LibMarkdown+LibSyntax: Add a Markdown syntax highlighter

It currently supports only headers and code blocks.

Userland/Libraries/LibMarkdown/SyntaxHighlighter.cpp

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) 2023, Maciej <sppmacd@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibMarkdown/SyntaxHighlighter.h>
+
+namespace Markdown {
+
+Syntax::Language SyntaxHighlighter::language() const
+{
+    return Syntax::Language::Markdown;
+}
+
+Optional<StringView> SyntaxHighlighter::comment_prefix() const
+{
+    return {};
+}
+
+Optional<StringView> SyntaxHighlighter::comment_suffix() const
+{
+    return {};
+}
+
+enum class Token {
+    Default,
+    Header,
+    Code
+};
+
+void SyntaxHighlighter::rehighlight(Palette const& palette)
+{
+    auto text = m_client->get_text();
+
+    Vector<GUI::TextDocumentSpan> spans;
+
+    auto append_header = [&](GUI::TextRange const& range) {
+        Gfx::TextAttributes attributes;
+        attributes.color = palette.base_text();
+        attributes.bold = true;
+        GUI::TextDocumentSpan span {
+            .range = range,
+            .attributes = attributes,
+            .data = static_cast<u32>(Token::Header),
+            .is_skippable = false
+        };
+        spans.append(span);
+    };
+
+    auto append_code_block = [&](GUI::TextRange const& range) {
+        Gfx::TextAttributes attributes;
+        attributes.color = palette.syntax_string();
+        GUI::TextDocumentSpan span {
+            .range = range,
+            .attributes = attributes,
+            .data = static_cast<u32>(Token::Code),
+            .is_skippable = false
+        };
+        spans.append(span);
+    };
+
+    // Headers, code blocks
+    {
+        size_t line_index = 0;
+        Optional<size_t> code_block_start;
+        for (auto const& line : StringView(text).lines()) {
+            if (line.starts_with("```"sv)) {
+                if (code_block_start.has_value()) {
+                    append_code_block({ { *code_block_start, 0 }, { line_index, line.length() } });
+                    code_block_start = {};
+                } else {
+                    code_block_start = line_index;
+                }
+            }
+
+            if (!code_block_start.has_value()) {
+                auto trimmed = line.trim_whitespace(TrimMode::Left);
+                size_t indent = line.length() - trimmed.length();
+                if (indent < 4 && trimmed.starts_with("#"sv)) {
+                    append_header({ { line_index, 0 }, { line_index, line.length() } });
+                }
+            }
+            line_index++;
+        }
+    }
+
+    // TODO: Highlight text nodes (em, strong, link, image)
+
+    m_client->do_set_spans(spans);
+}
+
+Vector<SyntaxHighlighter::MatchingTokenPair> SyntaxHighlighter::matching_token_pairs_impl() const
+{
+    return {};
+}
+
+bool SyntaxHighlighter::token_types_equal(u64 lhs, u64 rhs) const
+{
+    return static_cast<Token>(lhs) == static_cast<Token>(rhs);
+}
+
+}