underscore.js

//     Underscore.js 1.5.2
//     http://underscorejs.org
//     (c) 2009-2013 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
//     Underscore may be freely distributed under the MIT license.

(function() {

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

    // Establish the root object, `window` in the browser, or `exports` 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, FuncProto = Function.prototype;

    // Create quick reference variables for speed access to core prototypes.
    var
        push             = ArrayProto.push,
        slice            = ArrayProto.slice,
        concat           = ArrayProto.concat,
        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,
        nativeBind         = FuncProto.bind;

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

    // Export the Underscore object for **Node.js**, with
    // backwards-compatibility for the old `require()` API. If we're in
    // the browser, add `_` as a global object via a string identifier,
    // for Closure Compiler "advanced" mode.
    if (typeof exports !== 'undefined') {
        if (typeof module !== 'undefined' && module.exports) {
            exports = module.exports = _;
        }
        exports._ = _;
    } else {
        root._ = _;
    }

    // Current version.
    _.VERSION = '1.5.2';

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

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

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

    var reduceError = 'Reduce of empty array with no initial value';

    // **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 = arguments.length > 2;
        if (obj == null) obj = [];
        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) {
                memo = value;
                initial = true;
            } else {
                memo = iterator.call(context, memo, value, index, list);
            }
        });
        if (!initial) throw new TypeError(reduceError);
        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) {
        var initial = arguments.length > 2;
        if (obj == null) obj = [];
        if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
            if (context) iterator = _.bind(iterator, context);
            return initial ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
        }
        var length = obj.length;
        if (length !== +length) {
            var keys = _.keys(obj);
            length = keys.length;
        }
        each(obj, function(value, index, list) {
            index = keys ? keys[--length] : --length;
            if (!initial) {
                memo = obj[index];
                initial = true;
            } else {
                memo = iterator.call(context, memo, obj[index], index, list);
            }
        });
        if (!initial) throw new TypeError(reduceError);
        return memo;
    };

    // 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) {
        var results = [];
        if (obj == null) return results;
        if (nativeFilter && obj.filter === nativeFilter) return obj.filter(iterator, context);
        each(obj, function(value, index, list) {
            if (iterator.call(context, value, index, list)) results.push(value);
        });
        return results;
    };

    // Return all the elements for which a truth test fails.
    _.reject = function(obj, iterator, context) {
        return _.filter(obj, function(value, index, list) {
            return !iterator.call(context, value, index, list);
        }, context);
    };

    // 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);
        var result = true;
        if (obj == null) return result;
        if (nativeEvery && obj.every === nativeEvery) return obj.every(iterator, context);
        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);
        var result = false;
        if (obj == null) return result;
        if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
        each(obj, function(value, index, list) {
            if (result || (result = iterator.call(context, value, index, list))) return breaker;
        });
        return !!result;
    };

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

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

    // Convenience version of a common use case of `filter`: selecting only objects
    // containing specific `key:value` pairs.
    _.where = function(obj, attrs, first) {
        if (_.isEmpty(attrs)) return first ? void 0 : [];
        return _[first ? 'find' : 'filter'](obj, function(value) {
            for (var key in attrs) {
                if (attrs[key] !== value[key]) return false;
            }
            return true;
        });
    };

    // Convenience version of a common use case of `find`: getting the first object
    // containing specific `key:value` pairs.
    _.findWhere = function(obj, attrs) {
        return _.where(obj, attrs, true);
    };

    // Return the maximum element or (element-based computation).
    // Can't optimize arrays of integers longer than 65,535 elements.
    // See [WebKit Bug 80797](https://bugs.webkit.org/show_bug.cgi?id=80797)
    _.max = function(obj, iterator, context) {
        if (!iterator && _.isArray(obj) && obj[0] === +obj[0] && obj.length < 65535) {
            return Math.max.apply(Math, obj);
        }
        if (!iterator && _.isEmpty(obj)) return -Infinity;
        var result = {computed : -Infinity, value: -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) && obj[0] === +obj[0] && obj.length < 65535) {
            return Math.min.apply(Math, obj);
        }
        if (!iterator && _.isEmpty(obj)) return Infinity;
        var result = {computed : Infinity, value: 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;
    };

    // Shuffle an array, using the modern version of the
    // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle).
    _.shuffle = function(obj) {
        var rand;
        var index = 0;
        var shuffled = [];
        each(obj, function(value) {
            rand = _.random(index++);
            shuffled[index - 1] = shuffled[rand];
            shuffled[rand] = value;
        });
        return shuffled;
    };

