ytplayer/frontend/loudness.js

/* ============================================================================
 * loudness — EBU R128 integrated loudness (LUFS), measured in the browser.
 *
 * The per-sample work (two K-weighting biquads + 100 ms energy steps) runs in
 * WebAssembly (loudness-wasm.js, built from wasm/loudness.c); a plain JS loop
 * with identical math is the fallback. Gating (400 ms blocks, 75 % overlap,
 * absolute -70 LUFS, relative -10 LU) is done here on the step energies.
 *
 * Plain <script> global `Loudness` in the browser; `require`-able by node.
 * ========================================================================== */
(function (root) {
  'use strict';

  const CHUNK = 65536;

  // ITU-R BS.1770-4 K-weighting, recomputed for any sample rate.
  function coefficients(fs) {
    let f0 = 1681.974450955533, G = 3.999843853973347, Q = 0.7071752369554196;
    let K = Math.tan(Math.PI * f0 / fs);
    const Vh = Math.pow(10, G / 20), Vb = Math.pow(Vh, 0.4996667741545416);
    let a0 = 1 + K / Q + K * K;
    const s1 = [(Vh + Vb * K / Q + K * K) / a0, 2 * (K * K - Vh) / a0, (Vh - Vb * K / Q + K * K) / a0,
      2 * (K * K - 1) / a0, (1 - K / Q + K * K) / a0];
    f0 = 38.13547087602444; Q = 0.5003270373238773;
    K = Math.tan(Math.PI * f0 / fs);
    a0 = 1 + K / Q + K * K;
    const s2 = [1, -2, 1, 2 * (K * K - 1) / a0, (1 - K / Q + K * K) / a0];
    return [s1, s2];
  }

  // Gating over per-100 ms energies (already summed over channels).
  function integrate(stepE, n, stepLen) {
    const blocks = [];
    for (let i = 0; i + 4 <= n; i++) {
      const z = (stepE[i] + stepE[i + 1] + stepE[i + 2] + stepE[i + 3]) / (4 * stepLen);
      if (z > 0) blocks.push(z);
    }
    const L = (z) => -0.691 + 10 * Math.log10(z);
    const abs = blocks.filter((z) => L(z) > -70);
    if (!abs.length) return null;
    const rel = L(abs.reduce((a, b) => a + b, 0) / abs.length) - 10;
    const gated = abs.filter((z) => L(z) > rel);
    if (!gated.length) return null;
    return L(gated.reduce((a, b) => a + b, 0) / gated.length);
  }

  // ---- engines: { setup(nch, stepLen, s1, s2), input(ch) → Float32Array, feed(n), result() } ----
  function wasmEngine() {
    const b64 = root.LOUDNESS_WASM_B64;
    if (!b64 || typeof WebAssembly === 'undefined') return null;
    try {
      const bin = typeof atob === 'function'
        ? Uint8Array.from(atob(b64), (c) => c.charCodeAt(0))
        : Uint8Array.from(Buffer.from(b64, 'base64'));
      const inst = new WebAssembly.Instance(new WebAssembly.Module(bin), {});
      const x = inst.exports;
      const mem = () => x.memory.buffer;
      return {
        engine: 'wasm',
        setup(nch, stepLen, s1, s2) { x.setup(nch, stepLen, ...s1, ...s2); },
        input(ch) { return new Float32Array(mem(), x.inbuf(ch), CHUNK); },
        feed(n) { x.feed(n); },
        steps() { return [new Float64Array(mem(), x.steps(), x.count()), x.count()]; },
      };
    } catch { return null; }
  }

  function jsEngine() {
    const buf = [new Float32Array(CHUNK), new Float32Array(CHUNK)];
    let nch = 1, stepLen = 1, stepPos = 0, acc = 0, k1, k2, st;
    const out = [];
    return {
      engine: 'js',
      setup(c, sl, s1, s2) { nch = c > 1 ? 2 : 1; stepLen = sl; stepPos = 0; acc = 0; k1 = s1; k2 = s2; st = [new Float64Array(8), new Float64Array(8)]; out.length = 0; },
      input(ch) { return buf[ch]; },
      feed(n) {
        for (let i = 0; i < n; i++) {
          let e = 0;
          for (let c = 0; c < nch; c++) {
            const s = st[c], x = buf[c][i];
            const y = k1[0] * x + k1[1] * s[0] + k1[2] * s[1] - k1[3] * s[2] - k1[4] * s[3];
            s[1] = s[0]; s[0] = x; s[3] = s[2]; s[2] = y;
            const z = k2[0] * y + k2[1] * s[4] + k2[2] * s[5] - k2[3] * s[6] - k2[4] * s[7];
            s[5] = s[4]; s[4] = y; s[7] = s[6]; s[6] = z;
            e += z * z;
          }
          acc += e;
          if (++stepPos === stepLen) { out.push(acc); acc = 0; stepPos = 0; }
        }
      },
      steps() { return [out, out.length]; },
    };
  }

  let _engine = null;
  function engine(prefer) {
    if (prefer === 'js') return jsEngine();
    if (!_engine) _engine = wasmEngine() || jsEngine();
    return _engine;
  }

  /** Integrated loudness (LUFS) of channel data, or null for silence. */
  function measureChannels(channels, sampleRate, { prefer } = {}) {
    const e = engine(prefer);
    const nch = Math.min(2, channels.length);
    const stepLen = Math.max(1, Math.round(sampleRate / 10));
    const [s1, s2] = coefficients(sampleRate);
    e.setup(nch, stepLen, s1, s2);
    const len = channels[0].length;
    for (let pos = 0; pos < len; pos += CHUNK) {
      const n = Math.min(CHUNK, len - pos);
      for (let c = 0; c < nch; c++) e.input(c).set(channels[c].subarray(pos, pos + n));
      e.feed(n);
    }
    const [steps, n] = e.steps();
    return integrate(steps, n, stepLen);
  }

  const Loudness = { coefficients, integrate, measureChannels, engineName: () => engine().engine, TARGET: -14 };
  if (typeof module !== 'undefined' && module.exports) module.exports = Loudness;
  else root.Loudness = Loudness;
})(typeof globalThis !== 'undefined' ? globalThis : this);