fix(satteri): reset heading slugs between renders

.changeset/fresh-bugs-reset.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/markdown-satteri': patch
+---
+
+Fixes `satteriHeadingIdsPlugin` sharing Slugger state across documents when reused as a user hast plugin, which caused headings with the same text in different posts to receive incorrect numeric suffixes

packages/markdown/satteri/src/satteri-processor.ts

@@ -115,30 +115,33 @@ export function collectHastText(
 	return text;
 }
 
-export function createHeadingIdsPlugin(): HastPluginDefinition {
+export function createHeadingIdsPlugin(): HastPluginEntry {
 	const slugger = new Slugger();
-	// Collect headings in a separate array so we can make this idempotent
-	const headings: MarkdownHeading[] = [];
-	return {
-		name: 'heading-ids',
-		element: {
-			filter: ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'],
-			visit(node, ctx) {
-				const astro = ctx.data.astro;
-				const rawText = ctx.textContent(node);
-				const text = rawText.includes('frontmatter')
-					? collectHastText(node, astro?.frontmatter ?? {})
-					: rawText;
-				const existingId = node.properties?.id;
-				const slug = typeof existingId === 'string' ? existingId : slugger.slug(text);
-				const depth = Number.parseInt(node.tagName[1], 10);
-				headings.push({ depth, slug, text });
-				if (astro) astro.headings = headings;
-				if (typeof existingId !== 'string') {
-					ctx.setProperty(node, 'id', slug);
-				}
+	return () => {
+		slugger.reset();
+		// Collect headings in a separate array so we can make this idempotent
+		const headings: MarkdownHeading[] = [];
+		return {
+			name: 'heading-ids',
+			element: {
+				filter: ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'],
+				visit(node, ctx) {
+					const astro = ctx.data.astro;
+					const rawText = ctx.textContent(node);
+					const text = rawText.includes('frontmatter')
+						? collectHastText(node, astro?.frontmatter ?? {})
+						: rawText;
+					const existingId = node.properties?.id;
+					const slug = typeof existingId === 'string' ? existingId : slugger.slug(text);
+					const depth = Number.parseInt(node.tagName[1], 10);
+					headings.push({ depth, slug, text });
+					if (astro) astro.headings = headings;
+					if (typeof existingId !== 'string') {
+						ctx.setProperty(node, 'id', slug);
+					}
+				},
 			},
-		},
+		};
 	};
 }
 

packages/markdown/satteri/test/markdown.test.ts

@@ -94,6 +94,23 @@ describe('satteri markdown', () => {
 		]);
 	});
 
+	it('resets heading slugs across multiple renders when `satteriHeadingIdsPlugin()` is reused', async () => {
+		const plugin = satteriHeadingIdsPlugin();
+		const processor = await createSatteriMarkdownProcessor({
+			hastPlugins: [plugin],
+		});
+		const first = await processor.render('## Introduction\n\n## Summary');
+		const second = await processor.render('## Introduction\n\n## Summary');
+		assert.deepEqual(first.metadata.headings, [
+			{ depth: 2, slug: 'introduction', text: 'Introduction' },
+			{ depth: 2, slug: 'summary', text: 'Summary' },
+		]);
+		assert.deepEqual(second.metadata.headings, [
+			{ depth: 2, slug: 'introduction', text: 'Introduction' },
+			{ depth: 2, slug: 'summary', text: 'Summary' },
+		]);
+	});
+
 	it('respects heading IDs set by a user hast plugin in both DOM and `headings`', async () => {
 		const setIdPlugin: HastPluginDefinition = {
 			name: 'set-heading-id',