frontend/src/utils/wavEncoder.ts

const TARGET_SAMPLE_RATE = 16000;

function writeString(view: DataView, offset: number, text: string) {
  for (let i = 0; i < text.length; i++) {
    view.setUint8(offset + i, text.charCodeAt(i));
  }
}

export function encodeWav(samples: Float32Array, sampleRate: number): Blob {
  const numChannels = 1;
  const bitsPerSample = 16;
  const blockAlign = (numChannels * bitsPerSample) / 8;
  const byteRate = sampleRate * blockAlign;
  const dataSize = samples.length * blockAlign;
  const buffer = new ArrayBuffer(44 + dataSize);
  const view = new DataView(buffer);

  writeString(view, 0, "RIFF");
  view.setUint32(4, 36 + dataSize, true);
  writeString(view, 8, "WAVE");
  writeString(view, 12, "fmt ");
  view.setUint32(16, 16, true);
  view.setUint16(20, 1, true);
  view.setUint16(22, numChannels, true);
  view.setUint32(24, sampleRate, true);
  view.setUint32(28, byteRate, true);
  view.setUint16(32, blockAlign, true);
  view.setUint16(34, bitsPerSample, true);
  writeString(view, 36, "data");
  view.setUint32(40, dataSize, true);

  let offset = 44;
  for (let i = 0; i < samples.length; i++) {
    const s = Math.max(-1, Math.min(1, samples[i]));
    view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7fff, true);
    offset += 2;
  }

  return new Blob([buffer], { type: "audio/wav" });
}

export async function resampleTo16kMono(audioBuffer: AudioBuffer): Promise<Float32Array> {
  if (audioBuffer.sampleRate === TARGET_SAMPLE_RATE && audioBuffer.numberOfChannels === 1) {
    return audioBuffer.getChannelData(0).slice();
  }

  const duration = audioBuffer.duration;
  const offline = new OfflineAudioContext(
    1,
    Math.ceil(duration * TARGET_SAMPLE_RATE),
    TARGET_SAMPLE_RATE,
  );
  const source = offline.createBufferSource();
  source.buffer = audioBuffer;
  source.connect(offline.destination);
  source.start(0);
  const rendered = await offline.startRendering();
  return rendered.getChannelData(0).slice();
}

export { TARGET_SAMPLE_RATE };