// Use the fastest possible means to execute a task in a future turn
// of the event loop.

// linked list of tasks (single, with head node)
var head = {task: void 0, next: null};
var tail = head;
var flushing = false;
var requestFlush = void 0;
var isNodeJS = false;

function flush() {
    /* jshint loopfunc: true */

    while (head.next) {
        head = head.next;
        var task = head.task;
        head.task = void 0;
        var domain = head.domain;

        if (domain) {
            head.domain = void 0;
            domain.enter();
        }

        try {
            task();

        } catch (e) {
            if (isNodeJS) {
                // In node, uncaught exceptions are considered fatal errors.
                // Re-throw them synchronously to interrupt flushing!

                // Ensure continuation if the uncaught exception is suppressed
                // listening "uncaughtException" events (as domains does).
                // Continue in next event to avoid tick recursion.
                if (domain) {
                    domain.exit();
                }
                setTimeout(flush, 0);
                if (domain) {
                    domain.enter();
                }

                throw e;

            } else {
                // In browsers, uncaught exceptions are not fatal.
                // Re-throw them asynchronously to avoid slow-downs.
                setTimeout(function() {
                   throw e;
                }, 0);
            }
        }

        if (domain) {
            domain.exit();
        }
    }

    flushing = false;
}

if (typeof process !== "undefined" && process.nextTick) {
    // Node.js before 0.9. Note that some fake-Node environments, like the
    // Mocha test runner, introduce a `process` global without a `nextTick`.
    isNodeJS = true;

    requestFlush = function () {
        process.nextTick(flush);
    };

} else if (typeof setImmediate === "function") {
    // In IE10, Node.js 0.9+, or https://github.com/NobleJS/setImmediate
    if (typeof window !== "undefined") {
        requestFlush = setImmediate.bind(window, flush);
    } else {
        requestFlush = function () {
            setImmediate(flush);
        };
    }

} else if (typeof MessageChannel !== "undefined") {
    // modern browsers
    // http://www.nonblocking.io/2011/06/windownexttick.html
    var channel = new MessageChannel();
    channel.port1.onmessage = flush;
    requestFlush = function () {
        channel.port2.postMessage(0);
    };

} else {
    // old browsers
    requestFlush = function () {
        setTimeout(flush, 0);
    };
}

function asap(task) {
    tail = tail.next = {
        task: task,
        domain: isNodeJS && process.domain,
        next: null
    };

    if (!flushing) {
        flushing = true;
        requestFlush();
    }
};

module.exports = asap;