import type { LibrarySectionItem } from "@/lib/api";

/** 章 → 其下小节（保持平铺顺序；顺序沿用后端 sort_order） */
export function groupSections(sections: LibrarySectionItem[]) {
  const groups: { chapter: LibrarySectionItem; subs: LibrarySectionItem[] }[] = [];
  for (const s of sections) {
    if (s.parent_id == null) {
      groups.push({ chapter: s, subs: [] });
    } else {
      groups[groups.length - 1]?.subs.push(s);
    }
  }
  return groups;
}

export interface ChapterNav {
  /** 当前章下标（0 起） */
  index: number;
  /** 章总数 */
  total: number;
  prev: LibrarySectionItem | null;
  next: LibrarySectionItem | null;
}

/** 当前章在章节序列中的位置与相邻章（找不到回落第一章） */
export function chapterNav(
  groups: { chapter: LibrarySectionItem }[],
  currentId: number | null
): ChapterNav {
  const total = groups.length;
  let index = groups.findIndex((g) => g.chapter.id === currentId);
  if (index < 0) index = 0;
  return {
    index,
    total,
    prev: index > 0 ? groups[index - 1].chapter : null,
    next: index < total - 1 ? groups[index + 1].chapter : null,
  };
}