export const NUM_BARS = 150;
export const BAR_W = 3;
export const BAR_GAP = 1;
export const WAVEFORM_H = 48;
export const VIEWBOX_W = NUM_BARS * (BAR_W + BAR_GAP); // 600

// Module-level cache: survives StrictMode double-effect and re-renders
const peaksCache = new Map<string, Float32Array>();

export async function extractPeaks(
  url: string,
  n: number,
): Promise<Float32Array> {
  if (peaksCache.has(url)) return peaksCache.get(url)!;

  // /api/files is public (no auth middleware). Plain fetch avoids the CORS
  // preflight that credentials/Authorization would trigger — the server's
  // oakCors config doesn't set Allow-Credentials so credentialed fetches
  // are blocked, while <audio> bypasses CORS entirely.
  const res = await fetch(url);
  if (!res.ok) throw new Error(`fetch failed: ${res.status}`);
  const buf = await res.arrayBuffer();

  const actx = new AudioContext();
  try {
    const decoded = await actx.decodeAudioData(buf);
    const data = decoded.getChannelData(0);
    const blockSize = Math.floor(data.length / n);
    const peaks = new Float32Array(n);
    for (let i = 0; i < n; i++) {
      let peak = 0;
      const offset = i * blockSize;
      for (let j = 0; j < blockSize; j++) {
        const abs = Math.abs(data[offset + j]);
        if (abs > peak) peak = abs;
      }
      peaks[i] = peak;
    }
    peaksCache.set(url, peaks);
    return peaks;
  } finally {
    actx.close();
  }
}