// 两张 PNG 的数值对拍：PSNR / SSIM / 平均亮度差（0-255）。
// 用法：node tools/diff.mjs <a.png> <b.png> [diff-out.png]
// 判读：平均差 < 1 视为肉眼无差异；1-3 为细微；> 8 通常意味着构图/内容变了。

import { spawn } from "node:child_process";

const [a, b, outPng] = process.argv.slice(2);
if (!a || !b) {
  console.error("usage: node tools/diff.mjs <a.png> <b.png> [diff-out.png]");
  process.exit(2);
}

function ff(args) {
  return new Promise((resolve) => {
    const p = spawn("ffmpeg", ["-hide_banner", "-loglevel", "info", ...args], { stdio: ["ignore", "pipe", "pipe"] });
    let err = "";
    p.stderr.on("data", (d) => (err += d));
    p.stdout.on("data", () => {});
    p.on("close", () => resolve(err));
  });
}

const psnr = await ff(["-i", a, "-i", b, "-lavfi", "psnr", "-f", "null", "-"]);
const ssim = await ff(["-i", a, "-i", b, "-lavfi", "ssim", "-f", "null", "-"]);
const yavg = await ff([
  "-i", a, "-i", b,
  "-lavfi", "blend=all_mode=difference,signalstats,metadata=print:key=lavfi.signalstats.YAVG",
  "-f", "null", "-",
]);

const num = (text, re) => {
  const m = text.match(re);
  return m ? Number(m[1]) : NaN;
};

const yavgs = [...yavg.matchAll(/YAVG=([\d.]+)/g)].map((m) => Number(m[1]));

const report = {
  psnr_avg: /average:inf/.test(psnr) ? Infinity : num(psnr, /average:([\d.]+)/),
  ssim_all: num(ssim, /All:([\d.]+)/),
  mean_abs_diff: yavgs.length ? yavgs[yavgs.length - 1] : NaN,
};

console.log(`psnr=${report.psnr_avg} dB   ssim=${report.ssim_all}   mean|diff|=${report.mean_abs_diff}/255`);

if (outPng) {
  await ff(["-y", "-i", a, "-i", b, "-lavfi", "blend=all_mode=difference", "-frames:v", "1", outPng]);
  console.log(`diff image -> ${outPng}`);
}