// Shared YIN pitch estimation over a PCM buffer.
export function yinFrame(x, off, W, sr, fmin = 55, fmax = 400) {
  const tauMin = Math.floor(sr / fmax), tauMax = Math.min(Math.floor(sr / fmin), W - 1);
  if (off < 0 || off + W >= x.length || tauMax <= tauMin) return { f0: 0, conf: 0 };
  const d = new Float32Array(tauMax + 1);
  const N = W - tauMax;
  for (let tau = tauMin; tau <= tauMax; tau += 1) {
    let s = 0;
    for (let i = 0; i < N; i += 1) { const dv = x[off + i] - x[off + i + tau]; s += dv * dv; }
    d[tau] = s;
  }
  const dp = new Float32Array(tauMax + 1);
  let run = 0; dp[0] = 1;
  for (let tau = tauMin; tau <= tauMax; tau += 1) {
    run += d[tau];
    dp[tau] = run > 0 ? (d[tau] * (tau - tauMin + 1)) / run : 1;
  }
  let best = -1;
  for (let tau = tauMin + 1; tau < tauMax; tau += 1) if (dp[tau] < 0.15 && dp[tau] <= dp[tau + 1]) { best = tau; break; }
  if (best < 0) { let m = Infinity; for (let tau = tauMin; tau <= tauMax; tau += 1) if (dp[tau] < m) { m = dp[tau]; best = tau; } }
  if (best <= tauMin || best >= tauMax) return { f0: 0, conf: 0 };
  const a = dp[best - 1], b = dp[best], c = dp[best + 1];
  const den = a - 2 * b + c;
  const tau = best + (den !== 0 ? Math.max(-1, Math.min(1, (0.5 * (a - c)) / den)) : 0);
  return { f0: sr / tau, conf: 1 - dp[best] };
}

// Median f0 over a region, weighted to confident frames.
export function regionPitch(x, from, to, sr, fmin = 55, fmax = 400) {
  const W = Math.min(2048, Math.max(512, to - from));
  const hop = Math.max(128, Math.round(sr * 0.01));
  const vals = [];
  for (let off = from; off + W < to + W && off + W < x.length; off += hop) {
    const r = yinFrame(x, off, W, sr, fmin, fmax);
    if (r.conf >= 0.55 && r.f0 > 0) vals.push(r.f0);
    if (off > to) break;
  }
  if (!vals.length) return { f0: 0, n: 0 };
  vals.sort((a, b) => a - b);
  return { f0: vals[vals.length >> 1], n: vals.length };
}

export const midiOf = (hz) => 69 + 12 * Math.log2(hz / 440);