Add optional callable to drive extraction of node hash in distributor.

This is mainly in response to the longstanding issue #36 in which my
proposed solution was fine in terms of functionalities, but eventually
never made into the repository since it was far from being clean enough
for my taste.

Now developers can optionally pass a callable object when creating the
hashring instance to decide how the distributor should extract the hash
from a node (really a connection instance) to populate the ring:

  use Predis\Cluster\Distribution\HashRing;
  use Predis\Connection\PredisCluster;

  $servers = array(
    'tcp://10.0.0.1?alias=node01',
    'tcp://10.0.0.2?alias=node02',
  );

  $options = array(
    'nodehash' => function ($connection) {
      return $connection->getParameters()->alias;
    },
    'cluster' => function ($options) {
      $replicas = HashRing::DEFAULT_REPLICAS;
      $hashring = new HashRing($replicas, $options->nodehash);
      $cluster  = new PredisCluster($hashring);

      return $cluster;
    },
  );

  $client = new Predis\Client($servers, $options);

Both HashRing and KetamaPureRing in the Predis\Cluster\Distribution
namespace support this new approach.

lib/Predis/Cluster/Distribution/HashRing.php

@@ -26,19 +26,21 @@ class HashRing implements DistributionStrategyInterface, HashGeneratorInterface
     const DEFAULT_REPLICAS = 128;
     const DEFAULT_WEIGHT   = 100;
 
-    private $nodes;
     private $ring;
     private $ringKeys;
     private $ringKeysCount;
     private $replicas;
+    private $nodeHashCallback;
+    private $nodes = array();
 
     /**
      * @param int $replicas Number of replicas in the ring.
+     * @param mixed $nodeHashCallback Callback returning the string used to calculate the hash of a node.
      */
-    public function __construct($replicas = self::DEFAULT_REPLICAS)
+    public function __construct($replicas = self::DEFAULT_REPLICAS, $nodeHashCallback = null)
     {
         $this->replicas = $replicas;
-        $this->nodes = array();
+        $this->nodeHashCallback = $nodeHashCallback;
     }
 
     /**
@@ -164,7 +166,11 @@ class HashRing implements DistributionStrategyInterface, HashGeneratorInterface
      */
     protected function getNodeHash($nodeObject)
     {
-        return (string) $nodeObject;
+        if ($this->nodeHashCallback === null) {
+            return (string) $nodeObject;
+        }
+
+        return call_user_func($this->nodeHashCallback, $nodeObject);
     }
 
     /**

lib/Predis/Cluster/Distribution/KetamaPureRing.php

@@ -26,9 +26,9 @@ class KetamaPureRing extends HashRing
     /**
      *
      */
-    public function __construct()
+    public function __construct($nodeHashCallback = null)
     {
-        parent::__construct($this::DEFAULT_REPLICAS);
+        parent::__construct($this::DEFAULT_REPLICAS, $nodeHashCallback);
     }
 
     /**

tests/Predis/Cluster/Distribution/HashRingTest.php

@@ -129,4 +129,25 @@ class HashRingTest extends DistributionStrategyTestCase
         $this->assertSame($expected2, $actual2);
         $this->assertSame($expected3, $actual3);
     }
+
+    /**
+     * @todo This tests should be moved in Predis\Cluster\Distribution\DistributionStrategyTestCase
+     * @group disconnected
+     */
+    public function testCallbackToGetNodeHash()
+    {
+        $node = '127.0.0.1:7000';
+        $replicas = HashRing::DEFAULT_REPLICAS;
+        $callable = $this->getMock('stdClass', array('__invoke'));
+
+        $callable->expects($this->once())
+                 ->method('__invoke')
+                 ->with($node)
+                 ->will($this->returnValue($node));
+
+        $ring = new HashRing($replicas, $callable);
+        $ring->add($node);
+
+        $this->getNodes($ring);
+    }
 }

tests/Predis/Cluster/Distribution/KetamaPureRingTest.php

@@ -130,4 +130,24 @@ class KetamaPureRingTest extends DistributionStrategyTestCase
         $this->assertSame($expected2, $actual2);
         $this->assertSame($expected3, $actual3);
     }
+
+    /**
+     * @todo This tests should be moved in Predis\Cluster\Distribution\DistributionStrategyTestCase
+     * @group disconnected
+     */
+    public function testCallbackToGetNodeHash()
+    {
+        $node = '127.0.0.1:7000';
+        $callable = $this->getMock('stdClass', array('__invoke'));
+
+        $callable->expects($this->once())
+                 ->method('__invoke')
+                 ->with($node)
+                 ->will($this->returnValue($node));
+
+        $ring = new KetamaPureRing($callable);
+        $ring->add($node);
+
+        $this->getNodes($ring);
+    }
 }