(function(){

    var async = {};

    // global on the server, window in the browser
    var root = this;
    var previous_async = root.async;

    if(typeof module !== 'undefined' && module.exports) module.exports = async;
    else root.async = async;

    async.noConflict = function(){
        root.async = previous_async;
        return async;
    };

    //// cross-browser compatiblity functions ////

    var _forEach = function(arr, iterator){
        if(arr.forEach) return arr.forEach(iterator);
        for(var i=0; i<arr.length; i++){
            iterator(arr[i], i, arr);
        }
    };

    var _map = function(arr, iterator){
        if(arr.map) return arr.map(iterator);
        var results = [];
        _forEach(arr, function(x, i, a){
            results.push(iterator(x, i, a));
        })
        return results;
    };

    var _reduce = function(arr, iterator, memo){
        if(arr.reduce) return arr.reduce(iterator, memo);
        _forEach(arr, function(x, i, a){
            memo = iterator(memo, x, i, a);
        });
        return memo;
    };

    var _keys = function(obj){
        if(Object.keys) return Object.keys(obj);
        var keys = [];
        for(var k in obj){
            if(obj.hasOwnProperty(k)) keys.push(k);
        }
        return keys;
    };

    var _indexOf = function(arr, item){
        if(arr.indexOf) return arr.indexOf(item);
        for(var i=0; i<arr.length; i++){
            if(arr[i] === item) return i;
        }
        return -1;
    };

    //// exported async module functions ////

    //// nextTick implementation with browser-compatible fallback ////
    async.nextTick = function(fn){
        if(typeof process == 'undefined' || !(process.nextTick)){
            setTimeout(fn, 0);
        }
        else process.nextTick(fn);
    };

    async.forEach = function(arr, iterator, callback){
        if(!arr.length) return callback();
        var completed = 0;
        _forEach(arr, function(x){
            iterator(x, function(err){
                if(err){
                    callback(err);
                    callback = function(){};
                }
                else {
                    completed++;
                    if(completed == arr.length) callback();
                }
            });
        });
    };

    async.forEachSeries = function(arr, iterator, callback){
        if(!arr.length) return callback();
        var completed = 0;
        var iterate = function(){
            iterator(arr[completed], function(err){
                if(err){
                    callback(err);
                    callback = function(){};
                }
                else {
                    completed++;
                    if(completed == arr.length) callback();
                    else iterate();
                }
            });
        };
        iterate();
    };


    var doParallel = function(fn){
        return function(){
            var args = Array.prototype.slice.call(arguments);
            return fn.apply(null, [async.forEach].concat(args));
        };
    };
    var doSeries = function(fn){
        return function(){
            var args = Array.prototype.slice.call(arguments);
            return fn.apply(null, [async.forEachSeries].concat(args));
        };
    };


    var _asyncMap = function(eachfn, arr, iterator, callback){
        var results = [];
        arr = _map(arr, function(x, i){
            return {index: i, value: x};
        });
        eachfn(arr, function(x, callback){
            iterator(x.value, function(err, v){
                results[x.index] = v;
                callback(err);
            });
        }, function(err){
            callback(err, results);
        });
    };
    async.map = doParallel(_asyncMap);
    async.mapSeries = doSeries(_asyncMap);


    // reduce only has a series version, as doing reduce in parallel won't
    // work in many situations.
    async.reduce = function(arr, memo, iterator, callback){
        async.forEachSeries(arr, function(x, callback){
            iterator(memo, x, function(err, v){
                memo = v;
                callback(err);
            });
        }, function(err){
            callback(err, memo);
        });
    };
    // inject alias
    async.inject = async.reduce;
    // foldl alias
    async.foldl = async.reduce;

    async.reduceRight = function(arr, memo, iterator, callback){
        var reversed = _map(arr, function(x){return x;}).reverse();
        async.reduce(reversed, memo, iterator, callback);
    };
    // foldr alias
    async.foldr = async.reduceRight;

    var _filter = function(eachfn, arr, iterator, callback){
        var results = [];
        arr = _map(arr, function(x, i){
            return {index: i, value: x};
        });
        eachfn(arr, function(x, callback){
            iterator(x.value, function(v){
                if(v) results.push(x);
                callback();
            });
        }, function(err){
            callback(_map(results.sort(function(a,b){
                return a.index - b.index;
            }), function(x){
                return x.value;
            }));
        });
    };
    async.filter = doParallel(_filter);
    async.filterSeries = doSeries(_filter);
    // select alias
    async.select = async.filter;
    async.selectSeries = async.filterSeries;

    var _reject = function(eachfn, arr, iterator, callback){
        var results = [];
        arr = _map(arr, function(x, i){
            return {index: i, value: x};
        });
        eachfn(arr, function(x, callback){
            iterator(x.value, function(v){
                if(!v) results.push(x);
                callback();
            });
        }, function(err){
            callback(_map(results.sort(function(a,b){
                return a.index - b.index;
            }), function(x){
                return x.value;
            }));
        });
    };
    async.reject = doParallel(_reject);
    async.rejectSeries = doSeries(_reject);

