// lib/watch-progress.ts
// Single source of truth for watch-progress maths and the sequential unlock rule.
import { prisma } from './prisma';

/** A video counts as completed (and unlocks the next one) at this percentage. */
export const UNLOCK_THRESHOLD_PERCENT = 80;

export type SecRange = { startSec: number; endSec: number };

/**
 * Merges overlapping or touching ranges. Returns a new array sorted by start.
 * When ranges carry a `watchedAt`, the merged range keeps the most recent one.
 */
export function mergeRanges<T extends SecRange & { watchedAt?: Date }>(
  ranges: T[]
): Array<SecRange & { watchedAt?: Date }> {
  const sorted = ranges
    .filter((r) => r.endSec > r.startSec)
    .sort((a, b) => a.startSec - b.startSec);

  const merged: Array<SecRange & { watchedAt?: Date }> = [];
  for (const r of sorted) {
    const last = merged[merged.length - 1];
    if (last && r.startSec <= last.endSec) {
      last.endSec = Math.max(last.endSec, r.endSec);
      if (r.watchedAt && (!last.watchedAt || r.watchedAt > last.watchedAt)) {
        last.watchedAt = r.watchedAt;
      }
    } else {
      merged.push({ startSec: r.startSec, endSec: r.endSec, watchedAt: r.watchedAt });
    }
  }
  return merged;
}

export function totalSeconds(ranges: SecRange[]): number {
  return ranges.reduce((sum, r) => sum + Math.max(0, r.endSec - r.startSec), 0);
}

/**
 * Validates client-reported [start, end] pairs and snaps them to whole seconds
 * inside the video. Anything malformed is dropped.
 */
export function sanitizeRanges(input: unknown, durationSec: number): SecRange[] {
  if (!Array.isArray(input)) return [];
  const max = Math.ceil(durationSec);
  const out: SecRange[] = [];
  for (const item of input.slice(0, 500)) {
    if (!Array.isArray(item) || item.length < 2) continue;
    const s = Number(item[0]);
    const e = Number(item[1]);
    if (!Number.isFinite(s) || !Number.isFinite(e) || e <= s) continue;
    const startSec = Math.min(max, Math.max(0, Math.floor(s)));
    const endSec = Math.min(max, Math.max(0, Math.ceil(e)));
    if (endSec > startSec) out.push({ startSec, endSec });
  }
  return out;
}

type RestrictableVideo = {
  videoCourses?: Array<{ courseId: string; exclusive: boolean }>;
};

/** Drops videos that are exclusive to courses the user is not enrolled in. */
export function filterAccessibleVideos<T extends RestrictableVideo>(
  videos: T[],
  userCourseIds: string[]
): T[] {
  return videos.filter((video) => {
    const restricted = (video.videoCourses ?? [])
      .filter((a) => a.exclusive)
      .map((a) => a.courseId);
    if (restricted.length === 0) return true;
    return restricted.some((courseId) => userCourseIds.includes(courseId));
  });
}

type PlaylistVideo = {
  id: string;
  title: string;
  index: number;
  locked: boolean;
  instantAccess: boolean;
};

/**
 * The playlist's videos, in order, as this user sees them. Returns null when
 * the user is not enrolled in any course the playlist belongs to.
 */
export async function getPlaylistVideosForUser(
  userId: string,
  playlistId: string
): Promise<PlaylistVideo[] | null> {
  const [enrollments, playlist] = await Promise.all([
    prisma.enrollment.findMany({ where: { userId }, select: { courseId: true } }),
    prisma.playlist.findUnique({
      where: { id: playlistId },
      select: {
        courseId: true,
        courses: { select: { courseId: true } },
        videos: {
          orderBy: { index: 'asc' },
          select: {
            id: true,
            title: true,
            index: true,
            locked: true,
            instantAccess: true,
            videoCourses: { select: { courseId: true, exclusive: true } },
          },
        },
      },
    }),
  ]);
  if (!playlist) return null;

  const userCourseIds = enrollments.map((e) => e.courseId);
  const playlistCourseIds = [playlist.courseId, ...playlist.courses.map((c) => c.courseId)];
  if (!playlistCourseIds.some((id) => userCourseIds.includes(id))) return null;

  return filterAccessibleVideos(playlist.videos, userCourseIds).map(
    ({ id, title, index, locked, instantAccess }) => ({ id, title, index, locked, instantAccess })
  );
}

/**
 * Called when a user completes `video`: unlocks the next video in the playlist
 * that needs unlocking (skipping lecturer-locked and instant-access videos).
 */
export async function unlockNextVideo(
  userId: string,
  video: { id: string; playlistId: string }
): Promise<{ id: string; title: string } | null> {
  const videos = await getPlaylistVideosForUser(userId, video.playlistId);
  if (!videos) return null;
  const pos = videos.findIndex((v) => v.id === video.id);
  if (pos === -1) return null;

  const next = videos.slice(pos + 1).find((v) => !v.locked && !v.instantAccess);
  if (!next) return null;

  await prisma.videoUnlock.upsert({
    where: { userId_videoId: { userId, videoId: next.id } },
    update: {},
    create: { userId, videoId: next.id },
  });
  return { id: next.id, title: next.title };
}

export type UnlockCheck =
  | { ok: true; free: boolean }
  | {
      ok: false;
      status: number;
      reason: 'not_found' | 'locked' | 'no_access' | 'not_earned';
      error: string;
    };

/**
 * Decides whether a user may unlock a video by hand. Mirrors unlockNextVideo:
 * a video opens once any watchable video between it and the previous
 * unlock-gated video (inclusive) has been completed.
 * `free` means the video never needed an unlock record.
 */
export async function checkUnlockEligibility(
  userId: string,
  videoId: string
): Promise<UnlockCheck> {
  const video = await prisma.video.findUnique({
    where: { id: videoId },
    select: { id: true, playlistId: true, locked: true },
  });
  if (!video) return { ok: false, status: 404, reason: 'not_found', error: 'Video not found' };
  if (video.locked) {
    return {
      ok: false,
      status: 403,
      reason: 'locked',
      error: 'This video is locked by your lecturer.',
    };
  }

  const videos = await getPlaylistVideosForUser(userId, video.playlistId);
  const pos = videos ? videos.findIndex((v) => v.id === videoId) : -1;
  if (!videos || pos === -1) {
    return {
      ok: false,
      status: 403,
      reason: 'no_access',
      error: 'You do not have access to this video.',
    };
  }

  if (videos[pos].instantAccess || pos === 0) return { ok: true, free: true };

  // Walk back to the previous video that itself needs unlocking.
  let gateStart = 0;
  for (let i = pos - 1; i >= 0; i--) {
    if (!videos[i].locked && !videos[i].instantAccess) {
      gateStart = i;
      break;
    }
  }
  const gate = videos.slice(gateStart, pos).filter((v) => !v.locked);
  if (gate.length === 0) return { ok: true, free: true };

  const progress = await prisma.videoProgress.findMany({
    where: { userId, videoId: { in: gate.map((v) => v.id) } },
    select: { videoId: true, percent: true, completed: true },
  });
  const done = progress.some(
    (p) => p.completed || p.percent >= UNLOCK_THRESHOLD_PERCENT
  );
  if (done) return { ok: true, free: false };

  const previous = gate[gate.length - 1];
  const watched = progress.find((p) => p.videoId === previous.id)?.percent ?? 0;
  return {
    ok: false,
    status: 403,
    reason: 'not_earned',
    error: `Watch at least ${UNLOCK_THRESHOLD_PERCENT}% of "${previous.title}" to unlock this video (you are at ${watched}%).`,
  };
}