#!/usr/bin/env node
// Remove a span from a FINISHED video, picture and sound together.
//
//   node slice-out.mjs <in.mp4> <out.mp4> <at> <len> [fade]
//
// Used to shorten a long static screen in an intro. Cutting the finished file
// rather than re-rendering is not a shortcut, it is the safer edit: everything
// after the cut moves earlier by exactly the same amount in BOTH streams, so
// every sync already judged by ear -- the shouts against the game's silent gap,
// the gong, the fatality, the fanfare -- survives untouched. Re-rendering would
// mean re-deriving PREROLL and every absolute time in the finale envelope.
//
// Two conditions make it safe, and both must be checked before using it:
//   1. the PICTURE either side of the cut is the same, so the video splice is
//      invisible (a static screen; verify, do not assume);
//   2. the SOUND either side is rhythmically continuous, so the audio splice
//      does not stumble. Cut a whole number of musical periods -- measure the
//      period by autocorrelating the envelope, do not guess it from the tempo.
//
// LENGTH IS PRESERVED EXACTLY. acrossfade shortens its output by the fade, so
// each side is extended by half a fade first; the result is exactly len shorter
// than the input, matching the hard video cut. Getting this wrong drifts the
// audio against the picture by the fade duration for the whole rest of the film.
import { execFileSync } from "node:child_process";

const [IN, OUT, AT, LEN, FADE = "0.060"] = process.argv.slice(2);
if (!IN || !OUT || !AT || !LEN) {
  console.error("usage: node slice-out.mjs <in.mp4> <out.mp4> <at> <len> [fade]");
  process.exit(1);
}
const at = Number(AT), len = Number(LEN), fade = Number(FADE);

const dur = Number(execFileSync("ffprobe", ["-v", "error", "-show_entries", "format=duration",
  "-of", "csv=p=0", IN]).toString().trim());
const half = fade / 2;

// Video: hard cut at `at`, resuming at `at+len`. Audio: the same cut, but each
// side reaches half a fade further in so the crossfade costs nothing in length.
const fc = [
  `[0:v]trim=0:${at},setpts=PTS-STARTPTS[v0]`,
  `[0:v]trim=${at + len}:${dur},setpts=PTS-STARTPTS[v1]`,
  `[v0][v1]concat=n=2:v=1:a=0[v]`,
  `[0:a]atrim=0:${at + half},asetpts=PTS-STARTPTS[a0]`,
  `[0:a]atrim=${at + len - half}:${dur},asetpts=PTS-STARTPTS[a1]`,
  `[a0][a1]acrossfade=d=${fade}:c1=tri:c2=tri[a]`,
].join(";");

execFileSync("ffmpeg", ["-y", "-v", "warning", "-i", IN, "-filter_complex", fc,
  "-map", "[v]", "-map", "[a]",
  "-c:v", "libx264", "-preset", "medium", "-crf", "18", "-pix_fmt", "yuv420p",
  "-c:a", "aac", "-b:a", "192k", "-movflags", "+faststart", OUT], { stdio: "inherit" });

const got = Number(execFileSync("ffprobe", ["-v", "error", "-show_entries", "format=duration",
  "-of", "csv=p=0", OUT]).toString().trim());
const want = dur - len;
console.log(`${IN} ${dur.toFixed(3)}s -> ${OUT} ${got.toFixed(3)}s`);
console.log(`  cut ${len}s at ${at}s; predicted ${want.toFixed(3)}s, off by ${(got - want).toFixed(3)}s`);