    var _detect = function(eachfn, arr, iterator, main_callback){
        eachfn(arr, function(x, callback){
            iterator(x, function(result){
                if(result) main_callback(x);
                else callback();
            });
        }, function(err){
            main_callback();
        });
    };
    async.detect = doParallel(_detect);
    async.detectSeries = doSeries(_detect);

    async.some = function(arr, iterator, main_callback){
        async.forEach(arr, function(x, callback){
            iterator(x, function(v){
                if(v){
                    main_callback(true);
                    main_callback = function(){};
                }
                callback();
            });
        }, function(err){
            main_callback(false);
        });
    };
    // any alias
    async.any = async.some;

    async.every = function(arr, iterator, main_callback){
        async.forEach(arr, function(x, callback){
            iterator(x, function(v){
                if(!v){
                    main_callback(false);
                    main_callback = function(){};
                }
                callback();
            });
        }, function(err){
            main_callback(true);
        });
    };
    // all alias
    async.all = async.every;

    async.sortBy = function(arr, iterator, callback){
        async.map(arr, function(x, callback){
            iterator(x, function(err, criteria){
                if(err) callback(err);
                else callback(null, {value: x, criteria: criteria});
            });
        }, function(err, results){
            if(err) return callback(err);
            else callback(null, _map(results.sort(function(left, right){
                var a = left.criteria, b = right.criteria;
                return a < b ? -1 : a > b ? 1 : 0;
            }), function(x){return x.value;}));
        })
    };

    async.auto = function(tasks, callback){
        callback = callback || function(){};
        var keys = _keys(tasks);
        if(!keys.length) return callback(null);

        var completed = [];

        var listeners = [];
        var addListener = function(fn){
            listeners.unshift(fn);
        };
        var removeListener = function(fn){
            for(var i=0; i<listeners.length; i++){
                if(listeners[i] === fn){
                    listeners.splice(i, 1);
                    return;
                }
            }
        };
        var taskComplete = function(){
            _forEach(listeners, function(fn){fn();});
        };

        addListener(function(){
            if(completed.length == keys.length){
                callback(null);
            }
        });

        _forEach(keys, function(k){
            var task = (tasks[k] instanceof Function)? [tasks[k]]: tasks[k];
            var taskCallback = function(err){
                if(err){
                    callback(err);
                    // stop subsequent errors hitting callback multiple times
                    callback = function(){};
                }
                else {
                    completed.push(k);
                    taskComplete();
                }
            };
            var requires = task.slice(0, Math.abs(task.length-1)) || [];
            var ready = function(){
                return _reduce(requires, function(a,x){
                    return (a && _indexOf(completed, x) != -1);
                }, true);
            };
            if(ready()) task[task.length-1](taskCallback);
            else {
                var listener = function(){
                    if(ready()){
                        removeListener(listener);
                        task[task.length-1](taskCallback);
                    }
                };
                addListener(listener);
            }
        });
    };

    async.waterfall = function(tasks, callback){
        if(!tasks.length) return callback();
        callback = callback || function(){};
        var wrapIterator = function(iterator){
            return function(err){
                if(err){
                    callback(err);
                    callback = function(){};
                }
                else {
                    var args = Array.prototype.slice.call(arguments, 1);
                    var next = iterator.next();
                    if(next) args.push(wrapIterator(next));
                    else     args.push(callback);
                    async.nextTick(function(){iterator.apply(null, args);});
                }
            };
        };
        wrapIterator(async.iterator(tasks))();
    };

    async.parallel = function(tasks, callback){
        callback = callback || function(){};
        async.map(tasks, function(fn, callback){
            if(fn){
                fn(function(err){
                    var args = Array.prototype.slice.call(arguments,1);
                    if(args.length <= 1) args = args[0];
                    callback.call(null, err, args || null);
                });
            }
        }, callback);
    };

    async.series = function(tasks, callback){
        callback = callback || function(){};
        async.mapSeries(tasks, function(fn, callback){
            if(fn){
                fn(function(err){
                    var args = Array.prototype.slice.call(arguments,1);
                    if(args.length <= 1) args = args[0];
                    callback.call(null, err, args || null);
                });
            }
        }, callback);
    };

    async.iterator = function(tasks){
        var makeCallback = function(index){
            var fn = function(){
                if(tasks.length) tasks[index].apply(null, arguments);
                return fn.next();
            };
            fn.next = function(){
                return (index < tasks.length-1)? makeCallback(index+1): null;
            };
            return fn;
        };
        return makeCallback(0);
    };

    async.apply = function(fn){
        var args = Array.prototype.slice.call(arguments, 1);
        return function(){
            return fn.apply(
                null, args.concat(Array.prototype.slice.call(arguments))
            );
        };
    };

    var _console_fn = function(name){
        return function(fn){
            var args = Array.prototype.slice.call(arguments, 1);
            fn.apply(null, args.concat([function(err){
                var args = Array.prototype.slice.call(arguments, 1);
                if(typeof console != 'undefined'){
                    if(err){
                        if(console.error) console.error(err);
                    }
                    else if(console[name]){
                        _forEach(args, function(x){console[name](x);});
                    }
                }
            }]));
        };
    };
    async.log = _console_fn('log');
    async.dir = _console_fn('dir');
    /*async.info = _console_fn('info');
    async.warn = _console_fn('warn');
    async.error = _console_fn('error');*/

})();