(function(exports) {
    "use strict";

    exports.batchAsync = function(limit, arr, doStuff) {
        arr = arr.slice(0);
        return new Promise(function(resolve, reject) {
            var total = arr.length;
            var active = 0;
            var results = [];
            var error;

            function doMoreStuff() {
                // Don't take on any more tasks if we've errored,
                // or if too many are already in progress
                if (error || active > limit) {
                    return;
                }

                // If there are no more tasks to start, return
                if (!arr.length) {
                    // If everything is also *finished*, resolve
                    if (active < 1) {
                        resolve(results);
                    }
                    return;
                }

                // We need to dequeue the task here so the index is correct
                // (keep in mind we want to support sync and async)
                var index = total - arr.length;
                var task = arr.shift();
                active += 1;

                // Spawn another task immediately,
                // which will be stopped if we're at the limit
                doMoreStuff();

                var p;
                try {
                    p = doStuff(task, index, arr);
                } catch (e) {
                    // we need to handle, and bubble, synchronous errors
                    error = e;
                    reject(e);
                    throw e;
                }
                // Do stuff and then decrease the active counter when done
                // add support for sync by rapping in a promise
                Promise.resolve(p)
                    .then(function(result) {
                        if ("undefined" === typeof result) {
                            throw new Error(
                                "result was 'undefined'. Please return 'null' to signal that you didn't just forget to return another promise."
                            );
                        }
                        active -= 1;
                        results[index] = result;
                    })
                    .then(doMoreStuff)
                    .catch(function(e) {
                        // handle async errors
                        error = e;
                        reject(e);
                    });
            }

            doMoreStuff();
        });
    };
})("undefined" !== typeof window ? window : module.exports);