var global    = require('./$.global')
  , macrotask = require('./$.task').set
  , Observer  = global. MutationObserver || global. WebKitMutationObserver
  , process   = global.process
  , Promise   = global. Promise
  , isNode    = require('./$.cof')(process) == 'process'
  , head, last, notify;

var flush = function(){
  var parent, domain, fn;
  if(isNode && (parent = process.domain)){
    process.domain = null;
    parent.exit();
  }
  while(head){
    domain = head.domain;
    fn     = head.fn;
    if(domain)domain.enter();
    fn(); // <- currently we use it only for Promise - try / catch not required
    if(domain)domain.exit();
    head = head.next;
  } last = undefined;
  if(parent)parent.enter();
};

// Node.js
if(isNode){
  notify = function(){
    process.nextTick(flush);
  };
// browsers with MutationObserver
} else if(Observer){
  var toggle = 1
    , node   = document.createTextNode('');
  new Observer(flush).observe(node, {characterData: true}); // eslint-disable-line no-new
  notify = function(){
    node.data = toggle = -toggle;
  };
// environments with maybe non-completely correct, but existent Promise
} else if(Promise && Promise.resolve){
  notify = function(){
    Promise.resolve().then(flush);
  };
// for other environments - macrotask based on:
// - setImmediate
// - MessageChannel
// - window.postMessag
// - onreadystatechange
// - setTimeout
} else {
  notify = function(){
    // strange IE + webpack dev server bug - use .call(global)
    macrotask.call(global, flush);
  };
}

module.exports = function asap(fn){
  var task = {fn: fn, next: undefined, domain: isNode && process.domain};
  if(last)last.next = task;
  if(!head){
    head = task;
    notify();
  } last = task;
};