cycle.js

// cycle.js
// 2011-08-24

/*jslint evil: true, regexp: true */

/*members $ref, apply, call, decycle, hasOwnProperty, length, prototype, push,
    retrocycle, stringify, test, toString
*/

if (typeof JSON.decycle !== 'function') {
    JSON.decycle = function decycle(object) {
        'use strict';

// Make a deep copy of an object or array, assuring that there is at most
// one instance of each object or array in the resulting structure. The
// duplicate references (which might be forming cycles) are replaced with
// an object of the form
//      {$ref: PATH}
// where the PATH is a JSONPath string that locates the first occurance.
// So,
//      var a = [];
//      a[0] = a;
//      return JSON.stringify(JSON.decycle(a));
// produces the string '[{"$ref":"$"}]'.

// JSONPath is used to locate the unique object. $ indicates the top level of
// the object or array. [NUMBER] or [STRING] indicates a child member or
// property.

        var objects = [],   // Keep a reference to each unique object or array
            paths = [];     // Keep the path to each unique object or array

        return (function derez(value, path) {

// The derez recurses through the object, producing the deep copy.

            var i,          // The loop counter
                name,       // Property name
                nu;         // The new object or array

            switch (typeof value) {
            case 'object':

// typeof null === 'object', so get out if this value is not really an object.

                if (!value) {
                    return null;
                }

// If the value is an object or array, look to see if we have already
// encountered it. If so, return a $ref/path object. This is a hard way,
// linear search that will get slower as the number of unique objects grows.

                for (i = 0; i < objects.length; i += 1) {
                    if (objects[i] === value) {
                        return {$ref: paths[i]};
                    }
                }

// Otherwise, accumulate the unique value and its path.

                objects.push(value);
                paths.push(path);

// If it is an array, replicate the array.

                if (Object.prototype.toString.apply(value) === '[object Array]') {
                    nu = [];
                    for (i = 0; i < value.length; i += 1) {
                        nu[i] = derez(value[i], path + '[' + i + ']');
                    }
                } else {

// If it is an object, replicate the object.

                    nu = {};
                    for (name in value) {
                        if (Object.prototype.hasOwnProperty.call(value, name)) {
                            nu[name] = derez(value[name],
                                path + '[' + JSON.stringify(name) + ']');
                        }
                    }
                }
                return nu;
            case 'number':
            case 'string':
            case 'boolean':
                return value;
            }
        }(object, '$'));
    };
}


if (typeof JSON.retrocycle !== 'function') {
    JSON.retrocycle = function retrocycle($) {
        'use strict';

// Restore an object that was reduced by decycle. Members whose values are
// objects of the form
//      {$ref: PATH}
// are replaced with references to the value found by the PATH. This will
// restore cycles. The object will be mutated.

// The eval function is used to locate the values described by a PATH. The
// root object is kept in a $ variable. A regular expression is used to
// assure that the PATH is extremely well formed. The regexp contains nested
// * quantifiers. That has been known to have extremely bad performance
// problems on some browsers for very long strings. A PATH is expected to be
// reasonably short. A PATH is allowed to belong to a very restricted subset of
// Goessner's JSONPath.

// So,
//      var s = '[{"$ref":"$"}]';
//      return JSON.retrocycle(JSON.parse(s));
// produces an array containing a single element which is the array itself.

        var px =
            /^\$(?:\[(?:\d+|\"(?:[^\\\"\u0000-\u001f]|\\([\\\"\/bfnrt]|u[0-9a-zA-Z]{4}))*\")\])*$/;

        (function rez(value) {

// The rez function walks recursively through the object looking for $ref
// properties. When it finds one that has a value that is a path, then it
// replaces the $ref object with a reference to the value that is found by
// the path.

            var i, item, name, path;

            if (value && typeof value === 'object') {
                if (Object.prototype.toString.apply(value) === '[object Array]') {
                    for (i = 0; i < value.length; i += 1) {
                        item = value[i];
                        if (item && typeof item === 'object') {
                            path = item.$ref;
                            if (typeof path === 'string' && px.test(path)) {
                                value[i] = eval(path);
                            } else {
                                rez(item);
                            }
                        }
                    }
                } else {
                    for (name in value) {
                        if (typeof value[name] === 'object') {
                            item = value[name];
                            if (item) {
                                path = item.$ref;
                                if (typeof path === 'string' && px.test(path)) {
                                    value[name] = eval(path);
                                } else {
                                    rez(item);
                                }
                            }
                        }
                    }
                }
            }
        }($));
        return $;
    };
}