import identity from 'lodash/identity';
import rest from 'lodash/_baseRest';

import setImmediate from './internal/setImmediate';
import initialParams from './internal/initialParams';

function has(obj, key) {
    return key in obj;
}

/**
 * Caches the results of an `async` function. When creating a hash to store
 * function results against, the callback is omitted from the hash and an
 * optional hash function can be used.
 *
 * If no hash function is specified, the first argument is used as a hash key,
 * which may work reasonably if it is a string or a data type that converts to a
 * distinct string. Note that objects and arrays will not behave reasonably.
 * Neither will cases where the other arguments are significant. In such cases,
 * specify your own hash function.
 *
 * The cache of results is exposed as the `memo` property of the function
 * returned by `memoize`.
 *
 * @name memoize
 * @static
 * @memberOf module:Utils
 * @method
 * @category Util
 * @param {Function} fn - The function to proxy and cache results from.
 * @param {Function} hasher - An optional function for generating a custom hash
 * for storing results. It has all the arguments applied to it apart from the
 * callback, and must be synchronous.
 * @returns {Function} a memoized version of `fn`
 * @example
 *
 * var slow_fn = function(name, callback) {
 *     // do something
 *     callback(null, result);
 * };
 * var fn = async.memoize(slow_fn);
 *
 * // fn can now be used as if it were slow_fn
 * fn('some name', function() {
 *     // callback
 * });
 */
export default function memoize(fn, hasher) {
    var memo = Object.create(null);
    var queues = Object.create(null);
    hasher = hasher || identity;
    var memoized = initialParams(function memoized(args, callback) {
        var key = hasher.apply(null, args);
        if (has(memo, key)) {
            setImmediate(function() {
                callback.apply(null, memo[key]);
            });
        } else if (has(queues, key)) {
            queues[key].push(callback);
        } else {
            queues[key] = [callback];
            fn.apply(null, args.concat([rest(function(args) {
                memo[key] = args;
                var q = queues[key];
                delete queues[key];
                for (var i = 0, l = q.length; i < l; i++) {
                    q[i].apply(null, args);
                }
            })]));
        }
    });
    memoized.memo = memo;
    memoized.unmemoized = fn;
    return memoized;
}