/**
 *
 * iterateAsync
 * iterate array or object asynchronously.
 *
 * @author shinout
 *
 * @param Object / Array obj : object to iterate.
 * @param function fn        : function to apply to each key, value.
 *                             arguments order -> function(value, key)
 * @param function callback  : (optional) callback called when iteration is finished.
 * @return void
 *
 **/

function iterateAsync(obj, fn, callback) {
  // checking type
  if (typeof obj != "object")  throw new Error("first arguments must be an array or an object");
  if (typeof fn != "function") throw new Error("second arguments must be a function");

  var self = this; // scope to pass to fn

  // function to execute fn, returning a flag of whether to run next or not.
  var next = (function() {
    var k = 0;  // current key

    if (Array.isArray(obj)) { // if array, simply fetch a value by current key
      var last = obj.length;
      return function() {
        fn.call(self, obj[k], k++);
        return k < last;
      };
    }
    else { // if object
      var keys = Object.keys(obj), last = keys.length;
      return function() {
        fn.call(self, obj[keys[k]], keys[k++]);
        return k < last;
      };
    }
  })();

  // execution
  (function iterate() {
    if (next()) process.nextTick(iterate);
    else if (typeof callback == "function") callback();
  })();
}

module.exports = iterateAsync;


// test
function test() {
  var obj = {};

  for (var i=0; i<100; i++) obj["obj" + i] = i * 2;

  var arr = [];
  for (var i=0; i<100; i++) arr[i] = i * 3;


  iterateAsync(obj, function(v, k) {
    console.log(k, "* 2 = ", v);
  });


  iterateAsync(arr, function(v, k) {
    console.log(k, "* 3 = ", v);
  });
}

if (typeof exports == 'object' && exports === this) test();