    // Sample **n** random values from an array.
    // If **n** is not specified, returns a single random element from the array.
    // The internal `guard` argument allows it to work with `map`.
    _.sample = function(obj, n, guard) {
        if (arguments.length < 2 || guard) {
            return obj[_.random(obj.length - 1)];
        }
        return _.shuffle(obj).slice(0, Math.max(0, n));
    };

    // An internal function to generate lookup iterators.
    var lookupIterator = function(value) {
        return _.isFunction(value) ? value : function(obj){ return obj[value]; };
    };

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

    // An internal function used for aggregate "group by" operations.
    var group = function(behavior) {
        return function(obj, value, context) {
            var result = {};
            var iterator = value == null ? _.identity : lookupIterator(value);
            each(obj, function(value, index) {
                var key = iterator.call(context, value, index, obj);
                behavior(result, key, value);
            });
            return result;
        };
    };

    // Groups the object's values by a criterion. Pass either a string attribute
    // to group by, or a function that returns the criterion.
    _.groupBy = group(function(result, key, value) {
        (_.has(result, key) ? result[key] : (result[key] = [])).push(value);
    });

    // Indexes the object's values by a criterion, similar to `groupBy`, but for
    // when you know that your index values will be unique.
    _.indexBy = group(function(result, key, value) {
        result[key] = value;
    });

    // Counts instances of an object that group by a certain criterion. Pass
    // either a string attribute to count by, or a function that returns the
    // criterion.
    _.countBy = group(function(result, key) {
        _.has(result, key) ? result[key]++ : result[key] = 1;
    });

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

    // Safely create a real, live array from anything iterable.
    _.toArray = function(obj) {
        if (!obj) return [];
        if (_.isArray(obj)) return slice.call(obj);
        if (obj.length === +obj.length) return _.map(obj, _.identity);
        return _.values(obj);
    };

    // Return the number of elements in an object.
    _.size = function(obj) {
        if (obj == null) return 0;
        return (obj.length === +obj.length) ? obj.length : _.keys(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` and `take`. The **guard** check
    // allows it to work with `_.map`.
    _.first = _.head = _.take = function(array, n, guard) {
        if (array == null) return void 0;
        return (n == null) || guard ? array[0] : slice.call(array, 0, n);
    };

    // Returns everything but the last entry of the array. Especially useful on
    // the arguments object. Passing **n** will return all the values in
    // the array, excluding the last N. The **guard** check allows it to work with
    // `_.map`.
    _.initial = function(array, n, guard) {
        return slice.call(array, 0, array.length - ((n == null) || guard ? 1 : n));
    };

    // Get the last element of an array. Passing **n** will return the last N
    // values in the array. The **guard** check allows it to work with `_.map`.
    _.last = function(array, n, guard) {
        if (array == null) return void 0;
        if ((n == null) || guard) {
            return array[array.length - 1];
        } else {
            return slice.call(array, Math.max(array.length - n, 0));
        }
    };

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

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

    // Internal implementation of a recursive `flatten` function.
    var flatten = function(input, shallow, output) {
        if (shallow && _.every(input, _.isArray)) {
            return concat.apply(output, input);
        }
        each(input, function(value) {
            if (_.isArray(value) || _.isArguments(value)) {
                shallow ? push.apply(output, value) : flatten(value, shallow, output);
            } else {
                output.push(value);
            }
        });
        return output;
    };

    // Flatten out an array, either recursively (by default), or just one level.
    _.flatten = function(array, shallow) {
        return flatten(array, shallow, []);
    };

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

    // 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, iterator, context) {
        if (_.isFunction(isSorted)) {
            context = iterator;
            iterator = isSorted;
            isSorted = false;
        }
        var initial = iterator ? _.map(array, iterator, context) : array;
        var results = [];
        var seen = [];
        each(initial, function(value, index) {
            if (isSorted ? (!index || seen[seen.length - 1] !== value) : !_.contains(seen, value)) {
                seen.push(value);
                results.push(array[index]);
            }
        });
        return results;
    };

    // Produce an array that contains the union: each distinct element from all of
    // the passed-in arrays.
    _.union = function() {
        return _.uniq(_.flatten(arguments, true));
    };

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

    // Take the difference between one array and a number of other arrays.
    // Only the elements present in just the first array will remain.
    _.difference = function(array) {
        var rest = concat.apply(ArrayProto, slice.call(arguments, 1));
        return _.filter(array, function(value){ return !_.contains(rest, value); });
    };

