feat: Support section-end mark (line with only hashes)

This adds the following rule on sectionization:

- A line with only `#`s can be used to end the section whose depth matches the number of the `#`s.
  - e.g., the section starting with `### Heading 3` can end with `###`.

(related to #155, thanks @nosuke23 for the suggestion)

docs/ja/vfm.md

@@ -567,6 +567,8 @@ ruby rt {
 - 見出しの行が `#` ではじまり同数以上の `#` で終わる場合はセクションを分けません
   - `### Not Sectionize ###` （同じ数の `#` で囲まれている） -- セクション分けしない
   - `### Sectionize ##` （閉じの `#` の数が足りない） -- セクション分けする
+- `#` だけからなる行により `#` の数と一致する深さのセクションを終了させることができる
+  - 例: `### Heading 3` で開始したセクションは `###` で終了させられる
 - 親が `blockquote` の場合はセクションを分けません
 - 見出しの深さへ一致するように、セクションの `levelN` クラスを設定します
 - 見出しの `id` 属性値をセクションの `aria-labelledby` 属性へ値をコピーします

docs/vfm.md

@@ -567,6 +567,8 @@ Make the heading a hierarchical section.
 - Do not sectionize if the heading line starts with `#`s and ends with equal or greater number of `#`s.
   - `### Not Sectionize ###` (enclosed by equal number of `#`s) -- not sectionize
   - `### Sectionize ##` (insufficient number of closing `#`s) -- sectionize 
+- A line with only `#`s can be used to end the section whose depth matches the number of the `#`s.
+  - e.g., the section starting with `### Heading 3` can end with `###`.
 - Do not sectionize if parent is `blockquote`.
 - Set the `levelN` class in the section to match the heading depth.
 - Copy the value of the `id` attribute of the heading to the `aria-labelledby` attribute of the section.

src/plugins/section.ts

@@ -32,21 +32,37 @@ const createProperties = (depth: number, node: any): KeyValue => {
   return properties;
 };
 
+const getHeadingLine = (node: any, file: VFile): string => {
+  if (node?.type !== 'heading') {
+    return '';
+  }
+  const startOffset = node.position?.start.offset ?? 0;
+  const endOffset = node.position?.end.offset ?? 0;
+  const text = file.toString().slice(startOffset, endOffset);
+  return text.trim();
+};
+
 /**
  * Check if the heading has a non-section mark (sufficient number of closing hashes).
  * @param node Node of Markdown AST.
  * @param file Virtual file.
  * @returns `true` if the node has a non-section mark.
  */
 const hasNonSectionMark = (node: any, file: VFile): boolean => {
-  const startOffset = node.position?.start.offset ?? 0;
-  const endOffset = node.position?.end.offset ?? 0;
-  const text = file.toString().slice(startOffset, endOffset);
-  const depth = node.depth;
-  if ((/[ \t](#+)$/.exec(text)?.[1]?.length ?? 0) >= depth) {
-    return true;
-  }
-  return false;
+  const line = getHeadingLine(node, file);
+  return (
+    !!line && (/^#.*[ \t](#+)$/.exec(line)?.[1]?.length ?? 0) >= node.depth
+  );
+};
+
+/**
+ * Check if the node is a section-end mark (line with only hashes).
+ * @param node Node of Markdown AST.
+ * @returns `true` if the node is a section-end mark.
+ */
+const isSectionEndMark = (node: any, file: VFile): boolean => {
+  const line = getHeadingLine(node, file);
+  return !!line && /^(#+)$/.exec(line)?.[1]?.length === node.depth;
 };
 
 /**
@@ -131,7 +147,12 @@ const sectionizeIfRequired = (node: any, ancestors: Parent[], file: VFile) => {
     children: between,
   } as any;
 
-  parent.children.splice(startIndex, section.children.length, section);
+  parent.children.splice(
+    startIndex,
+    section.children.length +
+      (isSectionEndMark(end, file) && end.depth === depth ? 1 : 0),
+    section,
+  );
 };
 
 /**

tests/section.test.ts

@@ -219,3 +219,72 @@ This is a note.
 `;
   expect(received).toBe(expected);
 });
+
+it('Section-end marks', () => {
+  const md = `# Heading 1
+
+Depth 1
+
+## Heading 2
+
+Depth 2
+
+### Heading 3
+
+Depth 3
+
+#### Heading 4
+
+Depth 4
+
+##### Heading 5
+
+Depth 5
+
+###### Heading 6
+
+Depth 6
+
+######
+
+Depth 5 again
+
+####
+
+Depth 3 again
+
+##
+
+Depth 1 again`;
+  const received = stringify(md, { partial: true });
+  const expected = `
+<section class="level1" aria-labelledby="heading-1">
+  <h1 id="heading-1">Heading 1</h1>
+  <p>Depth 1</p>
+  <section class="level2" aria-labelledby="heading-2">
+    <h2 id="heading-2">Heading 2</h2>
+    <p>Depth 2</p>
+    <section class="level3" aria-labelledby="heading-3">
+      <h3 id="heading-3">Heading 3</h3>
+      <p>Depth 3</p>
+      <section class="level4" aria-labelledby="heading-4">
+        <h4 id="heading-4">Heading 4</h4>
+        <p>Depth 4</p>
+        <section class="level5" aria-labelledby="heading-5">
+          <h5 id="heading-5">Heading 5</h5>
+          <p>Depth 5</p>
+          <section class="level6" aria-labelledby="heading-6">
+            <h6 id="heading-6">Heading 6</h6>
+            <p>Depth 6</p>
+          </section>
+          <p>Depth 5 again</p>
+        </section>
+      </section>
+      <p>Depth 3 again</p>
+    </section>
+  </section>
+  <p>Depth 1 again</p>
+</section>
+`;
+  expect(received).toBe(expected);
+});