Extract language from fenced code blocks (#47)

Author: DongyuZhao

Sources/SwiftParser/Markdown/Builders/MarkdownFencedCodeBuilder.swift

@@ -10,7 +10,8 @@ public class MarkdownFencedCodeBuilder: CodeNodeBuilder {
               isStartOfLine(context) else { return false }
         context.consuming += 1
         let code = trimFence(token.text)
-        let node = CodeBlockNode(source: code, language: nil)
+        let language = extractLanguage(token.text)
+        let node = CodeBlockNode(source: code, language: language)
         context.current.append(node)
         if context.consuming < context.tokens.count,
            let nl = context.tokens[context.consuming] as? MarkdownToken,
@@ -30,6 +31,18 @@ public class MarkdownFencedCodeBuilder: CodeNodeBuilder {
         return lines.joined(separator: "\n")
     }
 
+    private func extractLanguage(_ text: String) -> String? {
+        guard let firstLine = text.split(separator: "\n", maxSplits: 1).first else {
+            return nil
+        }
+        var cleaned = firstLine.trimmingCharacters(in: .whitespaces)
+        while cleaned.starts(with: "`") {
+            cleaned.removeFirst()
+        }
+        let lang = cleaned.trimmingCharacters(in: .whitespaces)
+        return lang.isEmpty ? nil : lang
+    }
+
     private func isStartOfLine(_ context: CodeContext<MarkdownNodeElement, MarkdownTokenElement>) -> Bool {
         if context.consuming == 0 { return true }
         if let prev = context.tokens[context.consuming - 1] as? MarkdownToken {

Tests/SwiftParserTests/Markdown/Consumer/MarkdownBlockElementTests.swift

@@ -17,7 +17,11 @@ final class MarkdownBlockElementTests: XCTestCase {
         let (node, context) = parser.parse(input, root: root)
         XCTAssertTrue(context.errors.isEmpty)
         XCTAssertEqual(node.children.count, 1)
-        XCTAssertTrue(node.children.first is CodeBlockNode)
+        if let code = node.children.first as? CodeBlockNode {
+            XCTAssertEqual(code.language, "swift")
+        } else {
+            XCTFail("Expected CodeBlockNode")
+        }
     }
 
     func testHorizontalRule() {