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

    // Converts lists into objects. Pass either a single array of `[key, value]`
    // pairs, or two parallel arrays of the same length -- one of keys, and one of
    // the corresponding values.
    _.object = function(list, values) {
        if (list == null) return {};
        var result = {};
        for (var i = 0, length = list.length; i < length; i++) {
            if (values) {
                result[list[i]] = values[i];
            } else {
                result[list[i][0]] = list[i][1];
            }
        }
        return result;
    };

    // 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.
    // If the array is large and already in sort order, pass `true`
    // for **isSorted** to use binary search.
    _.indexOf = function(array, item, isSorted) {
        if (array == null) return -1;
        var i = 0, length = array.length;
        if (isSorted) {
            if (typeof isSorted == 'number') {
                i = (isSorted < 0 ? Math.max(0, length + isSorted) : isSorted);
            } else {
                i = _.sortedIndex(array, item);
                return array[i] === item ? i : -1;
            }
        }
        if (nativeIndexOf && array.indexOf === nativeIndexOf) return array.indexOf(item, isSorted);
        for (; i < length; i++) if (array[i] === item) return i;
        return -1;
    };

    // Delegates to **ECMAScript 5**'s native `lastIndexOf` if available.
    _.lastIndexOf = function(array, item, from) {
        if (array == null) return -1;
        var hasIndex = from != null;
        if (nativeLastIndexOf && array.lastIndexOf === nativeLastIndexOf) {
            return hasIndex ? array.lastIndexOf(item, from) : array.lastIndexOf(item);
        }
        var i = (hasIndex ? from : 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) {
        if (arguments.length <= 1) {
            stop = start || 0;
            start = 0;
        }
        step = arguments[2] || 1;

        var length = Math.max(Math.ceil((stop - start) / step), 0);
        var idx = 0;
        var range = new Array(length);

        while(idx < length) {
            range[idx++] = start;
            start += step;
        }

        return range;
    };

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

    // Reusable constructor function for prototype setting.
    var ctor = function(){};

    // Create a function bound to a given object (assigning `this`, and arguments,
    // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
    // available.
    _.bind = function(func, context) {
        var args, bound;
        if (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
        if (!_.isFunction(func)) throw new TypeError;
        args = slice.call(arguments, 2);
        return bound = function() {
            if (!(this instanceof bound)) return func.apply(context, args.concat(slice.call(arguments)));
            ctor.prototype = func.prototype;
            var self = new ctor;
            ctor.prototype = null;
            var result = func.apply(self, args.concat(slice.call(arguments)));
            if (Object(result) === result) return result;
            return self;
        };
    };

    // Partially apply a function by creating a version that has had some of its
    // arguments pre-filled, without changing its dynamic `this` context.
    _.partial = function(func) {
        var args = slice.call(arguments, 1);
        return function() {
            return func.apply(this, 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) throw new Error("bindAll must be passed function names");
        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 _.has(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
        };
    };

    // Delays a function for the given number of milliseconds, and then calls
    // it with the arguments supplied.
    _.delay = function(func, wait) {
        var args = slice.call(arguments, 2);
        return setTimeout(function(){ return func.apply(null, args); }, wait);
    };

    // Defers a function, scheduling it to run after the current call stack has
    // cleared.
    _.defer = function(func) {
        return _.delay.apply(_, [func, 1].concat(slice.call(arguments, 1)));
    };

    // Returns a function, that, when invoked, will only be triggered at most once
    // during a given window of time. Normally, the throttled function will run
    // as much as it can, without ever going more than once per `wait` duration;
    // but if you'd like to disable the execution on the leading edge, pass
    // `{leading: false}`. To disable execution on the trailing edge, ditto.
    _.throttle = function(func, wait, options) {
        var context, args, result;
        var timeout = null;
        var previous = 0;
        options || (options = {});
        var later = function() {
            previous = options.leading === false ? 0 : new Date;
            timeout = null;
            result = func.apply(context, args);
        };
        return function() {
            var now = new Date;
            if (!previous && options.leading === false) previous = now;
            var remaining = wait - (now - previous);
            context = this;
            args = arguments;
            if (remaining <= 0) {
                clearTimeout(timeout);
                timeout = null;
                previous = now;
                result = func.apply(context, args);
            } else if (!timeout && options.trailing !== false) {
                timeout = setTimeout(later, remaining);
            }
            return result;
        };
    };

