/**
 * The seed's sounds, built here instead of shipped or taken from the system: unlike the DejaVu
 * typefaces there is no audio file the host and the CI image are guaranteed to share, and a binary
 * in the repo would be a binary in the repo. WAV is the one format that can be written by hand.
 *
 * The bytes are a pure function of the arguments, so storage stays content-addressed: re-seeding
 * writes the same hash and the checks can name a file by its exact size.
 */

const SAMPLE_RATE = 8000;
const BITS = 16;
const CHANNELS = 1;

/**
 * A mono 16-bit PCM sine, faded in and out over a millisecond so it neither clicks on start nor on
 * stop – a decoder is happy either way, but a human listening to the editor's preview is not.
 */
export function sineWav(seconds, frequency = 440) {
  const frames = Math.round(SAMPLE_RATE * seconds);
  const data = Buffer.alloc(frames * 2);
  const fade = Math.max(1, Math.round(SAMPLE_RATE * 0.001));
  for (let i = 0; i < frames; i++) {
    const envelope = Math.min(1, i / fade, (frames - 1 - i) / fade);
    const value = Math.sin((2 * Math.PI * frequency * i) / SAMPLE_RATE) * envelope * 0.6;
    data.writeInt16LE(Math.round(value * 32767), i * 2);
  }
  return withRiffHeader(data);
}

function withRiffHeader(data) {
  const byteRate = (SAMPLE_RATE * CHANNELS * BITS) / 8;
  const header = Buffer.alloc(44);
  header.write('RIFF', 0);
  header.writeUInt32LE(36 + data.length, 4);
  header.write('WAVE', 8);
  header.write('fmt ', 12);
  header.writeUInt32LE(16, 16); // PCM header length
  header.writeUInt16LE(1, 20); // 1 = uncompressed PCM
  header.writeUInt16LE(CHANNELS, 22);
  header.writeUInt32LE(SAMPLE_RATE, 24);
  header.writeUInt32LE(byteRate, 28);
  header.writeUInt16LE((CHANNELS * BITS) / 8, 32); // block align
  header.writeUInt16LE(BITS, 34);
  header.write('data', 36);
  header.writeUInt32LE(data.length, 40);
  return Buffer.concat([header, data]);
}