wav-worker.js

self.onmessage = function(e) {
  const { channelData, sampleRate } = e.data;
  const targetSampleRate = 16000;

  try {
    // 1. Progress: Downsampling
    self.postMessage({ type: 'progress', status: 'downsampling', progress: 20 });
    const downsampled = downsample(channelData, sampleRate, targetSampleRate);

    // 2. Progress: Encoding
    self.postMessage({ type: 'progress', status: 'encoding', progress: 50 });
    const wavBuffer = encodeWAV(downsampled, targetSampleRate);

    // 3. Progress: Completed
    self.postMessage({ type: 'progress', status: 'completed', progress: 100 });
    self.postMessage({ type: 'done', buffer: wavBuffer }, [wavBuffer]);
  } catch (error) {
    self.postMessage({ type: 'error', message: error.message });
  }
};

// Simple average-based downsampler for speech recognition
function downsample(buffer, inputSampleRate, outputSampleRate) {
  if (inputSampleRate === outputSampleRate) {
    return buffer;
  }
  const sampleRateRatio = inputSampleRate / outputSampleRate;
  const newLength = Math.round(buffer.length / sampleRateRatio);
  const result = new Float32Array(newLength);
  
  let offsetResult = 0;
  let offsetBuffer = 0;
  
  while (offsetResult < result.length) {
    const nextOffsetBuffer = Math.round((offsetResult + 1) * sampleRateRatio);
    let accum = 0;
    let count = 0;
    for (let i = offsetBuffer; i < nextOffsetBuffer && i < buffer.length; i++) {
      accum += buffer[i];
      count++;
    }
    result[offsetResult] = count > 0 ? accum / count : 0;
    offsetResult++;
    offsetBuffer = nextOffsetBuffer;
  }
  return result;
}

// Write standard 16-bit PCM WAV file
function encodeWAV(samples, sampleRate) {
  const buffer = new ArrayBuffer(44 + samples.length * 2);
  const view = new DataView(buffer);

  /* RIFF identifier */
  writeString(view, 0, 'RIFF');
  /* file length */
  view.setUint32(4, 36 + samples.length * 2, true);
  /* RIFF type */
  writeString(view, 8, 'WAVE');
  /* format chunk identifier */
  writeString(view, 12, 'fmt ');
  /* format chunk length */
  view.setUint32(16, 16, true);
  /* sample format (raw PCM) */
  view.setUint16(20, 1, true);
  /* channel count */
  view.setUint16(22, 1, true); // Mono
  /* sample rate */
  view.setUint32(24, sampleRate, true);
  /* byte rate (sample rate * block align) */
  view.setUint32(28, sampleRate * 2, true);
  /* block align (channel count * bytes per sample) */
  view.setUint16(32, 2, true);
  /* bits per sample */
  view.setUint16(34, 16, true);
  /* data chunk identifier */
  writeString(view, 36, 'data');
  /* data chunk length */
  view.setUint32(40, samples.length * 2, true);

  // Write samples
  const volume = 1.0;
  let offset = 44;
  for (let i = 0; i < samples.length; i++, offset += 2) {
    const s = Math.max(-1, Math.min(1, samples[i]));
    view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7FFF, true);
  }

  return buffer;
}

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