/**
 * 课程工具 - 获取上一节/下一节
 */

export type LessonRef = { moduleIndex: number; lessonIndex: number };

export function getPrevNextLesson(
  modules: readonly { lessons: readonly unknown[] }[],
  moduleIndex: number,
  lessonIndex: number
): { prev: LessonRef | null; next: LessonRef | null } {
  const allLessons: LessonRef[] = [];
  modules.forEach((m, mi) => {
    m.lessons.forEach((_, li) => {
      allLessons.push({ moduleIndex: mi, lessonIndex: li });
    });
  });

  const idx = allLessons.findIndex((l) => l.moduleIndex === moduleIndex && l.lessonIndex === lessonIndex);
  if (idx < 0) return { prev: null, next: null };

  return {
    prev: idx > 0 ? allLessons[idx - 1] : null,
    next: idx < allLessons.length - 1 ? allLessons[idx + 1] : null,
  };
}