dorfteich/apps/api/src/import-export/export-markdown.ts

/**
 * Markdown rewrites for export (issue #65, ADR 0009). A page's cached Markdown
 * stores images as `![alt](<fileId>)` and wikilinks as `[[slug]]` /
 * `[[slug|text]]`; neither is portable as-is, so each export target rewrites
 * them: the pond ZIP to relative files, an office document to inline data.
 */

// A wikilink token — `[[slug]]` or `[[slug|display text]]`. Only real wikilink
// nodes serialize this way (literal brackets in text are escaped), so matching
// the raw token is safe.
const WIKILINK = /\[\[([^\]|]+)(?:\|([^\]]+))?\]\]/g;
// A Markdown image — `![alt](src)`. In cached page Markdown `src` is always a
// bare attachment id (no parentheses), so the group is unambiguous.
const IMAGE = /!\[([^\]]*)\]\(([^)]+)\)/g;

/** The image attachment ids referenced by a page's Markdown, in order. */
export function imageFileIds(markdown: string): string[] {
  const ids: string[] = [];
  for (const match of markdown.matchAll(IMAGE)) ids.push(match[2]!);
  return ids;
}

/**
 * Rewrite for the pond ZIP: a wikilink becomes a relative link to the target
 * page's `.md` file when that page is in the export (readable), else its plain
 * display text; an image source becomes a relative path into `media/`.
 */
export function markdownForZip(
  markdown: string,
  readableSlugs: Set<string>,
  mediaNameById: Map<string, string>,
): string {
  return markdown
    .replace(WIKILINK, (_whole, slug: string, text?: string) => {
      const label = (text ?? slug).trim();
      return readableSlugs.has(slug) ? `[${label}](${encodeURIComponent(slug)}.md)` : label;
    })
    .replace(IMAGE, (whole, alt: string, src: string) => {
      const name = mediaNameById.get(src);
      return name ? `![${alt}](media/${name})` : whole;
    });
}

/**
 * Rewrite for a standalone office document: a wikilink becomes plain display
 * text (there is no target document to link to), and an image source becomes an
 * inline `data:` URI so pandoc embeds the bytes.
 */
export function markdownForDocument(markdown: string, dataUriById: Map<string, string>): string {
  return markdown
    .replace(WIKILINK, (_whole, slug: string, text?: string) => (text ?? slug).trim())
    .replace(IMAGE, (whole, alt: string, src: string) => {
      const uri = dataUriById.get(src);
      return uri ? `![${alt}](${uri})` : whole;
    });
}

const EXTENSION_BY_MIME: Readonly<Record<string, string>> = {
  'image/png': 'png',
  'image/jpeg': 'jpg',
  'image/gif': 'gif',
  'image/webp': 'webp',
  'image/svg+xml': 'svg',
};

/** File extension for an attachment's stored bytes, from its MIME type. */
export function imageExtension(mimeType: string): string {
  return EXTENSION_BY_MIME[mimeType] ?? 'bin';
}