Implements SizeCalculator interface in FTP adapter

See #417

spec/Gaufrette/Adapter/FtpSpec.php

@@ -213,4 +213,16 @@ function it_fetches_keys_for_windows()
 
         $this->keys()->shouldReturn(array('archive', 'file1.zip', 'file2.zip'));
     }
+
+    function it_supports_sizecalculator()
+    {
+        $this->shouldImplement('Gaufrette\Adapter\SizeCalculator');
+
+        $this->size('/path')->shouldReturn(5000);
+    }
+
+    function it_throws_an_exception_when_size_cant_be_fetched()
+    {
+        $this->shouldThrow('RuntimeException')->during('size', ['/erroneous']);
+    }
 }

spec/Gaufrette/Adapter/functions.php

@@ -284,3 +284,12 @@ function apc_exists($path)
 
     return true;
 }
+
+function ftp_size($connection, $path)
+{
+    if ($path === '/erroneous') {
+        return -1;
+    }
+
+    return 5000;
+}

src/Gaufrette/Adapter/Ftp.php

@@ -13,7 +13,8 @@
  */
 class Ftp implements Adapter,
                      FileFactory,
-                     ListKeysAware
+                     ListKeysAware,
+                     SizeCalculator
 {
     protected $connection = null;
     protected $directory;
@@ -292,6 +293,24 @@ public function createFile($key, Filesystem $filesystem)
         return $file;
     }
 
+    /**
+     * @param string $key
+     *
+     * @return int
+     *
+     * @throws \RuntimeException
+     */
+    public function size($key)
+    {
+        $this->ensureDirectoryExists($this->directory, $this->create);
+
+        if (-1 === $size = ftp_size($this->connection, $key)) {
+            throw new \RuntimeException(sprintf('Unable to fetch the size of "%s".', $key));
+        }
+
+        return $size;
+    }
+
     /**
      * Ensures the specified directory exists. If it does not, and the create
      * parameter is set to TRUE, it tries to create it.