import path from 'node:path';
import { fileURLToPath } from 'node:url';

export const contentsRoot = fileURLToPath(new URL('../../contents/', import.meta.url));

export function normalizeBase(value = '/') {
  if (!/^\/(?:[A-Za-z0-9_-]+\/)*[A-Za-z0-9_-]*\/?$/.test(value)) {
    throw new Error('SITE_BASE must be / or an absolute path of URL-safe segments');
  }
  return value === '/' ? '/' : `/${value.split('/').filter(Boolean).join('/')}/`;
}

/** Rewrite published Markdown links; null means a source-only draft target. */
export function documentHref(href, sourceFile, base = '/') {
  if (/^(?:[a-z][a-z\d+.-]*:|\/\/|#)/i.test(href)) return href;
  const match = href.match(/^([^?#]+\.md)([?#].*)?$/i);
  if (!match) return href;
  const filename = path.resolve(path.dirname(sourceFile), decodeURIComponent(match[1]));
  const relative = path.relative(contentsRoot, filename);
  if (relative.startsWith('..') || path.isAbsolute(relative)) {
    throw new Error(`Markdown link outside contents/: ${href} in ${sourceFile}`);
  }
  if (relative.split(path.sep)[0] === 'draft') return null;
  const slug = relative.replace(/\.md$/i, '').split(path.sep).map(encodeURIComponent).join('/');
  return `${normalizeBase(base)}book/${slug}/${match[2] ?? ''}`;
}

export default function documentLinks({ base = '/' } = {}) {
  return (tree, file) => {
    const draftReferences = new Set();
    function collect(node) {
      if (node.type === 'definition' && documentHref(node.url, file.path, base) === null) draftReferences.add(node.identifier);
      for (const child of node.children ?? []) collect(child);
    }
    collect(tree);
    function rewrite(node) {
      if (node.children) node.children = node.children.flatMap(rewrite);
      if (node.type === 'linkReference' && draftReferences.has(node.identifier)) return node.children;
      if ((node.type === 'link' || node.type === 'definition') && typeof node.url === 'string') {
        const url = documentHref(node.url, file.path, base);
        // Retain authored link text/formatting, but expose no unpublished URL.
        if (url === null) return node.type === 'link' ? node.children : [];
        node.url = url;
      }
      return [node];
    }
    rewrite(tree);
  };
}