var hasOwn = Object.prototype.hasOwnProperty;
var toString = Object.prototype.toString;
var objKeys = require('amp-keys');
var isFunction = require('amp-is-function');


// 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;
    // Compare `[[Class]]` names.
    var className = toString.call(a);
    if (className !== toString.call(b)) return false;
    switch (className) {
        // Strings, numbers, regular expressions, dates, and booleans 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;
    }

    var areArrays = className === '[object Array]';
    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`.
    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);
    var size, result;
    // Recursively compare objects and arrays.
    if (areArrays) {
        // 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.
        var keys = objKeys(a), key;
        size = keys.length;
        // Ensure that both objects contain the same number of properties before comparing deep equality.
        result = objKeys(b).length === size;
        if (result) {
            while (size--) {
                // Deep compare each member
                key = keys[size];
                if (!(result = hasOwn.call(b, key) && eq(a[key], b[key], aStack, bStack))) break;
            }
        }
    }
    // Remove the first object from the stack of traversed objects.
    aStack.pop();
    bStack.pop();
    return result;
};


module.exports = function isEqual(a, b) {
    return eq(a, b, [], []);
};