    // 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. If `immediate` is passed, trigger the function on the
    // leading edge, instead of the trailing.
    _.debounce = function(func, wait, immediate) {
        var timeout, args, context, timestamp, result;
        return function() {
            context = this;
            args = arguments;
            timestamp = new Date();
            var later = function() {
                var last = (new Date()) - timestamp;
                if (last < wait) {
                    timeout = setTimeout(later, wait - last);
                } else {
                    timeout = null;
                    if (!immediate) result = func.apply(context, args);
                }
            };
            var callNow = immediate && !timeout;
            if (!timeout) {
                timeout = setTimeout(later, wait);
            }
            if (callNow) result = func.apply(context, args);
            return result;
        };
    };

    // Returns a function that will be executed at most one time, no matter how
    // often you call it. Useful for lazy initialization.
    _.once = function(func) {
        var ran = false, memo;
        return function() {
            if (ran) return memo;
            ran = true;
            memo = func.apply(this, arguments);
            func = null;
            return memo;
        };
    };

    // 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];
            push.apply(args, arguments);
            return wrapper.apply(this, 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 = arguments;
        return function() {
            var args = arguments;
            for (var i = funcs.length - 1; i >= 0; i--) {
                args = [funcs[i].apply(this, args)];
            }
            return args[0];
        };
    };

    // Returns a function that will only be executed after being called N times.
    _.after = function(times, func) {
        return function() {
            if (--times < 1) {
                return func.apply(this, arguments);
            }
        };
    };

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

    // Retrieve the names of an object's properties.
    // Delegates to **ECMAScript 5**'s native `Object.keys`
    _.keys = nativeKeys || function(obj) {
        if (obj !== Object(obj)) throw new TypeError('Invalid object');
        var keys = [];
        for (var key in obj) if (_.has(obj, key)) keys.push(key);
        return keys;
    };

    // Retrieve the values of an object's properties.
    _.values = function(obj) {
        var keys = _.keys(obj);
        var length = keys.length;
        var values = new Array(length);
        for (var i = 0; i < length; i++) {
            values[i] = obj[keys[i]];
        }
        return values;
    };

    // Convert an object into a list of `[key, value]` pairs.
    _.pairs = function(obj) {
        var keys = _.keys(obj);
        var length = keys.length;
        var pairs = new Array(length);
        for (var i = 0; i < length; i++) {
            pairs[i] = [keys[i], obj[keys[i]]];
        }
        return pairs;
    };

    // Invert the keys and values of an object. The values must be serializable.
    _.invert = function(obj) {
        var result = {};
        var keys = _.keys(obj);
        for (var i = 0, length = keys.length; i < length; i++) {
            result[obj[keys[i]]] = keys[i];
        }
        return result;
    };

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

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

    // Return a copy of the object only containing the whitelisted properties.
    _.pick = function(obj) {
        var copy = {};
        var keys = concat.apply(ArrayProto, slice.call(arguments, 1));
        each(keys, function(key) {
            if (key in obj) copy[key] = obj[key];
        });
        return copy;
    };

    // Return a copy of the object without the blacklisted properties.
    _.omit = function(obj) {
        var copy = {};
        var keys = concat.apply(ArrayProto, slice.call(arguments, 1));
        for (var key in obj) {
            if (!_.contains(keys, key)) copy[key] = obj[key];
        }
        return copy;
    };

    // Fill in a given object with default properties.
    _.defaults = function(obj) {
        each(slice.call(arguments, 1), function(source) {
            if (source) {
                for (var prop in source) {
                    if (obj[prop] === void 0) obj[prop] = source[prop];
                }
            }
        });
        return obj;
    };

    // Create a (shallow-cloned) duplicate of an object.
    _.clone = function(obj) {
        if (!_.isObject(obj)) return 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;
    };

