LibMarkdown: Refactor Document's parser into ContainerBlock

This will better allow us too do things like have Lists and blockquotes
support multiple blocks.

Userland/Libraries/LibMarkdown/ContainerBlock.cpp

@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibMarkdown/CodeBlock.h>
+#include <LibMarkdown/ContainerBlock.h>
+#include <LibMarkdown/Heading.h>
+#include <LibMarkdown/HorizontalRule.h>
+#include <LibMarkdown/List.h>
+#include <LibMarkdown/Paragraph.h>
+#include <LibMarkdown/Table.h>
+
+namespace Markdown {
+
+String ContainerBlock::render_to_html() const
+{
+    StringBuilder builder;
+
+    for (auto& block : m_blocks) {
+        auto s = block.render_to_html();
+        builder.append(s);
+    }
+
+    return builder.build();
+}
+
+String ContainerBlock::render_for_terminal(size_t view_width) const
+{
+    StringBuilder builder;
+
+    for (auto& block : m_blocks) {
+        auto s = block.render_for_terminal(view_width);
+        builder.append(s);
+    }
+
+    return builder.build();
+}
+
+template<typename BlockType>
+static bool try_parse_block(Vector<StringView>::ConstIterator& lines, NonnullOwnPtrVector<Block>& blocks)
+{
+    OwnPtr<BlockType> block = BlockType::parse(lines);
+    if (!block)
+        return false;
+    blocks.append(block.release_nonnull());
+    return true;
+}
+
+OwnPtr<ContainerBlock> ContainerBlock::parse(Vector<StringView>::ConstIterator& lines)
+{
+    NonnullOwnPtrVector<Block> blocks;
+
+    StringBuilder paragraph_text;
+
+    auto flush_paragraph = [&] {
+        if (paragraph_text.is_empty())
+            return;
+        auto paragraph = make<Paragraph>(Text::parse(paragraph_text.build()));
+        blocks.append(move(paragraph));
+        paragraph_text.clear();
+    };
+
+    while (true) {
+        if (lines.is_end())
+            break;
+
+        if ((*lines).is_empty()) {
+            ++lines;
+
+            flush_paragraph();
+            continue;
+        }
+
+        bool any = try_parse_block<Table>(lines, blocks) || try_parse_block<List>(lines, blocks) || try_parse_block<CodeBlock>(lines, blocks)
+            || try_parse_block<Heading>(lines, blocks) || try_parse_block<HorizontalRule>(lines, blocks);
+
+        if (any) {
+            if (!paragraph_text.is_empty()) {
+                auto last_block = blocks.take_last();
+                flush_paragraph();
+                blocks.append(move(last_block));
+            }
+            continue;
+        }
+
+        if (!paragraph_text.is_empty())
+            paragraph_text.append("\n");
+        paragraph_text.append(*lines++);
+    }
+
+    flush_paragraph();
+
+    return make<ContainerBlock>(move(blocks));
+}
+
+}