typst - use pandoc ast to search for level one headings

Author: cscheid

src/core/lib/markdown-analysis/level-one-headings.ts

@@ -0,0 +1,24 @@
+/*
+ * level-one-headings.ts
+ *
+ * Copyright (C) 2025 Posit Software, PBC
+ */
+
+import { join } from "../../../deno_ral/path.ts";
+import { execProcess } from "../../process.ts";
+import { pandocBinaryPath, resourcePath } from "../../resources.ts";
+
+export async function hasLevelOneHeadings(markdown: string): Promise<boolean> {
+  // this is O(n * m) where n is the number of blocks and m is the number of matches
+  // we could do better but won't until we profile and show it's a problem
+
+  const path = pandocBinaryPath();
+  const filterPath = resourcePath(
+    join("filters", "quarto-internals", "leveloneanalysis.lua"),
+  );
+  const result = await execProcess({
+    cmd: [path, "-f", "markdown", "-t", "markdown", "-L", filterPath],
+    stdout: "piped",
+  }, markdown);
+  return result.stdout === "true";
+}

src/format/typst/format-typst.ts

@@ -29,6 +29,7 @@ import {
 } from "../../config/types.ts";
 import { formatResourcePath } from "../../core/resources.ts";
 import { createFormat } from "../formats-shared.ts";
+import { hasLevelOneHeadings as hasL1Headings } from "../../core/lib/markdown-analysis/level-one-headings.ts";
 
 export function typstFormat(): Format {
   return createFormat("Typst", "pdf", {
@@ -44,14 +45,14 @@ export function typstFormat(): Format {
       [kCiteproc]: false,
     },
     resolveFormat: typstResolveFormat,
-    formatExtras: (
+    formatExtras: async (
       _input: string,
       markdown: string,
       flags: PandocFlags,
       format: Format,
       _libDir: string,
       _services: RenderServices,
-    ): FormatExtras => {
+    ): Promise<FormatExtras> => {
       const pandoc: FormatPandoc = {};
       const metadata: Metadata = {};
 
@@ -68,7 +69,7 @@ export function typstFormat(): Format {
 
       // unless otherwise specified, pdfs with only level 2 or greater headings get their
       // heading level shifted by -1.
-      const hasLevelOneHeadings = !!markdown.match(/\n^#\s.*$/gm);
+      const hasLevelOneHeadings = await hasL1Headings(markdown);
       if (
         !hasLevelOneHeadings &&
         flags?.[kShiftHeadingLevelBy] === undefined &&

src/resources/filters/quarto-internals/leveloneanalysis.lua

@@ -0,0 +1,18 @@
+local found = false
+function Header(el)
+  found = found or el.level == 1
+  return nil
+end
+
+function Pandoc(doc)
+  if found then
+    doc.blocks = pandoc.Blocks({
+      pandoc.Str("true")
+    })
+  else
+    doc.blocks = pandoc.Blocks({
+      pandoc.Str("false")
+    })
+  end
+  return doc
+end