/**
 * Нативный ZIP — упаковка и распаковка без внешних библиотек.
 * 
 * Использование:
 *   const files = await unzip(zipBlob);         // {name: Blob, ...}
 *   const zip = await buildZip([{name, data}]); // Blob
 */

/**
 * Распаковать ZIP blob → {filename: Blob}.
 * Поддерживает только STORE (без сжатия) — как отдаёт drhider.
 */
async function unzip(zb) {
  const buf = await zb.arrayBuffer();
  const dv = new DataView(buf);
  const files = {};
  let pos = 0;

  while (pos < buf.byteLength - 4) {
    const sig = dv.getUint32(pos, true);
    if (sig === 0x04034b50) {
      // Local file header
      const start = pos;
      pos += 26;
      const nameLen = dv.getUint16(pos, true);
      pos += 2;
      const extraLen = dv.getUint16(pos, true);
      pos += 2;
      const name = new TextDecoder().decode(
        new Uint8Array(buf, pos, nameLen)
      );
      pos += nameLen + extraLen;
      const compSize = dv.getUint32(start + 18, true);
      files[name] = new Blob([new Uint8Array(buf, pos, compSize)]);
      pos += compSize;
    } else {
      break; // Central directory или EOCD — конец файлов
    }
  }

  return files;
}

/**
 * Создать ZIP blob из entries [{name, data}].
 * Метод: STORE (без сжатия), UTF-8 имена.
 */
async function buildZip(entries) {
  const parts = [];
  const centralDir = [];
  let offset = 0;

  for (const e of entries) {
    const data = new Uint8Array(await e.data.arrayBuffer());
    const nameBytes = new TextEncoder().encode(e.name);

    // Local file header (30 bytes + filename)
    const lh = new Uint8Array(30 + nameBytes.length);
    const lv = new DataView(lh.buffer);
    lv.setUint32(0, 0x04034b50, true);   // signature
    lv.setUint16(6, 0x800, true);         // flags: UTF-8
    lv.setUint32(18, data.length, true);  // compressed size
    lv.setUint32(22, data.length, true);  // uncompressed size
    lv.setUint16(26, nameBytes.length, true);
    lh.set(nameBytes, 30);
    parts.push(lh, data);

    // Central directory entry (46 bytes + filename)
    const ce = new Uint8Array(46 + nameBytes.length);
    const cv = new DataView(ce.buffer);
    cv.setUint32(0, 0x02014b50, true);
    cv.setUint16(8, 0x800, true);
    cv.setUint32(20, data.length, true);
    cv.setUint32(24, data.length, true);
    cv.setUint16(28, nameBytes.length, true);
    cv.setUint32(42, offset, true);
    ce.set(nameBytes, 46);
    centralDir.push(ce);

    offset += 30 + nameBytes.length + data.length;
  }

  // End of central directory (22 bytes)
  const cdSize = centralDir.reduce((s, c) => s + 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);

  return new Blob([...parts, ...centralDir, eocd], { type: 'application/zip' });
}