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JavaScript's functional programming helper library.
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define(['./underscore', './_setup', './_getByteLength', './isTypedArray', './isFunction', './_stringTagBug', './isDataView', './keys', './_has', './_toBufferView'], function (underscore, _setup, _getByteLength, isTypedArray, isFunction, _stringTagBug, isDataView, keys, _has, _toBufferView) {

  // We use this string twice, so give it a name for minification.
  var tagDataView = '[object DataView]';

  // Internal recursive comparison function for `_.isEqual`.
  function eq(a, b, aStack, bStack) {
    // Identical objects are equal. `0 === -0`, but they aren't identical.
    // See the [Harmony `egal` proposal](https://wiki.ecmascript.org/doku.php?id=harmony:egal).
    if (a === b) return a !== 0 || 1 / a === 1 / b;
    // `null` or `undefined` only equal to itself (strict comparison).
    if (a == null || b == null) return false;
    // `NaN`s are equivalent, but non-reflexive.
    if (a !== a) return b !== b;
    // Exhaust primitive checks
    var type = typeof a;
    if (type !== 'function' && type !== 'object' && typeof b != 'object') return false;
    return deepEq(a, b, aStack, bStack);
  }

  // Internal recursive comparison function for `_.isEqual`.
  function deepEq(a, b, aStack, bStack) {
    // Unwrap any wrapped objects.
    if (a instanceof underscore) a = a._wrapped;
    if (b instanceof underscore) b = b._wrapped;
    // Compare `[[Class]]` names.
    var className = _setup.toString.call(a);
    if (className !== _setup.toString.call(b)) return false;
    // Work around a bug in IE 10 - Edge 13.
    if (_stringTagBug.hasDataViewBug && className == '[object Object]' && isDataView(a)) {
      if (!isDataView(b)) return false;
      className = tagDataView;
    }
    switch (className) {
      // These types are compared by value.
      case '[object RegExp]':
        // RegExps are coerced to strings for comparison (Note: '' + /a/i === '/a/i')
      case '[object String]':
        // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
        // equivalent to `new String("5")`.
        return '' + a === '' + b;
      case '[object Number]':
        // `NaN`s are equivalent, but non-reflexive.
        // Object(NaN) is equivalent to NaN.
        if (+a !== +a) return +b !== +b;
        // An `egal` comparison is performed for other numeric values.
        return +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;
      case '[object Symbol]':
        return _setup.SymbolProto.valueOf.call(a) === _setup.SymbolProto.valueOf.call(b);
      case '[object ArrayBuffer]':
      case tagDataView:
        // Coerce to typed array so we can fall through.
        return deepEq(_toBufferView(a), _toBufferView(b), aStack, bStack);
    }

    var areArrays = className === '[object Array]';
    if (!areArrays && isTypedArray(a)) {
        var byteLength = _getByteLength(a);
        if (byteLength !== _getByteLength(b)) return false;
        if (a.buffer === b.buffer && a.byteOffset === b.byteOffset) return true;
        areArrays = true;
    }
    if (!areArrays) {
      if (typeof a != 'object' || typeof b != 'object') return false;

      // Objects with different constructors are not equivalent, but `Object`s or `Array`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)
                          && ('constructor' in a && 'constructor' in b)) {
        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`.

    // Initializing stack of traversed objects.
    // It's done here since we only need them for objects and arrays comparison.
    aStack = aStack || [];
    bStack = bStack || [];
    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;
    }

    // Add the first object to the stack of traversed objects.
    aStack.push(a);
    bStack.push(b);

    // Recursively compare objects and arrays.
    if (areArrays) {
      // Compare array lengths to determine if a deep comparison is necessary.
      length = a.length;
      if (length !== b.length) return false;
      // Deep compare the contents, ignoring non-numeric properties.
      while (length--) {
        if (!eq(a[length], b[length], aStack, bStack)) return false;
      }
    } else {
      // Deep compare objects.
      var _keys = keys(a), key;
      length = _keys.length;
      // Ensure that both objects contain the same number of properties before comparing deep equality.
      if (keys(b).length !== length) return false;
      while (length--) {
        // Deep compare each member
        key = _keys[length];
        if (!(_has(b, key) && eq(a[key], b[key], aStack, bStack))) return false;
      }
    }
    // Remove the first object from the stack of traversed objects.
    aStack.pop();
    bStack.pop();
    return true;
  }

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

  return isEqual;

});