Speed up JsonConverter again

src/JsonConverter.php

@@ -104,8 +104,8 @@ final class JsonConverter
     public readonly int $depth;
     /** @var int<1, max> */
     public readonly int $indentSize;
-    /** @var Closure(T, array-key): mixed */
-    public readonly Closure $formatter;
+    /** @var ?Closure(T, array-key): mixed */
+    public readonly ?Closure $formatter;
     /** @var int<1, max> */
     public readonly int $chunkSize;
     /** @var non-empty-string */
@@ -127,20 +127,20 @@ public static function create(): self
             flags: 0,
             depth: 512,
             indentSize: 4,
-            formatter: fn (mixed $value, int|string $offset) => $value,
+            formatter: null,
             chunkSize: 500
         );
     }
 
     /**
      * @param int<1, max> $depth
      * @param int<1, max> $indentSize
-     * @param Closure(T, array-key): mixed $formatter
+     * @param ?Closure(T, array-key): mixed $formatter
      * @param int<1, max> $chunkSize
      *
      * @throws InvalidArgumentException
      */
-    private function __construct(int $flags, int $depth, int $indentSize, Closure $formatter, int $chunkSize)
+    private function __construct(int $flags, int $depth, int $indentSize, ?Closure $formatter, int $chunkSize)
     {
         json_encode([], $flags & ~JSON_THROW_ON_ERROR, $depth);
 
@@ -347,8 +347,6 @@ public function chunkSize(int $chunkSize): self
      */
     public function formatter(?Closure $formatter): self
     {
-        $formatter ??= fn (mixed $value, int|string $offset) => $value;
-
         return new self($this->flags, $this->depth, $this->indentSize, $formatter, $this->chunkSize);
     }
 
@@ -444,32 +442,37 @@ public function save(iterable $records, mixed $destination, $context = null): in
      */
     public function convert(iterable $records): Iterator
     {
-        $iterator = MapIterator::fromIterable($records, $this->formatter);
+        $iterator = match ($this->formatter) {
+            null => MapIterator::toIterator($records),
+            default => MapIterator::fromIterable($records, $this->formatter)
+        };
+
         $iterator->rewind();
         if (!$iterator->valid()) {
             yield $this->emptyIterable;
 
             return;
         }
 
-        $incr = 0;
         $chunk = [];
+        $chunkOffset = 0;
         $offset = 0;
         $current = $iterator->current();
         $iterator->next();
 
         yield $this->start;
 
         while ($iterator->valid()) {
-            if ($incr === $this->chunkSize) {
+            if ($chunkOffset === $this->chunkSize) {
                 yield ($this->jsonEncodeChunk)($chunk).$this->separator;
 
-                $incr = 0;
+                $chunkOffset = 0;
                 $chunk = [];
             }
 
-            ++$incr;
-            $chunk[++$offset] = $current;
+            $chunk[$offset] = $current;
+            ++$chunkOffset;
+            ++$offset;
             $current = $iterator->current();
             $iterator->next();
         }