underscore.js

//     Underscore.js 1.1.3
//     (c) 2010 Jeremy Ashkenas, Caolan McMahon.
//     Underscore is freely distributable under the MIT license.
//     Portions of Underscore are inspired or borrowed from Prototype,
//     Oliver Steele's Functional, and John Resig's Micro-Templating.
//     For all details and documentation:
//     http://documentcloud.github.com/underscore

(function() {

  // Baseline setup
  // --------------

  // Establish the root object, `window` in the browser, or `global` on the server.
  var root = this;

  // Save the previous value of the `_` variable.
  var previousUnderscore = root._;

  // Establish the object that gets returned to break out of a loop iteration.
  var breaker = {};

  // Save bytes in the minified (but not gzipped) version:
  var ArrayProto = Array.prototype, ObjProto = Object.prototype;

  // Create quick reference variables for speed access to core prototypes.
  var slice            = ArrayProto.slice,
      unshift          = ArrayProto.unshift,
      toString         = ObjProto.toString,
      hasOwnProperty   = ObjProto.hasOwnProperty;

  // All **ECMAScript 5** native function implementations that we hope to use
  // are declared here.
  var
    nativeForEach      = ArrayProto.forEach,
    nativeMap          = ArrayProto.map,
    nativeReduce       = ArrayProto.reduce,
    nativeReduceRight  = ArrayProto.reduceRight,
    nativeFilter       = ArrayProto.filter,
    nativeEvery        = ArrayProto.every,
    nativeSome         = ArrayProto.some,
    nativeIndexOf      = ArrayProto.indexOf,
    nativeLastIndexOf  = ArrayProto.lastIndexOf,
    nativeIsArray      = Array.isArray,
    nativeKeys         = Object.keys;

  // Create a safe reference to the Underscore object for use below.
  var _ = function(obj) { return new wrapper(obj); };

  // Export the Underscore object for **CommonJS**, with backwards-compatibility
  // for the old `require()` API. If we're not in CommonJS, add `_` to the
  // global object.
  if (typeof module !== 'undefined' && module.exports) {
    module.exports = _;
    _._ = _;
  } else {
    root._ = _;
  }

  // Current version.
  _.VERSION = '1.1.3';

  // Collection Functions
  // --------------------

  // The cornerstone, an `each` implementation, aka `forEach`.
  // Handles objects implementing `forEach`, arrays, and raw objects.
  // Delegates to **ECMAScript 5**'s native `forEach` if available.
  var each = _.each = _.forEach = function(obj, iterator, context) {
    var value;
    if (nativeForEach && obj.forEach === nativeForEach) {
      obj.forEach(iterator, context);
    } else if (_.isNumber(obj.length)) {
      for (var i = 0, l = obj.length; i < l; i++) {
        if (iterator.call(context, obj[i], i, obj) === breaker) return;
      }
    } else {
      for (var key in obj) {
        if (hasOwnProperty.call(obj, key)) {
          if (iterator.call(context, obj[key], key, obj) === breaker) return;
        }
      }
    }
  };

  // Return the results of applying the iterator to each element.
  // Delegates to **ECMAScript 5**'s native `map` if available.
  _.map = function(obj, iterator, context) {
    if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
    var results = [];
    each(obj, function(value, index, list) {
      results[results.length] = iterator.call(context, value, index, list);
    });
    return results;
  };

  // **Reduce** builds up a single result from a list of values, aka `inject`,
  // or `foldl`. Delegates to **ECMAScript 5**'s native `reduce` if available.
  _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
    var initial = memo !== void 0;
    if (nativeReduce && obj.reduce === nativeReduce) {
      if (context) iterator = _.bind(iterator, context);
      return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
    }
    each(obj, function(value, index, list) {
      if (!initial && index === 0) {
        memo = value;
      } else {
        memo = iterator.call(context, memo, value, index, list);
      }
    });
    return memo;
  };

  // The right-associative version of reduce, also known as `foldr`.
  // Delegates to **ECMAScript 5**'s native `reduceRight` if available.
  _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
    if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
      if (context) iterator = _.bind(iterator, context);
      return memo !== void 0 ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
    }
    var reversed = (_.isArray(obj) ? obj.slice() : _.toArray(obj)).reverse();
    return _.reduce(reversed, iterator, memo, context);
  };

