/**
 * Groups the quiz list for the "All" tab:
 *
 *   Books                     → one group per book, in the library's order
 *   Lectures & Conversations  → one group per year, oldest first, undated last
 *   Other quizzes             → stand-alone quizzes
 *
 * Only groups with quizzes in them are returned, so an empty category or
 * sub-category never renders.
 */

import { BOOK_NAMES } from "@/lib/constants";
import type { QuizCard } from "@/lib/quiz";

export type QuizGroup = { key: string; label: string; quizzes: QuizCard[] };

export type QuizSections = {
  books: QuizGroup[];
  lectures: QuizGroup[];
  other: QuizCard[];
};

const BOOK_ORDER = Object.keys(BOOK_NAMES);

/** "bg" from /books/bg/2 — the book a chapter quiz belongs to. */
function bookCode(path?: string | null): string | null {
  const m = path?.match(/^\/books\/([^/]+)/);
  return m ? m[1] : null;
}

/**
 * The recording's year. Saved on the quiz since target_year existed; for
 * older quizzes, read from the slug's yymmdd prefix (720720sb-london → 1972)
 * when it falls in Śrīla Prabhupāda's recorded years.
 */
function lectureYear(q: QuizCard): number | null {
  if (q.target_year) return q.target_year;
  const m = q.target_path?.match(/^\/transcriptions\/(\d{2})\d{4}/);
  if (!m) return null;
  const y = 1900 + Number(m[1]);
  return y >= 1966 && y <= 1977 ? y : null;
}

export function groupQuizzes(cards: QuizCard[]): QuizSections {
  const books = new Map<string, QuizCard[]>();
  const years = new Map<number | null, QuizCard[]>();
  const other: QuizCard[] = [];

  for (const q of cards) {
    if (q.target_kind === "chapter") {
      const code = bookCode(q.target_path) ?? "other";
      books.set(code, [...(books.get(code) ?? []), q]);
    } else if (
      q.target_kind === "lecture" ||
      q.target_kind === "conversation"
    ) {
      const y = lectureYear(q);
      years.set(y, [...(years.get(y) ?? []), q]);
    } else {
      other.push(q);
    }
  }

  const rank = (code: string) => {
    const i = BOOK_ORDER.indexOf(code);
    return i < 0 ? BOOK_ORDER.length : i;
  };

  return {
    books: [...books.entries()]
      .sort(([a], [b]) => rank(a) - rank(b) || a.localeCompare(b))
      .map(([code, quizzes]) => ({
        key: code,
        label: BOOK_NAMES[code] ?? code.toUpperCase(),
        // Chapter order (/books/bg/2 before /books/bg/10), not newest first.
        quizzes: [...quizzes].sort((a, b) =>
          (a.target_path ?? "").localeCompare(b.target_path ?? "", undefined, {
            numeric: true,
          }),
        ),
      })),
    lectures: [...years.entries()]
      // Oldest first; undated (null) last.
      .sort(([a], [b]) => (a ?? 9999) - (b ?? 9999))
      .map(([year, quizzes]) => ({
        key: String(year ?? "undated"),
        label: year ? String(year) : "Undated",
        quizzes,
      })),
    other,
  };
}