algorithm
        // http://connalle.blogspot.com/2013/10/topological-sortingkahn-algorithm.html
        var currentTask;
        var counter = 0;
        while (readyToCheck.length) {
            currentTask = readyToCheck.pop();
            counter++;
            arrayEach(getDependents(currentTask), function (dependent) {
                if (--uncheckedDependencies[dependent] === 0) {
                    readyToCheck.push(dependent);
                }
            });
        }

        if (counter !== numTasks) {
            throw new Error(
                'async.auto cannot execute tasks due to a recursive dependency'
            );
        }
    }

    function getDependents(taskName) {
        var result = [];
        baseForOwn(tasks, function (task, key) {
            if (isArray(task) && baseIndexOf(task, taskName, 0) >= 0) {
                result.push(key);
            }
        });
        return result;
    }
}
Determines the best order for running the {@link AsyncFunction}s in `tasks`, based on
their requirements. Each function can optionally depend on other functions
being completed first, and each function is run as soon as its requirements
are satisfied.

If any of the {@link AsyncFunction}s pass an error to their callback, the `auto` sequence
will stop. Further tasks will not execute (so any other functions depending
on it will not run), and the main `callback` is immediately called with the
error.

{@link AsyncFunction}s also receive an object containing the results of functions which
have completed so far as the first argument, if they have dependencies. If a
task function has no dependencies, it will only be passed a callback.

@name auto
@static
@memberOf module:ControlFlow
@method
@category Control Flow
@param {Object} tasks - An object. Each of its properties is either a
function or an array of requirements, with the {@link AsyncFunction} itself the last item
in the array. The object's key of a property serves as the name of the task
defined by that property, i.e. can be used when specifying requirements for
other tasks. The function receives one or two arguments:
* a `results` object, containing the results of the previously executed
  functions, only passed if the task has any dependencies,
* a `callback(err, result)` function, which must be called when finished,
  passing an `error` (which can be `null`) and the result of the function's
  execution.
@param {number} [concurrency=Infinity] - An optional `integer` for
determining the maximum number of tasks that can be run in parallel. By
default, as many as possible.
@param {Function} [callback] - An optional callback which is called when all
the tasks have been completed. It receives the `err` argument if any `tasks`
pass an error to their callback. Results are always returned; however, if an
error occurs, no further `tasks` will be performed, and the results object
will only contain partial results. Invoked with (err, results).
@returns undefined
@example

async.auto({
    // this function will just be passed a callback
    readData: async.apply(fs.readFile, 'data.txt', 'utf-8'),
    showData: ['readData', function(results, cb) {
        // results.readData is the file's contents
        // ...
    }]
}, callback);

async.auto({
    get_data: function(callback) {
        console.log('in get_data');
        // async code to get some data
        callback(null, 'data', 'converted to array');
    },
    make_folder: function(callback) {
        console.log('in make_folder');
        // async code to create a directory to store a file in
        // this is run at the same time as getting the data
        callback(null, 'folder');
    },
    write_file: ['get_data', 'make_folder', function(results, callback) {
        console.log('in write_file', JSON.stringify(results));
        // once there is some data and the directory exists,
        // write the data to a file in the directory
        callback(null, 'filename');
    }],
    email_link: ['write_file', function(results, callback) {
        console.log('in email_link', JSON.stringify(results));
        // once the file is written let's email a link to it...
        // results.write_file contains the filename returned by write_file.
        callback(null, {'file':results.write_file, 'email':'user@example.com'});
    }]
}, function(err, results) {
    console.log('err = ', err);
    console.log('results = ', results);
});