  // Return the first value which passes a truth test. Aliased as `detect`.
  _.find = _.detect = function(obj, iterator, context) {
    var result;
    any(obj, function(value, index, list) {
      if (iterator.call(context, value, index, list)) {
        result = value;
        return true;
      }
    });
    return result;
  };

  // Return all the elements that pass a truth test.
  // Delegates to **ECMAScript 5**'s native `filter` if available.
  // Aliased as `select`.
  _.filter = _.select = function(obj, iterator, context) {
    if (nativeFilter && obj.filter === nativeFilter) return obj.filter(iterator, context);
    var results = [];
    each(obj, function(value, index, list) {
      if (iterator.call(context, value, index, list)) results[results.length] = value;
    });
    return results;
  };

  // Return all the elements for which a truth test fails.
  _.reject = function(obj, iterator, context) {
    var results = [];
    each(obj, function(value, index, list) {
      if (!iterator.call(context, value, index, list)) results[results.length] = value;
    });
    return results;
  };

  // Determine whether all of the elements match a truth test.
  // Delegates to **ECMAScript 5**'s native `every` if available.
  // Aliased as `all`.
  _.every = _.all = function(obj, iterator, context) {
    iterator = iterator || _.identity;
    if (nativeEvery && obj.every === nativeEvery) return obj.every(iterator, context);
    var result = true;
    each(obj, function(value, index, list) {
      if (!(result = result && iterator.call(context, value, index, list))) return breaker;
    });
    return result;
  };

  // Determine if at least one element in the object matches a truth test.
  // Delegates to **ECMAScript 5**'s native `some` if available.
  // Aliased as `any`.
  var any = _.some = _.any = function(obj, iterator, context) {
    iterator = iterator || _.identity;
    if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
    var result = false;
    each(obj, function(value, index, list) {
      if (result = iterator.call(context, value, index, list)) return breaker;
    });
    return result;
  };

  // Determine if a given value is included in the array or object using `===`.
  // Aliased as `contains`.
  _.include = _.contains = function(obj, target) {
    if (nativeIndexOf && obj.indexOf === nativeIndexOf) return obj.indexOf(target) != -1;
    var found = false;
    any(obj, function(value) {
      if (found = value === target) return true;
    });
    return found;
  };

  // Invoke a method (with arguments) on every item in a collection.
  _.invoke = function(obj, method) {
    var args = slice.call(arguments, 2);
    return _.map(obj, function(value) {
      return (method ? value[method] : value).apply(value, args);
    });
  };

  // Convenience version of a common use case of `map`: fetching a property.
  _.pluck = function(obj, key) {
    return _.map(obj, function(value){ return value[key]; });
  };

  // Return the maximum element or (element-based computation).
  _.max = function(obj, iterator, context) {
    if (!iterator && _.isArray(obj)) return Math.max.apply(Math, obj);
    var result = {computed : -Infinity};
    each(obj, function(value, index, list) {
      var computed = iterator ? iterator.call(context, value, index, list) : value;
      computed >= result.computed && (result = {value : value, computed : computed});
    });
    return result.value;
  };

  // Return the minimum element (or element-based computation).
  _.min = function(obj, iterator, context) {
    if (!iterator && _.isArray(obj)) return Math.min.apply(Math, obj);
    var result = {computed : Infinity};
    each(obj, function(value, index, list) {
      var computed = iterator ? iterator.call(context, value, index, list) : value;
      computed < result.computed && (result = {value : value, computed : computed});
    });
    return result.value;
  };

  // Sort the object's values by a criterion produced by an iterator.
  _.sortBy = function(obj, iterator, context) {
    return _.pluck(_.map(obj, function(value, index, list) {
      return {
        value : value,
        criteria : iterator.call(context, value, index, list)
      };
    }).sort(function(left, right) {
      var a = left.criteria, b = right.criteria;
      return a < b ? -1 : a > b ? 1 : 0;
    }), 'value');
  };

  // Use a comparator function to figure out at what index an object should
  // be inserted so as to maintain order. Uses binary search.
  _.sortedIndex = function(array, obj, iterator) {
    iterator = iterator || _.identity;
    var low = 0, high = array.length;
    while (low < high) {
      var mid = (low + high) >> 1;
      iterator(array[mid]) < iterator(obj) ? low = mid + 1 : high = mid;
    }
    return low;
  };

