Retwis ported to modern Redis data types.

The example (and related documentation at redis.io) was not updated for
5 years. Redis had no sorted sets and hashes when the original code was
written (!). An update was really needed.

We also use a modern PHP client now: Predis. A copy is shipped within
this repository to make life easier to newcomers trying Redis for the
first time via this example.

Predis/Cluster/CommandHashStrategyInterface.php

@@ -0,0 +1,42 @@
+<?php
+
+/*
+ * This file is part of the Predis package.
+ *
+ * (c) Daniele Alessandri <suppakilla@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Predis\Cluster;
+
+use Predis\Command\CommandInterface;
+
+/**
+ * Interface for classes defining the strategy used to calculate an hash
+ * out of keys extracted from supported commands.
+ *
+ * This is mostly useful to support clustering via client-side sharding.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+interface CommandHashStrategyInterface
+{
+    /**
+     * Returns the hash for the given command using the specified algorithm, or null
+     * if the command cannot be hashed.
+     *
+     * @param  CommandInterface $command Command to be hashed.
+     * @return int
+     */
+    public function getHash(CommandInterface $command);
+
+    /**
+     * Returns the hash for the given key using the specified algorithm.
+     *
+     * @param  string $key Key to be hashed.
+     * @return string
+     */
+    public function getKeyHash($key);
+}