[Markdoc] `headings` and heading IDs (#7095)

* deps: markdown-remark

* wip: heading-ids function

* chore: add `@astrojs/markdoc` to external

* feat: `headings` support

* fix: allow `render` config on headings

* fix: nonexistent `userConfig`

* test: headings, toc, astro component render

* docs: README

* chore: changeset

* refactor: expose Markdoc helpers from runtime

* fix: bad named exports (commonjsssss)

* refactor: defaultNodes -> nodes

* deps: github-slugger

* fix: reset slugger cache on each render

* fix: bad astroNodes import

* docs: explain headingSlugger export

* docs: add back double stringify comment

* chore: bump to minor for internal exports change

1 files changed, 78 insertions, 0 deletions

diff --git a/packages/integrations/markdoc/src/runtime.ts b/packages/integrations/markdoc/src/runtime.ts
new file mode 100644
index 000000000..dadb73cd6
--- /dev/null
+++ b/packages/integrations/markdoc/src/runtime.ts
@@ -0,0 +1,78 @@
+import type { MarkdownHeading } from '@astrojs/markdown-remark';
+import Markdoc, {
+	type RenderableTreeNode,
+	type ConfigType as MarkdocConfig,
+} from '@markdoc/markdoc';
+import type { ContentEntryModule } from 'astro';
+import { nodes as astroNodes } from './nodes/index.js';
+
+/** Used to reset Slugger cache on each build at runtime */
+export { headingSlugger } from './nodes/index.js';
+export { default as Markdoc } from '@markdoc/markdoc';
+
+export function applyDefaultConfig(
+	config: MarkdocConfig,
+	entry: ContentEntryModule
+): MarkdocConfig {
+	return {
+		...config,
+		variables: {
+			entry,
+			...config.variables,
+		},
+		nodes: {
+			...astroNodes,
+			...config.nodes,
+		},
+		// TODO: Syntax highlighting
+	};
+}
+
+/**
+ * Get text content as a string from a Markdoc transform AST
+ */
+export function getTextContent(childNodes: RenderableTreeNode[]): string {
+	let text = '';
+	for (const node of childNodes) {
+		if (typeof node === 'string' || typeof node === 'number') {
+			text += node;
+		} else if (typeof node === 'object' && Markdoc.Tag.isTag(node)) {
+			text += getTextContent(node.children);
+		}
+	}
+	return text;
+}
+
+const headingLevels = [1, 2, 3, 4, 5, 6] as const;
+
+/**
+ * Collect headings from Markdoc transform AST
+ * for `headings` result on `render()` return value
+ */
+export function collectHeadings(children: RenderableTreeNode[]): MarkdownHeading[] {
+	let collectedHeadings: MarkdownHeading[] = [];
+	for (const node of children) {
+		if (typeof node !== 'object' || !Markdoc.Tag.isTag(node)) continue;
+
+		if (node.attributes.__collectHeading === true && typeof node.attributes.level === 'number') {
+			collectedHeadings.push({
+				slug: node.attributes.id,
+				depth: node.attributes.level,
+				text: getTextContent(node.children),
+			});
+			continue;
+		}
+
+		for (const level of headingLevels) {
+			if (node.name === 'h' + level) {
+				collectedHeadings.push({
+					slug: node.attributes.id,
+					depth: level,
+					text: getTextContent(node.children),
+				});
+			}
+		}
+		collectedHeadings.concat(collectHeadings(node.children));
+	}
+	return collectedHeadings;
+}