  // Safely convert anything iterable into a real, live array.
  _.toArray = function(iterable) {
    if (!iterable)                return [];
    if (iterable.toArray)         return iterable.toArray();
    if (_.isArray(iterable))      return iterable;
    if (_.isArguments(iterable))  return slice.call(iterable);
    return _.values(iterable);
  };

  // Return the number of elements in an object.
  _.size = function(obj) {
    return _.toArray(obj).length;
  };

  // Array Functions
  // ---------------

  // Get the first element of an array. Passing **n** will return the first N
  // values in the array. Aliased as `head`. The **guard** check allows it to work
  // with `_.map`.
  _.first = _.head = function(array, n, guard) {
    return n && !guard ? slice.call(array, 0, n) : array[0];
  };

  // Returns everything but the first entry of the array. Aliased as `tail`.
  // Especially useful on the arguments object. Passing an **index** will return
  // the rest of the values in the array from that index onward. The **guard**
  // check allows it to work with `_.map`.
  _.rest = _.tail = function(array, index, guard) {
    return slice.call(array, _.isUndefined(index) || guard ? 1 : index);
  };

  // Get the last element of an array.
  _.last = function(array) {
    return array[array.length - 1];
  };

  // Trim out all falsy values from an array.
  _.compact = function(array) {
    return _.filter(array, function(value){ return !!value; });
  };

  // Return a completely flattened version of an array.
  _.flatten = function(array) {
    return _.reduce(array, function(memo, value) {
      if (_.isArray(value)) return memo.concat(_.flatten(value));
      memo[memo.length] = value;
      return memo;
    }, []);
  };

  // Return a version of the array that does not contain the specified value(s).
  _.without = function(array) {
    var values = slice.call(arguments, 1);
    return _.filter(array, function(value){ return !_.include(values, value); });
  };

  // Produce a duplicate-free version of the array. If the array has already
  // been sorted, you have the option of using a faster algorithm.
  // Aliased as `unique`.
  _.uniq = _.unique = function(array, isSorted) {
    return _.reduce(array, function(memo, el, i) {
      if (0 == i || (isSorted === true ? _.last(memo) != el : !_.include(memo, el))) memo[memo.length] = el;
      return memo;
    }, []);
  };

  // Produce an array that contains every item shared between all the
  // passed-in arrays.
  _.intersect = function(array) {
    var rest = slice.call(arguments, 1);
    return _.filter(_.uniq(array), function(item) {
      return _.every(rest, function(other) {
        return _.indexOf(other, item) >= 0;
      });
    });
  };

  // Zip together multiple lists into a single array -- elements that share
  // an index go together.
  _.zip = function() {
    var args = slice.call(arguments);
    var length = _.max(_.pluck(args, 'length'));
    var results = new Array(length);
    for (var i = 0; i < length; i++) results[i] = _.pluck(args, "" + i);
    return results;
  };

  // If the browser doesn't supply us with indexOf (I'm looking at you, **MSIE**),
  // we need this function. Return the position of the first occurrence of an
  // item in an array, or -1 if the item is not included in the array.
  // Delegates to **ECMAScript 5**'s native `indexOf` if available.
  _.indexOf = function(array, item) {
    if (nativeIndexOf && array.indexOf === nativeIndexOf) return array.indexOf(item);
    for (var i = 0, l = array.length; i < l; i++) if (array[i] === item) return i;
    return -1;
  };


  // Delegates to **ECMAScript 5**'s native `lastIndexOf` if available.
  _.lastIndexOf = function(array, item) {
    if (nativeLastIndexOf && array.lastIndexOf === nativeLastIndexOf) return array.lastIndexOf(item);
    var i = array.length;
    while (i--) if (array[i] === item) return i;
    return -1;
  };

  // Generate an integer Array containing an arithmetic progression. A port of
  // the native Python `range()` function. See
  // [the Python documentation](http://docs.python.org/library/functions.html#range).
  _.range = function(start, stop, step) {
    var args  = slice.call(arguments),
        solo  = args.length <= 1,
        start = solo ? 0 : args[0],
        stop  = solo ? args[0] : args[1],
        step  = args[2] || 1,
        len   = Math.max(Math.ceil((stop - start) / step), 0),
        idx   = 0,
        range = new Array(len);
    while (idx < len) {
      range[idx++] = start;
      start += step;
    }
    return range;
  };

