/** GitHub-style heading slug: lowercase, markdown stripped, punctuation dropped, spaces to hyphens. */
export function slugify(heading: string): string {
  return heading
    .replace(/!?\[([^\]]*)\]\([^)]*\)/g, "$1") // links and images keep their text
    .replace(/[`*~]/g, "") // code spans and emphasis markers
    .replace(/<[^>]+>/g, "") // inline html
    .trim()
    .toLowerCase()
    .replace(/[^\p{L}\p{N}\p{M}\s_-]/gu, "")
    .replace(/\s/g, "-");
}

interface Heading {
  line: number;
  level: number;
  text: string;
}

function headings(lines: string[]): Heading[] {
  const out: Heading[] = [];
  let fence: string | null = null;
  lines.forEach((line, i) => {
    const f = /^ {0,3}(`{3,}|~{3,})/.exec(line);
    if (f) {
      const marker = f[1]![0]!;
      if (fence === null) fence = marker;
      else if (fence === marker) fence = null;
      return;
    }
    if (fence !== null) return;
    const h = /^ {0,3}(#{1,6})\s+(.*?)\s*#*\s*$/.exec(line);
    if (h) out.push({ line: i, level: h[1]!.length, text: h[2]! });
  });
  return out;
}

/** Heading line plus everything up to the next heading of the same or a higher level. */
export function extractSection(markdown: string, anchor: string): string | null {
  const lines = markdown.split(/\r?\n/);
  const hs = headings(lines);
  const seen = new Map<string, number>();
  for (let i = 0; i < hs.length; i++) {
    const h = hs[i]!;
    const base = slugify(h.text);
    const n = seen.get(base) ?? 0;
    seen.set(base, n + 1);
    const slug = n === 0 ? base : `${base}-${n}`; // GitHub numbers duplicates
    if (slug !== anchor) continue;
    const next = hs.slice(i + 1).find((x) => x.level <= h.level);
    return lines.slice(h.line, next ? next.line : lines.length).join("\n").trimEnd();
  }
  return null;
}

/** `<details><summary>…</summary>…</details>` blocks whose summary starts with `match`, in document order. */
export function extractHints(markdown: string, match: string): { label: string; body: string }[] {
  const out: { label: string; body: string }[] = [];
  const re = /<details>\s*<summary>([\s\S]*?)<\/summary>([\s\S]*?)<\/details>/g;
  for (const m of markdown.matchAll(re)) {
    const summary = m[1]!.trim();
    if (!summary.startsWith(match)) continue;
    const label = summary.slice(match.length).replace(/^\s*(?:—|-|\()\s*/, "").trim();
    out.push({ label, body: m[2]!.trim() });
  }
  return out;
}