    // Internal recursive comparison function for `isEqual`.
    var eq = function(a, b, aStack, bStack) {
        // Identical objects are equal. `0 === -0`, but they aren't identical.
        // See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
        if (a === b) return a !== 0 || 1 / a == 1 / b;
        // A strict comparison is necessary because `null == undefined`.
        if (a == null || b == null) return a === b;
        // Unwrap any wrapped objects.
        if (a instanceof _) a = a._wrapped;
        if (b instanceof _) b = b._wrapped;
        // Compare `[[Class]]` names.
        var className = toString.call(a);
        if (className != toString.call(b)) return false;
        switch (className) {
            // Strings, numbers, dates, and booleans are compared by value.
            case '[object String]':
                // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
                // equivalent to `new String("5")`.
                return a == String(b);
            case '[object Number]':
                // `NaN`s are equivalent, but non-reflexive. An `egal` comparison is performed for
                // other numeric values.
                return a != +a ? b != +b : (a == 0 ? 1 / a == 1 / b : a == +b);
            case '[object Date]':
            case '[object Boolean]':
                // Coerce dates and booleans to numeric primitive values. Dates are compared by their
                // millisecond representations. Note that invalid dates with millisecond representations
                // of `NaN` are not equivalent.
                return +a == +b;
            // RegExps are compared by their source patterns and flags.
            case '[object RegExp]':
                return a.source == b.source &&
                    a.global == b.global &&
                    a.multiline == b.multiline &&
                    a.ignoreCase == b.ignoreCase;
        }
        if (typeof a != 'object' || typeof b != 'object') return false;
        // Assume equality for cyclic structures. The algorithm for detecting cyclic
        // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
        var length = aStack.length;
        while (length--) {
            // Linear search. Performance is inversely proportional to the number of
            // unique nested structures.
            if (aStack[length] == a) return bStack[length] == b;
        }
        // Objects with different constructors are not equivalent, but `Object`s
        // from different frames are.
        var aCtor = a.constructor, bCtor = b.constructor;
        if (aCtor !== bCtor && !(_.isFunction(aCtor) && (aCtor instanceof aCtor) &&
            _.isFunction(bCtor) && (bCtor instanceof bCtor))) {
            return false;
        }
        // Add the first object to the stack of traversed objects.
        aStack.push(a);
        bStack.push(b);
        var size = 0, result = true;
        // Recursively compare objects and arrays.
        if (className == '[object Array]') {
            // Compare array lengths to determine if a deep comparison is necessary.
            size = a.length;
            result = size == b.length;
            if (result) {
                // Deep compare the contents, ignoring non-numeric properties.
                while (size--) {
                    if (!(result = eq(a[size], b[size], aStack, bStack))) break;
                }
            }
        } else {
            // Deep compare objects.
            for (var key in a) {
                if (_.has(a, key)) {
                    // Count the expected number of properties.
                    size++;
                    // Deep compare each member.
                    if (!(result = _.has(b, key) && eq(a[key], b[key], aStack, bStack))) break;
                }
            }
            // Ensure that both objects contain the same number of properties.
            if (result) {
                for (key in b) {
                    if (_.has(b, key) && !(size--)) break;
                }
                result = !size;
            }
        }
        // Remove the first object from the stack of traversed objects.
        aStack.pop();
        bStack.pop();
        return result;
    };

    // Perform a deep comparison to check if two objects are equal.
    _.isEqual = function(a, b) {
        return eq(a, b, [], []);
    };

    // Is a given array, string, or object empty?
    // An "empty" object has no enumerable own-properties.
    _.isEmpty = function(obj) {
        if (obj == null) return true;
        if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
        for (var key in obj) if (_.has(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 toString.call(obj) == '[object Array]';
    };

    // Is a given variable an object?
    _.isObject = function(obj) {
        return obj === Object(obj);
    };

    // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp.
    each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp'], function(name) {
        _['is' + name] = function(obj) {
            return toString.call(obj) == '[object ' + name + ']';
        };
    });

    // Define a fallback version of the method in browsers (ahem, IE), where
    // there isn't any inspectable "Arguments" type.
    if (!_.isArguments(arguments)) {
        _.isArguments = function(obj) {
            return !!(obj && _.has(obj, 'callee'));
        };
    }

    // Optimize `isFunction` if appropriate.
    if (typeof (/./) !== 'function') {
        _.isFunction = function(obj) {
            return typeof obj === 'function';
        };
    }

    // Is a given object a finite number?
    _.isFinite = function(obj) {
        return isFinite(obj) && !isNaN(parseFloat(obj));
    };

    // Is the given value `NaN`? (NaN is the only number which does not equal itself).
    _.isNaN = function(obj) {
        return _.isNumber(obj) && obj != +obj;
    };

    // Is a given value a boolean?
    _.isBoolean = function(obj) {
        return obj === true || obj === false || toString.call(obj) == '[object Boolean]';
    };

    // 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;
    };

    // Shortcut function for checking if an object has a given property directly
    // on itself (in other words, not on a prototype).
    _.has = function(obj, key) {
        return hasOwnProperty.call(obj, key);
    };

    // 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) {
        var accum = Array(Math.max(0, n));
        for (var i = 0; i < n; i++) accum[i] = iterator.call(context, i);
        return accum;
    };