  // Function (ahem) Functions
  // ------------------

  // Create a function bound to a given object (assigning `this`, and arguments,
  // optionally). Binding with arguments is also known as `curry`.
  _.bind = function(func, obj) {
    var args = slice.call(arguments, 2);
    return function() {
      return func.apply(obj || {}, args.concat(slice.call(arguments)));
    };
  };

  // Bind all of an object's methods to that object. Useful for ensuring that
  // all callbacks defined on an object belong to it.
  _.bindAll = function(obj) {
    var funcs = slice.call(arguments, 1);
    if (funcs.length == 0) funcs = _.functions(obj);
    each(funcs, function(f) { obj[f] = _.bind(obj[f], obj); });
    return obj;
  };

  // Memoize an expensive function by storing its results.
  _.memoize = function(func, hasher) {
    var memo = {};
    hasher = hasher || _.identity;
    return function() {
      var key = hasher.apply(this, arguments);
      return key in memo ? memo[key] : (memo[key] = func.apply(this, arguments));
    };
  };

  // Defers a function, scheduling it to run after the current call stack has
  // cleared. If we're running on Node.js, this is implemented with `nextTick`,
  // otherwise, use a `setTimeout`.
  _.defer = function(func) {
    if (typeof process !== 'undefined' && process.nextTick) {
      process.nextTick(func);
    } else {
      setTimeout(func, 0);
    }
  };

  // Internal function used to implement `_.throttle` and `_.debounce`.
  var limit = function(func, wait, debounce) {
    var timeout;
    return function() {
      var context = this, args = arguments;
      var throttler = function() {
        timeout = null;
        func.apply(context, args);
      };
      if (debounce) clearTimeout(timeout);
      if (debounce || !timeout) timeout = setTimeout(throttler, wait);
    };
  };

  // Returns a function, that, when invoked, will only be triggered at most once
  // during a given window of time.
  _.throttle = function(func, wait) {
    return limit(func, wait, false);
  };

  // Returns a function, that, as long as it continues to be invoked, will not
  // be triggered. The function will be called after it stops being called for
  // N milliseconds.
  _.debounce = function(func, wait) {
    return limit(func, wait, true);
  };

  // Returns the first function passed as an argument to the second,
  // allowing you to adjust arguments, run code before and after, and
  // conditionally execute the original function.
  _.wrap = function(func, wrapper) {
    return function() {
      var args = [func].concat(slice.call(arguments));
      return wrapper.apply(wrapper, args);
    };
  };

  // Returns a function that is the composition of a list of functions, each
  // consuming the return value of the function that follows.
  _.compose = function() {
    var funcs = slice.call(arguments);
    return function() {
      var args = slice.call(arguments);
      for (var i=funcs.length-1; i >= 0; i--) {
        args = [funcs[i].apply(this, args)];
      }
      return args[0];
    };
  };

  // Object Functions
  // ----------------

  // Retrieve the names of an object's properties.
  // Delegates to **ECMAScript 5**'s native `Object.keys`
  _.keys = nativeKeys || function(obj) {
    if (_.isArray(obj)) return _.range(0, obj.length);
    var keys = [];
    for (var key in obj) if (hasOwnProperty.call(obj, key)) keys[keys.length] = key;
    return keys;
  };

  // Retrieve the values of an object's properties.
  _.values = function(obj) {
    return _.map(obj, _.identity);
  };

  // Return a sorted list of the function names available on the object.
  // Aliased as `methods`
  _.functions = _.methods = function(obj) {
    return _.filter(_.keys(obj), function(key){ return _.isFunction(obj[key]); }).sort();
  };

  // Extend a given object with all the properties in passed-in object(s).
  _.extend = function(obj) {
    each(slice.call(arguments, 1), function(source) {
      for (var prop in source) obj[prop] = source[prop];
    });
    return obj;
  };

  // Create a (shallow-cloned) duplicate of an object.
  _.clone = function(obj) {
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  };

  // Invokes interceptor with the obj, and then returns obj.
  // The primary purpose of this method is to "tap into" a method chain, in
  // order to perform operations on intermediate results within the chain.
  _.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
  };

