Micro-optimizations: cache active block parser, ASCII fast path in isLetter

src/Parser/MarkdownParser.php

@@ -53,6 +53,9 @@ final class MarkdownParser implements MarkdownParserInterface
     /** @psalm-readonly-allow-private-mutation */
     private Cursor $cursor;
 
+    /** @psalm-readonly-allow-private-mutation */
+    private BlockContinueParserInterface $activeBlockParser;
+
     /**
      * @var array<int, BlockContinueParserInterface>
      *
@@ -297,6 +300,7 @@ private function addChild(BlockContinueParserInterface $blockParser, ?int $start
     private function activateBlockParser(BlockContinueParserInterface $blockParser): void
     {
         $this->activeBlockParsers[] = $blockParser;
+        $this->activeBlockParser    = $blockParser;
     }
 
     /**
@@ -309,6 +313,11 @@ private function deactivateBlockParser(): BlockContinueParserInterface
             throw new ParserLogicException('The last block parser should not be deactivated');
         }
 
+        $last = \end($this->activeBlockParsers);
+        if ($last !== false) {
+            $this->activeBlockParser = $last;
+        }
+
         return $popped;
     }
 
@@ -346,11 +355,6 @@ private function updateReferenceMap(iterable $references): void
      */
     public function getActiveBlockParser(): BlockContinueParserInterface
     {
-        $active = \end($this->activeBlockParsers);
-        if ($active === false) {
-            throw new ParserLogicException('No active block parsers are available');
-        }
-
-        return $active;
+        return $this->activeBlockParser;
     }
 }

src/Util/RegexHelper.php

@@ -98,7 +98,12 @@ public static function isLetter(?string $character): bool
             return false;
         }
 
-        return \preg_match('/[\pL]/u', $character) === 1;
+        // Fast path for ASCII (the common case in Markdown); ctype_alpha is locale-independent for single bytes
+        if (\strlen($character) === 1) {
+            return \ctype_alpha($character);
+        }
+
+        return \preg_match('/^\pL/u', $character) === 1;
     }
 
     /**