fix: apply GFM flanking rules to strikethrough delimiters

src/Rules/StrikethroughRule.php

@@ -27,13 +27,55 @@ public function parse(Parser $parser): Token
     {
         $opening = $parser->consumeWhile('~');
 
-        if (strpos($parser->content, '~', $parser->position) === false) {
+        $closing = $this->closingPosition($parser, strlen($opening));
+
+        if ($closing === null) {
             return new TextToken($opening);
         }
 
-        $buffer = $parser->consumeUntil('~');
-        $parser->consumeWhile('~');
+        $buffer = $parser->consume($closing - $parser->position);
+
+        $parser->consume(strlen($opening));
 
         return new StrikethroughToken($buffer);
     }
+
+    /**
+     * The position of the run that closes $length tildes, or null when there
+     * is none. A run of more than two tildes never delimits strikethrough, an
+     * opening run may not be followed by whitespace, and a closing run may not
+     * be preceded by it.
+     *
+     * @see https://github.github.com/gfm/#strikethrough-extension-
+     */
+    private function closingPosition(Parser $parser, int $length): ?int
+    {
+        if ($length > 2) {
+            return null;
+        }
+
+        $next = $parser->current;
+
+        if ($next === null || str_contains(Parser::WHITESPACE, $next)) {
+            return null;
+        }
+
+        $content = $parser->content;
+        $position = $parser->position;
+
+        while (($position = strpos($content, '~', $position)) !== false) {
+            $run = strspn($content, '~', $position);
+
+            if (
+                $run === $length
+                && ! str_contains(Parser::WHITESPACE, $content[$position - 1])
+            ) {
+                return $position;
+            }
+
+            $position += $run;
+        }
+
+        return null;
+    }
 }

tests/Rules/StrikethroughRuleTest.php

@@ -2,6 +2,7 @@
 
 namespace Tempest\Markdown\Tests\Rules;
 
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\Attributes\Test;
 use Tempest\Markdown\Markdown;
 use Tempest\Markdown\Parser;
@@ -57,6 +58,51 @@ public function unmatched_tildes_remain_literal(): void
         );
     }
 
+    #[Test]
+    #[DataProvider('provideFlanking')]
+    public function delimiters_follow_the_flanking_rules(
+        string $markdown,
+        string $expected,
+    ): void {
+        $this->assertSame(
+            $expected,
+            new Markdown(highlighter: null)->parse($markdown)->html,
+        );
+    }
+
+    public static function provideFlanking(): iterable
+    {
+        yield 'one tilde' => ['~struck~', '<p><s>struck</s></p>'];
+        yield 'two tildes' => ['~~struck~~', '<p><s>struck</s></p>'];
+        yield 'inside a word' => ['a~struck~b', '<p>a<s>struck</s>b</p>'];
+
+        // The opening run may not be followed by whitespace, and the closing
+        // run may not be preceded by it.
+        yield 'space after the opening run' => [
+            '~ struck ~',
+            '<p>~ struck ~</p>',
+        ];
+        yield 'space before the closing run' => [
+            '~11km / ~750m',
+            '<p>~11km / ~750m</p>',
+        ];
+        yield 'approximations in a list item' => [
+            "- Distance : ~120 km\n- Dénivelé : ~6 700 m D+ / ~7 300 m D-",
+            '<ul><li>Distance : ~120 km</li>'
+                . '<li>Dénivelé : ~6 700 m D+ / ~7 300 m D-</li></ul>',
+        ];
+
+        yield 'approximations in a table cell' => [
+            "| a |\n| - |\n| ~120 et ~700 |",
+            '<table><thead><tr><th>a</th></tr></thead>'
+                . '<tbody><tr><td>~120 et ~700</td></tr></tbody></table>',
+        ];
+
+        // Opening and closing runs have to be the same length.
+        yield 'one tilde closed by two' => ['~struck~~', '<p>~struck~~</p>'];
+        yield 'two tildes closed by one' => ['~~struck~', '<p>~~struck~</p>'];
+    }
+
     #[Test]
     public function test_unmatched_run_is_consumed_as_one_literal_token(): void
     {