  // Perform a deep comparison to check if two objects are equal.
  _.isEqual = function(a, b) {
    // Check object identity.
    if (a === b) return true;
    // Different types?
    var atype = typeof(a), btype = typeof(b);
    if (atype != btype) return false;
    // Basic equality test (watch out for coercions).
    if (a == b) return true;
    // One is falsy and the other truthy.
    if ((!a && b) || (a && !b)) return false;
    // Unwrap any wrapped objects.
    if (a._chain) a = a.value();
    if (b._chain) b = b.value();
    // One of them implements an isEqual()?
    if (a.isEqual) return a.isEqual(b);
    // Check dates' integer values.
    if (_.isDate(a) && _.isDate(b)) return a.getTime() === b.getTime();
    // Both are NaN?
    if (_.isNaN(a) && _.isNaN(b)) return false;
    // Compare regular expressions.
    if (_.isRegExp(a) && _.isRegExp(b))
      return a.source     === b.source &&
             a.global     === b.global &&
             a.ignoreCase === b.ignoreCase &&
             a.multiline  === b.multiline;
    // If a is not an object by this point, we can't handle it.
    if (atype !== 'object') return false;
    // Check for different array lengths before comparing contents.
    if (a.length && (a.length !== b.length)) return false;
    // Nothing else worked, deep compare the contents.
    var aKeys = _.keys(a), bKeys = _.keys(b);
    // Different object sizes?
    if (aKeys.length != bKeys.length) return false;
    // Recursive comparison of contents.
    for (var key in a) if (!(key in b) || !_.isEqual(a[key], b[key])) return false;
    return true;
  };

  // Is a given array or object empty?
  _.isEmpty = function(obj) {
    if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
    for (var key in obj) if (hasOwnProperty.call(obj, key)) return false;
    return true;
  };

  // Is a given value a DOM element?
  _.isElement = function(obj) {
    return !!(obj && obj.nodeType == 1);
  };

  // Is a given value an array?
  // Delegates to ECMA5's native Array.isArray
  _.isArray = nativeIsArray || function(obj) {
    return !!(obj && obj.concat && obj.unshift && !obj.callee);
  };

  // Is a given variable an arguments object?
  _.isArguments = function(obj) {
    return !!(obj && obj.callee);
  };

  // Is a given value a function?
  _.isFunction = function(obj) {
    return !!(obj && obj.constructor && obj.call && obj.apply);
  };

  // Is a given value a string?
  _.isString = function(obj) {
    return !!(obj === '' || (obj && obj.charCodeAt && obj.substr));
  };

  // Is a given value a number?
  _.isNumber = function(obj) {
    return !!(obj === 0 || (obj && obj.toExponential && obj.toFixed));
  };

  // Is the given value NaN -- this one is interesting. NaN != NaN, and
  // isNaN(undefined) == true, so we make sure it's a number first.
  _.isNaN = function(obj) {
    return toString.call(obj) === '[object Number]' && isNaN(obj);
  };

  // Is a given value a boolean?
  _.isBoolean = function(obj) {
    return obj === true || obj === false;
  };

  // Is a given value a date?
  _.isDate = function(obj) {
    return !!(obj && obj.getTimezoneOffset && obj.setUTCFullYear);
  };

  // Is the given value a regular expression?
  _.isRegExp = function(obj) {
    return !!(obj && obj.test && obj.exec && (obj.ignoreCase || obj.ignoreCase === false));
  };

  // Is a given value equal to null?
  _.isNull = function(obj) {
    return obj === null;
  };

  // Is a given variable undefined?
  _.isUndefined = function(obj) {
    return obj === void 0;
  };

  // Utility Functions
  // -----------------

  // Run Underscore.js in *noConflict* mode, returning the `_` variable to its
  // previous owner. Returns a reference to the Underscore object.
  _.noConflict = function() {
    root._ = previousUnderscore;
    return this;
  };

  // Keep the identity function around for default iterators.
  _.identity = function(value) {
    return value;
  };

  // Run a function **n** times.
  _.times = function (n, iterator, context) {
    for (var i = 0; i < n; i++) iterator.call(context, i);
  };

  // Add your own custom functions to the Underscore object, ensuring that
  // they're correctly added to the OOP wrapper as well.
  _.mixin = function(obj) {
    each(_.functions(obj), function(name){
      addToWrapper(name, _[name] = obj[name]);
    });
  };

  // Generate a unique integer id (unique within the entire client session).
  // Useful for temporary DOM ids.
  var idCounter = 0;
  _.uniqueId = function(prefix) {
    var id = idCounter++;
    return prefix ? prefix + id : id;
  };

