Add Pipeline::buffer()

src/Internal/ConcurrentFlatMapIterator.php

@@ -24,9 +24,14 @@ final class ConcurrentFlatMapIterator implements ConcurrentIterator
      * @param ConcurrentIterator<T> $iterator
      * @param \Closure(T, int):iterable<R> $flatMap
      */
-    public function __construct(ConcurrentIterator $iterator, int $concurrency, bool $ordered, \Closure $flatMap)
-    {
-        $queue = new QueueState;
+    public function __construct(
+        ConcurrentIterator $iterator,
+        int $bufferSize,
+        int $concurrency,
+        bool $ordered,
+        \Closure $flatMap,
+    ) {
+        $queue = new QueueState($bufferSize);
         $this->iterator = new ConcurrentQueueIterator($queue);
         $order = $ordered ? new Sequence : null;
 

src/Internal/ConcurrentIterableIterator.php

@@ -20,7 +20,7 @@ final class ConcurrentIterableIterator implements ConcurrentIterator
     /**
      * @param iterable<T> $iterable
      */
-    public function __construct(iterable $iterable)
+    public function __construct(iterable $iterable, int $bufferSize = 0)
     {
         if (\is_array($iterable)) {
             $this->iterator = new ConcurrentArrayIterator($iterable);
@@ -36,7 +36,7 @@ public function __construct(iterable $iterable)
             $iterable = $iterable->getIterator();
         }
 
-        $queue = new QueueState();
+        $queue = new QueueState($bufferSize);
         $this->iterator = new ConcurrentQueueIterator($queue);
 
         async(static function () use ($queue, $iterable): void {

src/Internal/FlatMapOperation.php

@@ -23,6 +23,7 @@ public static function getStopMarker(): object
      * @param \Closure(T, int):iterable<R> $flatMap
      */
     public function __construct(
+        private readonly int $bufferSize,
         private readonly int $concurrency,
         private readonly bool $ordered,
         private readonly \Closure $flatMap
@@ -45,9 +46,15 @@ public function __invoke(ConcurrentIterator $source): ConcurrentIterator
                         yield $item;
                     }
                 }
-            })());
+            })(), $this->bufferSize);
         }
 
-        return new ConcurrentFlatMapIterator($source, $this->concurrency, $this->ordered, $this->flatMap);
+        return new ConcurrentFlatMapIterator(
+            $source,
+            $this->bufferSize,
+            $this->concurrency,
+            $this->ordered,
+            $this->flatMap,
+        );
     }
 }

src/Pipeline.php

@@ -9,6 +9,7 @@
 use Amp\Pipeline\Internal\ConcurrentIterableIterator;
 use Amp\Pipeline\Internal\ConcurrentMergedIterator;
 use Amp\Pipeline\Internal\FlatMapOperation;
+use Amp\Pipeline\Internal\IntermediateOperation;
 use Amp\Pipeline\Internal\Sequence;
 use Amp\Pipeline\Internal\SortOperation;
 use function Amp\delay;
@@ -123,10 +124,15 @@ private static function mapToConcurrentIterators(array $iterables): array
         return \array_map(static fn (iterable $pipeline) => self::fromIterable($pipeline)->getIterator(), $iterables);
     }
 
+    /** @var non-negative-int */
+    private int $bufferSize = 0;
+
+    /** @var positive-int */
     private int $concurrency = 1;
 
     private bool $ordered = true;
 
+    /** @var list<IntermediateOperation> */
     private array $intermediateOperations = [];
 
     private bool $used = false;
@@ -146,6 +152,17 @@ public function __destruct()
         }
     }
 
+    public function buffer(int $bufferSize): self
+    {
+        if ($bufferSize < 0) {
+            throw new \ValueError('Argument #1 ($bufferSize) must be non-negative, got ' . $bufferSize);
+        }
+
+        $this->bufferSize = $bufferSize;
+
+        return $this;
+    }
+
     public function concurrent(int $concurrency): self
     {
         if ($concurrency < 1) {
@@ -345,7 +362,12 @@ public function flatMap(\Closure $flatMap): self
             throw new \Error('Pipeline consumption has already been started');
         }
 
-        $this->intermediateOperations[] = new FlatMapOperation($this->concurrency, $this->ordered, $flatMap);
+        $this->intermediateOperations[] = new FlatMapOperation(
+            $this->bufferSize,
+            $this->concurrency,
+            $this->ordered,
+            $flatMap,
+        );
 
         /** @var self<R> */
         return $this;