    // Return a random integer between min and max (inclusive).
    _.random = function(min, max) {
        if (max == null) {
            max = min;
            min = 0;
        }
        return min + Math.floor(Math.random() * (max - min + 1));
    };

    // List of HTML entities for escaping.
    var entityMap = {
        escape: {
            '&': '&amp;',
            '<': '&lt;',
            '>': '&gt;',
            '"': '&quot;',
            "'": '&#x27;'
        }
    };
    entityMap.unescape = _.invert(entityMap.escape);

    // Regexes containing the keys and values listed immediately above.
    var entityRegexes = {
        escape:   new RegExp('[' + _.keys(entityMap.escape).join('') + ']', 'g'),
        unescape: new RegExp('(' + _.keys(entityMap.unescape).join('|') + ')', 'g')
    };

    // Functions for escaping and unescaping strings to/from HTML interpolation.
    _.each(['escape', 'unescape'], function(method) {
        _[method] = function(string) {
            if (string == null) return '';
            return ('' + string).replace(entityRegexes[method], function(match) {
                return entityMap[method][match];
            });
        };
    });

    // If the value of the named `property` is a function then invoke it with the
    // `object` as context; otherwise, return it.
    _.result = function(object, property) {
        if (object == null) return void 0;
        var value = object[property];
        return _.isFunction(value) ? value.call(object) : value;
    };

    // Add your own custom functions to the Underscore object.
    _.mixin = function(obj) {
        each(_.functions(obj), function(name) {
            var func = _[name] = obj[name];
            _.prototype[name] = function() {
                var args = [this._wrapped];
                push.apply(args, arguments);
                return result.call(this, func.apply(_, args));
            };
        });
    };

    // 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,
        escape      : /<%-([\s\S]+?)%>/g
    };

    // When customizing `templateSettings`, if you don't want to define an
    // interpolation, evaluation or escaping regex, we need one that is
    // guaranteed not to match.
    var noMatch = /(.)^/;

    // Certain characters need to be escaped so that they can be put into a
    // string literal.
    var escapes = {
        "'":      "'",
        '\\':     '\\',
        '\r':     'r',
        '\n':     'n',
        '\t':     't',
        '\u2028': 'u2028',
        '\u2029': 'u2029'
    };

    var escaper = /\\|'|\r|\n|\t|\u2028|\u2029/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(text, data, settings) {
        var render;
        settings = _.defaults({}, settings, _.templateSettings);

        // Combine delimiters into one regular expression via alternation.
        var matcher = new RegExp([
            (settings.escape || noMatch).source,
            (settings.interpolate || noMatch).source,
            (settings.evaluate || noMatch).source
        ].join('|') + '|$', 'g');

        // Compile the template source, escaping string literals appropriately.
        var index = 0;
        var source = "__p+='";
        text.replace(matcher, function(match, escape, interpolate, evaluate, offset) {
            source += text.slice(index, offset)
                .replace(escaper, function(match) { return '\\' + escapes[match]; });

            if (escape) {
                source += "'+\n((__t=(" + escape + "))==null?'':_.escape(__t))+\n'";
            }
            if (interpolate) {
                source += "'+\n((__t=(" + interpolate + "))==null?'':__t)+\n'";
            }
            if (evaluate) {
                source += "';\n" + evaluate + "\n__p+='";
            }
            index = offset + match.length;
            return match;
        });
        source += "';\n";

        // If a variable is not specified, place data values in local scope.
        if (!settings.variable) source = 'with(obj||{}){\n' + source + '}\n';

        source = "var __t,__p='',__j=Array.prototype.join," +
            "print=function(){__p+=__j.call(arguments,'');};\n" +
            source + "return __p;\n";

        try {
            render = new Function(settings.variable || 'obj', '_', source);
        } catch (e) {
            e.source = source;
            throw e;
        }

        if (data) return render(data, _);
        var template = function(data) {
            return render.call(this, data, _);
        };

        // Provide the compiled function source as a convenience for precompilation.
        template.source = 'function(' + (settings.variable || 'obj') + '){\n' + source + '}';

        return template;
    };

    // Add a "chain" function, which will delegate to the wrapper.
    _.chain = function(obj) {
        return _(obj).chain();
    };

    // OOP
    // ---------------
    // 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.

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

    // 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];
        _.prototype[name] = function() {
            var obj = this._wrapped;
            method.apply(obj, arguments);
            if ((name == 'shift' || name == 'splice') && obj.length === 0) delete obj[0];
            return result.call(this, obj);
        };
    });

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

    _.extend(_.prototype, {

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

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

    });

}).call(this);