LibMarkdown: Add support for indented code blocks

Userland/Libraries/LibMarkdown/CodeBlock.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2019-2020, Sergey Bugaev <bugaevc@serenityos.org>
+ * Copyright (c) 2022, Peter Elliott <pelliott@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -76,16 +77,52 @@ RecursionDecision CodeBlock::walk(Visitor& visitor) const
 
 static Regex<ECMA262> style_spec_re("\\s*([\\*_]*)\\s*([^\\*_\\s]*).*");
 
+static constexpr auto tick_tick_tick = "```";
+
+static Optional<int> line_block_prefix(StringView const& line)
+{
+    int characters = 0;
+    int indents = 0;
+
+    for (char ch : line) {
+        if (indents == 4)
+            break;
+
+        if (ch == ' ') {
+            ++characters;
+            ++indents;
+        } else if (ch == '\t') {
+            ++characters;
+            indents = 4;
+        } else {
+            break;
+        }
+    }
+
+    if (indents == 4)
+        return characters;
+
+    return {};
+}
+
 OwnPtr<CodeBlock> CodeBlock::parse(LineIterator& lines)
 {
     if (lines.is_end())
         return {};
 
-    constexpr auto tick_tick_tick = "```";
-
     StringView line = *lines;
-    if (!line.starts_with(tick_tick_tick))
+    if (line.starts_with(tick_tick_tick))
+        return parse_backticks(lines);
+
+    if (line_block_prefix(line).has_value())
+        return parse_indent(lines);
+
     return {};
+}
+
+OwnPtr<CodeBlock> CodeBlock::parse_backticks(LineIterator& lines)
+{
+    StringView line = *lines;
 
     // Our Markdown extension: we allow
     // specifying a style and a language
@@ -124,4 +161,29 @@ OwnPtr<CodeBlock> CodeBlock::parse(LineIterator& lines)
     return make<CodeBlock>(language, style, builder.build());
 }
 
+OwnPtr<CodeBlock> CodeBlock::parse_indent(LineIterator& lines)
+{
+    bool first = true;
+    StringBuilder builder;
+
+    while (true) {
+        if (lines.is_end())
+            break;
+        StringView line = *lines;
+
+        auto prefix_length = line_block_prefix(line);
+        if (!prefix_length.has_value())
+            break;
+
+        line = line.substring_view(prefix_length.value());
+        ++lines;
+
+        if (!first)
+            builder.append('\n');
+        builder.append(line);
+        first = false;
+    }
+
+    return make<CodeBlock>("", "", builder.build());
+}
 }

Userland/Libraries/LibMarkdown/CodeBlock.h

@@ -33,6 +33,9 @@ private:
     String m_code;
     String m_language;
     String m_style;
+
+    static OwnPtr<CodeBlock> parse_backticks(LineIterator& lines);
+    static OwnPtr<CodeBlock> parse_indent(LineIterator& lines);
 };
 
 }