/**
 * A recorded trace as a .wav file.
 *
 * The trace holds one sample per timestep, 1/dt of them a second — around
 * 150 kHz at the default grid, which is a legal but eccentric rate for a
 * .wav. It is resampled to 48 kHz by linear interpolation, which is harmless
 * here: the content is band-limited far below either rate (the grid stops
 * resolving sound around 5 kHz). Normalized to peak, like playback, and
 * faded a few milliseconds at each end so the file does not open and close
 * with a click.
 */

export const WAV_RATE = 48000;

export function traceToWav(trace: Float32Array, dt: number): Blob {
  const n = Math.max(1, Math.round(trace.length * dt * WAV_RATE));
  let peak = 0;
  for (const v of trace) peak = Math.max(peak, Math.abs(v));
  const gain = peak > 0 ? 0.9 / peak : 0;

  const samples = new Float32Array(n);
  for (let i = 0; i < n; i++) {
    const s = i / (WAV_RATE * dt);
    const k = Math.min(trace.length - 2, Math.floor(s));
    const f = Math.min(1, s - k);
    samples[i] = gain * ((1 - f) * trace[k] + f * trace[k + 1]);
  }
  const fade = Math.min(Math.round(0.005 * WAV_RATE), Math.floor(n / 2));
  for (let i = 0; i < fade; i++) {
    const w = i / fade;
    samples[i] *= w;
    samples[n - 1 - i] *= w;
  }

  const bytes = 44 + 2 * n;
  const buf = new ArrayBuffer(bytes);
  const view = new DataView(buf);
  const str = (off: number, s: string): void => {
    for (let i = 0; i < s.length; i++) view.setUint8(off + i, s.charCodeAt(i));
  };
  str(0, 'RIFF');
  view.setUint32(4, bytes - 8, true);
  str(8, 'WAVE');
  str(12, 'fmt ');
  view.setUint32(16, 16, true); // PCM chunk size
  view.setUint16(20, 1, true); // PCM
  view.setUint16(22, 1, true); // mono
  view.setUint32(24, WAV_RATE, true);
  view.setUint32(28, 2 * WAV_RATE, true); // byte rate
  view.setUint16(32, 2, true); // block align
  view.setUint16(34, 16, true); // bits per sample
  str(36, 'data');
  view.setUint32(40, 2 * n, true);
  for (let i = 0; i < n; i++) {
    const v = Math.max(-1, Math.min(1, samples[i]));
    view.setInt16(44 + 2 * i, Math.round(v * 32767), true);
  }
  return new Blob([buf], { type: 'audio/wav' });
}