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/Distribution/KetamaPureRing.php

@@ -0,0 +1,71 @@
+<?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\Distribution;
+
+/**
+ * This class implements an hashring-based distributor that uses the same
+ * algorithm of libketama to distribute keys in a cluster using client-side
+ * sharding.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ * @author Lorenzo Castelli <lcastelli@gmail.com>
+ */
+class KetamaPureRing extends HashRing
+{
+    const DEFAULT_REPLICAS = 160;
+
+    /**
+     * @param mixed $nodeHashCallback Callback returning the string used to calculate the hash of a node.
+     */
+    public function __construct($nodeHashCallback = null)
+    {
+        parent::__construct($this::DEFAULT_REPLICAS, $nodeHashCallback);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function addNodeToRing(&$ring, $node, $totalNodes, $replicas, $weightRatio)
+    {
+        $nodeObject = $node['object'];
+        $nodeHash = $this->getNodeHash($nodeObject);
+        $replicas = (int) floor($weightRatio * $totalNodes * ($replicas / 4));
+
+        for ($i = 0; $i < $replicas; $i++) {
+            $unpackedDigest = unpack('V4', md5("$nodeHash-$i", true));
+
+            foreach ($unpackedDigest as $key) {
+                $ring[$key] = $nodeObject;
+            }
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function hash($value)
+    {
+        $hash = unpack('V', md5($value, true));
+
+        return $hash[1];
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function wrapAroundStrategy($upper, $lower, $ringKeysCount)
+    {
+        // Binary search for the first item in _ringkeys with a value greater
+        // or equal to the key. If no such item exists, return the first item.
+        return $lower < $ringKeysCount ? $lower : 0;
+    }
+}