  // By default, Underscore uses ERB-style template delimiters, change the
  // following template settings to use alternative delimiters.
  _.templateSettings = {
    evaluate    : /<%([\s\S]+?)%>/g,
    interpolate : /<%=([\s\S]+?)%>/g
  };

  // JavaScript micro-templating, similar to John Resig's implementation.
  // Underscore templating handles arbitrary delimiters, preserves whitespace,
  // and correctly escapes quotes within interpolated code.
  _.template = function(str, data) {
    var c  = _.templateSettings;
    var tmpl = 'var __p=[],print=function(){__p.push.apply(__p,arguments);};' +
      'with(obj||{}){__p.push(\'' +
      str.replace(/\\/g, '\\\\')
         .replace(/'/g, "\\'")
         .replace(c.interpolate, function(match, code) {
           return "'," + code.replace(/\\'/g, "'") + ",'";
         })
         .replace(c.evaluate || null, function(match, code) {
           return "');" + code.replace(/\\'/g, "'")
                              .replace(/[\r\n\t]/g, ' ') + "__p.push('";
         })
         .replace(/\r/g, '\\r')
         .replace(/\n/g, '\\n')
         .replace(/\t/g, '\\t')
         + "');}return __p.join('');";
    var func = function (data) {
        data = data || {};
        data._ = _;
        return new Function('obj', tmpl)(data);
    };
    return data ? func(data) : func;
  };

  // Async Functions
  // ---------------

  // Asynchronous version of `each`.
  _.asyncEach = _.asyncForEach = 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 += 1;
          if (completed === arr.length) {
            callback();
          }
        }
      });
    });
  };

  _.asyncEachSeries = _.asyncForEachSeries = 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 += 1;
          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, [_.asyncForEach].concat(args));
    };
  };
  var doSeries = function (fn) {
    return function () {
      var args = Array.prototype.slice.call(arguments);
      return fn.apply(null, [_.asyncForEachSeries].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);
    });
  };
  _.asyncMap = doParallel(_asyncMap);
  _.asyncMapSeries = doSeries(_asyncMap);


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


  _.asyncReduceRight = function (arr, memo, iterator, callback) {
    var reversed = _.map(arr, function (x) {
      return x;
    }).reverse();
    _.asyncReduce(reversed, memo, iterator, callback);
  };
  // foldr alias
  _.asyncFoldr = _.asyncReduceRight;


  var _asyncFilter = 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;
      }));
    });
  };
  _.asyncFilter = doParallel(_asyncFilter);
  _.asyncFilterSeries = doSeries(_asyncFilter);
  // select alias
  _.asyncSelect = _.asyncFilter;
  _.asyncSelectSeries = _.asyncFilterSeries;


  var _asyncReject = 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;
      }));
    });
  };
  _.asyncReject = doParallel(_asyncReject);
  _.asyncRejectSeries = doSeries(_asyncReject);


  var _asyncDetect = 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();
    });
  };
  _.asyncDetect = doParallel(_asyncDetect);
  _.asyncDetectSeries = doSeries(_asyncDetect);


  _.asyncSome = function (arr, iterator, main_callback) {
    _.asyncForEach(arr, function (x, callback) {
      iterator(x, function (v) {
        if (v) {
          main_callback(true);
          main_callback = function () {};
        }
        callback();
      });
    }, function (err) {
      main_callback(false);
    });
  };
  // any alias
  _.asyncAny = _.asyncSome;


  _.asyncEvery = function (arr, iterator, main_callback) {
    _.asyncForEach(arr, function (x, callback) {
      iterator(x, function (v) {
        if (!v) {
          main_callback(false);
          main_callback = function () {};
        }
        callback();
      });
    }, function (err) {
      main_callback(true);
    });
  };
  // all alias
  _.asyncAll = _.asyncEvery;


  _.asyncSortBy = function (arr, iterator, callback) {
    _.asyncMap(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 {
        var fn = function (left, right) {
          var a = left.criteria, b = right.criteria;
          return a < b ? -1 : a > b ? 1 : 0;
        };
        callback(null, _.map(results.sort(fn), function (x) {
          return x.value;
        }));
      }
    });
  };


  _.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);
  };


  // The OOP Wrapper
  // ---------------

  // If Underscore is called as a function, it returns a wrapped object that
  // can be used OO-style. This wrapper holds altered versions of all the
  // underscore functions. Wrapped objects may be chained.
  var wrapper = function(obj) { this._wrapped = obj; };

