fix: distribute shard tests round-robin to avoid PCRE backtrack limit

src/Plugins/Shard.php

@@ -113,8 +113,7 @@ public function handleArguments(array $arguments): array
             self::$timeBalanced = true;
             self::$shardsOutdated = $newTests !== [];
         } else {
-            $isInCurrentShard = fn (int $key): bool => $key % $total === ($index - 1);
-            $testsToRun = array_values(array_filter($tests, $isInCurrentShard, ARRAY_FILTER_USE_KEY));
+            $testsToRun = $this->distribute($tests, $index, $total);
         }
 
         self::$shard = [
@@ -444,6 +443,17 @@ private function loadShardsFile(): ?array
         return $data['timings'];
     }
 
+    /**
+     * @param  list<string>  $tests
+     * @return list<string>
+     */
+    private function distribute(array $tests, int $index, int $total): array
+    {
+        $isInCurrentShard = fn (int $key): bool => $key % $total === ($index - 1);
+
+        return array_values(array_filter($tests, $isInCurrentShard, ARRAY_FILTER_USE_KEY));
+    }
+
     /**
      * @param  list<string>  $tests
      * @param  array<string, float>  $timings

tests/Unit/Plugins/Shard.php

@@ -142,6 +142,44 @@
     });
 });
 
+describe('distribute', function (): void {
+    it('splits tests round-robin across shards', function (): void {
+        $output = new BufferedOutput;
+        $shard = new Shard($output);
+
+        $reflection = new ReflectionClass($shard);
+        $method = $reflection->getMethod('distribute');
+
+        $tests = ['A', 'B', 'C', 'D', 'E', 'F'];
+
+        expect($method->invoke($shard, $tests, 1, 3))->toBe(['A', 'D'])
+            ->and($method->invoke($shard, $tests, 2, 3))->toBe(['B', 'E'])
+            ->and($method->invoke($shard, $tests, 3, 3))->toBe(['C', 'F']);
+    });
+
+    it('returns an empty list when a shard gets no tests', function (): void {
+        $output = new BufferedOutput;
+        $shard = new Shard($output);
+
+        $reflection = new ReflectionClass($shard);
+        $method = $reflection->getMethod('distribute');
+
+        expect($method->invoke($shard, ['A', 'B'], 3, 4))->toBe([]);
+    });
+
+    it('puts every test in the single shard when total is 1', function (): void {
+        $output = new BufferedOutput;
+        $shard = new Shard($output);
+
+        $reflection = new ReflectionClass($shard);
+        $method = $reflection->getMethod('distribute');
+
+        $tests = ['A', 'B', 'C'];
+
+        expect($method->invoke($shard, $tests, 1, 1))->toBe($tests);
+    });
+});
+
 describe('ensureFilterLengthIsSafe', function (): void {
     it('accepts filter within length limit', function (): void {
         $output = new BufferedOutput;