Solve performance issues with very large chunks

Very large chunks need to be copied every time there's a partial write, which is pretty problematic. Instead of doing an almost full copy of the full chunk every time, this patch splits very large chunks into multiple smaller chunks automatically.

Fixes #41.

lib/ResourceOutputStream.php

@@ -14,6 +14,7 @@ use Amp\Success;
 final class ResourceOutputStream implements OutputStream
 {
     const MAX_CONSECUTIVE_EMPTY_WRITES = 3;
+    const LARGE_CHUNK_SIZE = 128 * 1024;
 
     /** @var resource */
     private $resource;
@@ -203,6 +204,16 @@ final class ResourceOutputStream implements OutputStream
         }
 
         $deferred = new Deferred;
+
+        if ($length - $written > self::LARGE_CHUNK_SIZE) {
+            $chunks = \str_split($data, self::LARGE_CHUNK_SIZE);
+            $data = \array_pop($chunks);
+            foreach ($chunks as $chunk) {
+                $this->writes->push([$chunk, $written, new Deferred]);
+                $written += self::LARGE_CHUNK_SIZE;
+            }
+        }
+
         $this->writes->push([$data, $written, $deferred]);
         Loop::enable($this->watcher);
         $promise = $deferred->promise();