  // Expose `wrapper.prototype` as `_.prototype`
  _.prototype = wrapper.prototype;

  // Helper function to continue chaining intermediate results.
  var result = function(obj, chain) {
    return chain ? _(obj).chain() : obj;
  };

  // A method to easily add functions to the OOP wrapper.
  var addToWrapper = function(name, func) {
    wrapper.prototype[name] = function() {
      var args = slice.call(arguments);
      unshift.call(args, this._wrapped);
      return result(func.apply(_, args), this._chain);
    };
  };

  _.parallel = function (tasks, callback) {
    callback = callback || function () {};
    if (tasks.constructor === Array) {
      _.asyncMap(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);
    }
    else {
      var results = {};
      _.asyncForEach(_.keys(tasks), function (k, callback) {
        tasks[k](function (err) {
          var args = Array.prototype.slice.call(arguments, 1);
          if (args.length <= 1) {
            args = args[0];
          }
          results[k] = args;
          callback(err);
        });
      }, function (err) {
        callback(err, results);
      });
    }
  };


  _.series = function (tasks, callback) {
    callback = callback || function () {};
    if (tasks.constructor === Array) {
      _.asyncMapSeries(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);
    }
    else {
      var results = {};
      _.asyncForEachSeries(_.keys(tasks), function (k, callback) {
        tasks[k](function (err) {
          var args = Array.prototype.slice.call(arguments, 1);
          if (args.length <= 1) {
            args = args[0];
          }
          results[k] = args;
          callback(err);
        });
      }, function (err) {
        callback(err, results);
      });
    }
  };


  _.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);
          }
          _.defer(function () {
            iterator.apply(null, args);
          });
        }
      };
    };
    wrapIterator(_.iterator(tasks))();
  };


  var _asyncConcat = function (eachfn, arr, fn, callback) {
    var r = [];
    eachfn(arr, function (x, cb) {
      fn(x, function (err, y) {
        r = r.concat(y || []);
        cb(err);
      });
    }, function (err) {
      callback(err, r);
    });
  };
  _.asyncConcat = doParallel(_asyncConcat);
  _.asyncConcatSeries = doSeries(_asyncConcat);


  _.asyncQueue = function (worker, concurrency) {
    var workers = 0;
    var tasks = [];
    var q = {
      concurrency: concurrency,
      push: function (data, callback) {
        tasks.push({data: data, callback: callback});
        _.defer(q.process);
      },
      process: function () {
        if (workers < q.concurrency && tasks.length) {
          var task = tasks.splice(0, 1)[0];
          workers += 1;
          worker(task.data, function () {
            workers -= 1;
            if (task.callback) {
              task.callback.apply(task, arguments);
            }
            q.process();
          });
        }
      },
      length: function () {
        return tasks.length;
      }
    };
    return q;
  };


  _.asyncMemoize = function (fn, hasher) {
    var memo = {};
    hasher = hasher || _.identity;
    return function () {
      var args = Array.prototype.slice.call(arguments);
      var callback = args.pop();
      var key = hasher.apply(null, args);
      if (key in memo) {
        callback.apply(null, memo[key]);
      }
      else {
        fn.apply(null, args.concat([function () {
          memo[key] = arguments;
          callback.apply(null, arguments);
        }]));
      }
    };
  };

  _.asyncWhile = function (test, iterator, callback) {
    if (test()) {
      iterator(function (err) {
        if (err) {
          return callback(err);
        }
        _.asyncWhile(test, iterator, callback);
      });
    }
    else {
      callback();
    }
  };


  // Add all of the Underscore functions to the wrapper object.
  _.mixin(_);

  // Add all mutator Array functions to the wrapper.
  each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
    var method = ArrayProto[name];
    wrapper.prototype[name] = function() {
      method.apply(this._wrapped, arguments);
      return result(this._wrapped, this._chain);
    };
  });

  // Add all accessor Array functions to the wrapper.
  each(['concat', 'join', 'slice'], function(name) {
    var method = ArrayProto[name];
    wrapper.prototype[name] = function() {
      return result(method.apply(this._wrapped, arguments), this._chain);
    };
  });

  // Start chaining a wrapped Underscore object.
  wrapper.prototype.chain = function() {
    this._chain = true;
    return this;
  };

  // Extracts the result from a wrapped and chained object.
  wrapper.prototype.value = function() {
    return this._wrapped;
  };

})();