/**
 * Store-only ZIP writer, enough for the dataset download without a
 * dependency. WAV float data barely compresses, so storing costs little;
 * every entry gets a real CRC so ordinary unzip tools verify it.
 */
interface ZipEntry { name: string; data: Uint8Array; }

const CRC_TABLE = (() => {
  const t = new Uint32Array(256);
  for (let n = 0; n < 256; n++) {
    let c = n;
    for (let k = 0; k < 8; k++) c = c & 1 ? 0xedb88320 ^ (c >>> 1) : c >>> 1;
    t[n] = c >>> 0;
  }
  return t;
})();

function crc32(data: Uint8Array): number {
  let c = 0xffffffff;
  for (let i = 0; i < data.length; i++) c = CRC_TABLE[(c ^ data[i]) & 0xff] ^ (c >>> 8);
  return (c ^ 0xffffffff) >>> 0;
}

export function zipStore(entries: ZipEntry[]): Uint8Array {
  const enc = new TextEncoder();
  const parts: Uint8Array[] = [];
  const central: Uint8Array[] = [];
  let offset = 0;

  for (const e of entries) {
    const name = enc.encode(e.name);
    const crc = crc32(e.data);
    const local = new Uint8Array(30 + name.length);
    const lv = new DataView(local.buffer);
    lv.setUint32(0, 0x04034b50, true);
    lv.setUint16(4, 20, true);              // version needed
    lv.setUint16(6, 0x0800, true);          // utf-8 names
    lv.setUint16(8, 0, true);               // stored
    lv.setUint16(10, 0, true);              // time
    lv.setUint16(12, 0x21, true);           // date (1980-01-01)
    lv.setUint32(14, crc, true);
    lv.setUint32(18, e.data.length, true);
    lv.setUint32(22, e.data.length, true);
    lv.setUint16(26, name.length, true);
    lv.setUint16(28, 0, true);
    local.set(name, 30);
    parts.push(local, e.data);

    const cd = new Uint8Array(46 + name.length);
    const cv = new DataView(cd.buffer);
    cv.setUint32(0, 0x02014b50, true);
    cv.setUint16(4, 20, true);
    cv.setUint16(6, 20, true);
    cv.setUint16(8, 0x0800, true);
    cv.setUint16(10, 0, true);
    cv.setUint16(12, 0, true);
    cv.setUint16(14, 0x21, true);
    cv.setUint32(16, crc, true);
    cv.setUint32(20, e.data.length, true);
    cv.setUint32(24, e.data.length, true);
    cv.setUint16(28, name.length, true);
    cv.setUint32(42, offset, true);
    cd.set(name, 46);
    central.push(cd);
    offset += local.length + e.data.length;
  }

  const cdSize = central.reduce((a, c) => a + c.length, 0);
  const eocd = new Uint8Array(22);
  const ev = new DataView(eocd.buffer);
  ev.setUint32(0, 0x06054b50, true);
  ev.setUint16(8, entries.length, true);
  ev.setUint16(10, entries.length, true);
  ev.setUint32(12, cdSize, true);
  ev.setUint32(16, offset, true);

  const total = offset + cdSize + 22;
  const out = new Uint8Array(total);
  let p = 0;
  for (const part of [...parts, ...central, eocd]) { out.set(part, p); p += part.length; }
  return out;
}