/**
 * @author Don Griffin
 *
 * This class is a base for all id generators. It also provides lookup of id generators by
 * their id.
 * 
 * Generally, id generators are used to generate a primary key for new model instances. There
 * are different approaches to solving this problem, so this mechanism has both simple use
 * cases and is open to custom implementations. A {@link Ext.data.Model} requests id generation
 * using the {@link Ext.data.Model#idgen} property.
 *
 * # Identity, Type and Shared IdGenerators
 *
 * It is often desirable to share IdGenerators to ensure uniqueness or common configuration.
 * This is done by giving IdGenerator instances an id property by which they can be looked
 * up using the {@link #get} method. To configure two {@link Ext.data.Model Model} classes
 * to share one {@link Ext.data.SequentialIdGenerator sequential} id generator, you simply
 * assign them the same id:
 *
 *     Ext.define('MyApp.data.MyModelA', {
 *         extend: 'Ext.data.Model',
 *         idgen: {
 *             type: 'sequential',
 *             id: 'foo'
 *         }
 *     });
 *
 *     Ext.define('MyApp.data.MyModelB', {
 *         extend: 'Ext.data.Model',
 *         idgen: {
 *             type: 'sequential',
 *             id: 'foo'
 *         }
 *     });
 *
 * To make this as simple as possible for generator types that are shared by many (or all)
 * Models, the IdGenerator types (such as 'sequential' or 'uuid') are also reserved as
 * generator id's. This is used by the {@link Ext.data.UuidGenerator} which has an id equal
 * to its type ('uuid'). In other words, the following Models share the same generator:
 *
 *     Ext.define('MyApp.data.MyModelX', {
 *         extend: 'Ext.data.Model',
 *         idgen: 'uuid'
 *     });
 *
 *     Ext.define('MyApp.data.MyModelY', {
 *         extend: 'Ext.data.Model',
 *         idgen: 'uuid'
 *     });
 *
 * This can be overridden (by specifying the id explicitly), but there is no particularly
 * good reason to do so for this generator type.
 *
 * # Creating Custom Generators
 * 
 * An id generator should derive from this class and implement the {@link #generate} method.
 * The constructor will apply config properties on new instances, so a constructor is often
 * not necessary.
 *
 * To register an id generator type, a derived class should provide an `alias` like so:
 *
 *     Ext.define('MyApp.data.CustomIdGenerator', {
 *         extend: 'Ext.data.IdGenerator',
 *         alias: 'idgen.custom',
 *
 *         configProp: 42, // some config property w/default value
 *
 *         generate: function () {
 *             return ... // a new id
 *         }
 *     });
 *
 * Using the custom id generator is then straightforward:
 *
 *     Ext.define('MyApp.data.MyModel', {
 *         extend: 'Ext.data.Model',
 *         idgen: 'custom'
 *     });
 *     // or...
 *
 *     Ext.define('MyApp.data.MyModel', {
 *         extend: 'Ext.data.Model',
 *         idgen: {
 *             type: 'custom',
 *             configProp: value
 *         }
 *     });
 *
 * It is not recommended to mix shared generators with generator configuration. This leads
 * to unpredictable results unless all configurations match (which is also redundant). In
 * such cases, a custom generator with a default id is the best approach.
 *
 *     Ext.define('MyApp.data.CustomIdGenerator', {
 *         extend: 'Ext.data.SequentialIdGenerator',
 *         alias: 'idgen.custom',
 *
 *         id: 'custom', // shared by default
 *
 *         prefix: 'ID_',
 *         seed: 1000
 *     });
 *
 *     Ext.define('MyApp.data.MyModelX', {
 *         extend: 'Ext.data.Model',
 *         idgen: 'custom'
 *     });
 *
 *     Ext.define('MyApp.data.MyModelY', {
 *         extend: 'Ext.data.Model',
 *         idgen: 'custom'
 *     });
 *
 *     // the above models share a generator that produces ID_1000, ID_1001, etc..
 *
 */
Ext.define('Ext.data.IdGenerator', {

    /**
     * @property {Boolean} isGenerator
     * `true` in this class to identify an object as an instantiated IdGenerator, or subclass thereof.
     */
    isGenerator: true,

    /**
     * Initializes a new instance.
     * @param {Object} config (optional) Configuration object to be applied to the new instance.
     */
    constructor: function(config) {
        var me = this;

        Ext.apply(me, config);

        if (me.id) {
            Ext.data.IdGenerator.all[me.id] = me;
        }
    },

    /**
     * @cfg {String} id
     * The id by which to register a new instance. This instance can be found using the
     * {@link Ext.data.IdGenerator#get} static method.
     */

    getRecId: function (rec) {
        return rec.modelName + '-' + rec.internalId;
    },

    /**
     * Generates and returns the next id. This method must be implemented by the derived
     * class.
     *
     * @return {String} The next id.
     * @method generate
     * @abstract
     */

    statics: {
        /**
         * @property {Object} all
         * This object is keyed by id to lookup instances.
         * @private
         * @static
         */
        all: {},

        /**
         * Returns the IdGenerator given its config description.
         * @param {String/Object} config If this parameter is an IdGenerator instance, it is
         * simply returned. If this is a string, it is first used as an id for lookup and
         * then, if there is no match, as a type to create a new instance. This parameter
         * can also be a config object that contains a `type` property (among others) that
         * are used to create and configure the instance.
         * @static
         */
        get: function (config) {
            var generator,
                id,
                type;

            if (typeof config == 'string') {
                id = type = config;
                config = null;
            } else if (config.isGenerator) {
                return config;
            } else {
                id = config.id || config.type;
                type = config.type;
            }

            generator = this.all[id];
            if (!generator) {
                generator = Ext.create('idgen.' + type, config);
            }

            return generator;
        }
    }
});