/**
 * Minimal ZIP writer (stored, no compression). Images are already compressed,
 * so deflating them gains nothing, and this avoids a dependency.
 */

const CRC_TABLE = (() => {
  const table = 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;
    table[n] = c >>> 0;
  }
  return table;
})();

function crc32(bytes) {
  let crc = 0xffffffff;
  for (let i = 0; i < bytes.length; i++) {
    crc = CRC_TABLE[(crc ^ bytes[i]) & 0xff] ^ (crc >>> 8);
  }
  return (crc ^ 0xffffffff) >>> 0;
}

function header(size, write) {
  const view = new DataView(new ArrayBuffer(size));
  write(view);
  return new Uint8Array(view.buffer);
}

/** @param {{name: string, data: Uint8Array}[]} files @returns {Blob} */
export function createZip(files) {
  const encoder = new TextEncoder();
  const now = new Date();
  const time = (now.getHours() << 11) | (now.getMinutes() << 5) | (now.getSeconds() >> 1);
  const date = ((now.getFullYear() - 1980) << 9) | ((now.getMonth() + 1) << 5) | now.getDate();

  const parts = [];
  const central = [];
  let offset = 0;

  for (const { name, data } of files) {
    const nameBytes = encoder.encode(name);
    const crc = crc32(data);

    const local = header(30, (v) => {
      v.setUint32(0, 0x04034b50, true);
      v.setUint16(4, 20, true);
      v.setUint16(6, 0x0800, true); // UTF-8 names
      v.setUint16(8, 0, true); // stored
      v.setUint16(10, time, true);
      v.setUint16(12, date, true);
      v.setUint32(14, crc, true);
      v.setUint32(18, data.length, true);
      v.setUint32(22, data.length, true);
      v.setUint16(26, nameBytes.length, true);
    });
    parts.push(local, nameBytes, data);

    central.push(
      header(46, (v) => {
        v.setUint32(0, 0x02014b50, true);
        v.setUint16(4, 20, true);
        v.setUint16(6, 20, true);
        v.setUint16(8, 0x0800, true);
        v.setUint16(10, 0, true);
        v.setUint16(12, time, true);
        v.setUint16(14, date, true);
        v.setUint32(16, crc, true);
        v.setUint32(20, data.length, true);
        v.setUint32(24, data.length, true);
        v.setUint16(28, nameBytes.length, true);
        v.setUint32(42, offset, true);
      }),
      nameBytes
    );

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

  const centralSize = central.reduce((sum, part) => sum + part.length, 0);
  const end = header(22, (v) => {
    v.setUint32(0, 0x06054b50, true);
    v.setUint16(8, files.length, true);
    v.setUint16(10, files.length, true);
    v.setUint32(12, centralSize, true);
    v.setUint32(16, offset, true);
  });

  return new Blob([...parts, ...central, end], { type: "application/zip" });
}