// Playback level. The synthesized signal is a sound pressure in pascals; the
// gain maps it to full scale either at a fixed calibration (absolute, 1 Pa
// at 0 dBFS plus a user gain), by its peak, or by its loudness (RMS of the
// sustained part). A soft clipper above 0.9 of full scale keeps the output
// bounded whatever the gain.

export type LevelMode = "absolute" | "loudness" | "peak";

const KNEE = 0.9;
const PEAK_TARGET = 10 ** (-1 / 20); // -1 dBFS
const LOUDNESS_TARGET = 10 ** (-20 / 20); // -20 dBFS RMS
const LOUDNESS_SKIP_S = 0.05;

function peakAbs(samples: Float32Array): number {
  let max = 0;
  for (let k = 0; k < samples.length; k++) {
    const a = Math.abs(samples[k]);
    if (a > max) max = a;
  }
  return max;
}

/**
 * Gain that maps the pressure signal [Pa] to full scale: absolute = 1 Pa at
 * 0 dBFS shifted by gainDb; peak = normalize the peak to -1 dBFS; loudness =
 * normalize the RMS of the signal after the first 50 ms to -20 dBFS. A silent
 * signal gets unit gain in the normalizing modes (there is nothing to scale).
 */
export function levelGain(samples: Float32Array, fs: number, mode: LevelMode, gainDb: number): number {
  if (mode === "absolute") return 10 ** (gainDb / 20);
  if (mode === "peak") {
    const max = peakAbs(samples);
    return max > 0 ? PEAK_TARGET / max : 1;
  }
  const start = Math.min(samples.length, Math.round(LOUDNESS_SKIP_S * fs));
  const count = samples.length - start;
  if (count <= 0) return 1;
  let sum = 0;
  for (let k = start; k < samples.length; k++) sum += samples[k] * samples[k];
  const rms = Math.sqrt(sum / count);
  return rms > 0 ? LOUDNESS_TARGET / rms : 1;
}

/**
 * Applies the gain and a soft clipper and returns a new array. Below the
 * knee (0.9 of full scale) the clipper is the identity; above it the excess
 * is squashed with a tanh so that the output stays within (-1, 1) and the
 * slope is continuous at the knee.
 */
export function applyLevel(samples: Float32Array, gain: number): Float32Array {
  const out = new Float32Array(samples.length);
  const room = 1 - KNEE;
  for (let k = 0; k < samples.length; k++) {
    const y = samples[k] * gain;
    const a = Math.abs(y);
    if (a <= KNEE) out[k] = y;
    else {
      const c = KNEE + room * Math.tanh((a - KNEE) / room);
      out[k] = y < 0 ? -c : c;
    }
  }
  return out;
}

/** Peak level in dBFS of a signal (-Infinity when silent). */
export function peakDbfs(samples: Float32Array): number {
  const max = peakAbs(samples);
  return max > 0 ? 20 * Math.log10(max) : -Infinity;
}

/** Peak sound pressure level in dB SPL re 20 uPa of a pressure signal [Pa]. */
export function peakSpl(samples: Float32Array): number {
  const max = peakAbs(samples);
  return max > 0 ? 20 * Math.log10(max / 20e-6) : -Infinity;
}