import { createHash } from "node:crypto";
import { execFile } from "node:child_process";
import { promisify } from "node:util";
import { mkdir, rename, stat } from "node:fs/promises";
import { existsSync } from "node:fs";
import path from "node:path";
import { CACHE_DIR } from "./paths";
import { probeCached } from "./browse";

const run = promisify(execFile);

// ---------------------------------------------------------------------------
// A frame to look at, for a cut or a variant.
//
// thumbs/ covers the SONG level (see lib/browse.ts) and nothing else -- those
// are hand-composed 1280x720 covers, not per-cut posters. So every cut and
// every variant gets a frame pulled out of the file itself.
//
// The frames go in CACHE_DIR, NOT next to the mp4s. `videos/<song>/` is a
// directory the user drags into a player expecting to see the current cuts and
// nothing else; a litter of .poster.jpg files would break that, and it is the
// kind of thing nobody notices until it is annoying. CACHE_DIR is already
// documented as derived output, safe to delete at any time, and it sits inside
// SONG_DATA -- hence inside MEDIA_ROOTS -- so the frames are servable.
// ---------------------------------------------------------------------------

export const POSTER_CACHE = path.join(CACHE_DIR, "browse");

/**
 * Where into the file to grab.
 *
 * A frame at 0s is the intro card on every Mortal Kombat cut -- four posters of
 * the same Sub-Zero bio screen, which tells you nothing about which cut you are
 * looking at. A quarter of the way in is past every intro in the tree.
 */
const POSTER_AT = 0.25;
const DEFAULT_WIDTH = 480;

const keyFor = (abs: string, mtimeMs: number, size: number, width: number) =>
  createHash("sha1")
    .update(`${abs}|${Math.round(mtimeMs)}|${size}|${width}|v1`)
    .digest("hex")
    .slice(0, 16);

export type PosterResult = { file: string; cached: boolean };

/** Extract (or reuse) a poster frame. Returns null if the file will not decode. */
export async function posterFrame(abs: string, width = DEFAULT_WIDTH): Promise<PosterResult | null> {
  let st;
  try {
    st = await stat(abs);
  } catch {
    return null;
  }
  const file = path.join(POSTER_CACHE, `${keyFor(abs, st.mtimeMs, st.size, width)}.jpg`);
  if (existsSync(file)) return { file, cached: true };

  const info = await probeCached(abs);
  if (!info || !info.hasVideo) return null;
  const at = info.duration > 0 ? +(info.duration * POSTER_AT).toFixed(3) : 0;

  await mkdir(POSTER_CACHE, { recursive: true });
  const tmp = `${file}.tmp-${process.pid}-${Math.random().toString(36).slice(2, 8)}.jpg`;
  try {
    // -ss BEFORE -i is the fast seek: it jumps in the container rather than
    // decoding up to the mark, which on a 66 MB render is the difference
    // between a poster in 200ms and one in several seconds.
    await run(
      "ffmpeg",
      [
        "-nostdin", "-v", "error", "-y",
        "-ss", String(at),
        "-i", abs,
        "-frames:v", "1",
        "-vf", `scale=${width}:-2`,
        "-q:v", "4",
        tmp,
      ],
      { maxBuffer: 1 << 20 },
    );
  } catch {
    return null;
  }
  await rename(tmp, file).catch(() => {});
  return existsSync(file) ? { file, cached: false } : null;
}