mr-drews-assignment-creator/lib/canvas/zip.js

// lib/canvas/zip.js — minimal, zero-dependency ZIP writer (STORE method, no compression).
// Canvas accepts uncompressed QTI packages, so we skip DEFLATE entirely and avoid any
// dependency. Output is a standard .zip as a Uint8Array. Deterministic (fixed timestamps).

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;
})();

// CRC-32 (IEEE 802.3). crc32 of "123456789" === 0xCBF43926 (unit-test anchor).
export function crc32(bytes) {
  let c = 0xffffffff;
  for (let i = 0; i < bytes.length; i++) c = CRC_TABLE[(c ^ bytes[i]) & 0xff] ^ (c >>> 8);
  return (c ^ 0xffffffff) >>> 0;
}

const enc = new TextEncoder();
const toBytes = (data) => (typeof data === "string" ? enc.encode(data) : data);

// files: [{ name: string, data: string|Uint8Array }] -> Uint8Array (.zip bytes)
export function storeZip(files) {
  const DOS_TIME = 0;
  const DOS_DATE = 0x0021; // 1980-01-01, deterministic
  const chunks = [];
  const central = [];
  let offset = 0;

  for (const f of files) {
    const nameBytes = enc.encode(f.name);
    const data = toBytes(f.data);
    const crc = crc32(data);

    const local = new Uint8Array(30 + nameBytes.length);
    const lv = new DataView(local.buffer);
    lv.setUint32(0, 0x04034b50, true); // local file header signature
    lv.setUint16(4, 20, true);         // version needed to extract
    lv.setUint16(6, 0x0800, true);     // flags: bit 11 = UTF-8 filenames
    lv.setUint16(8, 0, true);          // compression method 0 = store
    lv.setUint16(10, DOS_TIME, true);
    lv.setUint16(12, DOS_DATE, true);
    lv.setUint32(14, crc, true);
    lv.setUint32(18, data.length, true); // compressed size (== uncompressed for store)
    lv.setUint32(22, data.length, true); // uncompressed size
    lv.setUint16(26, nameBytes.length, true);
    lv.setUint16(28, 0, true);           // extra field length
    local.set(nameBytes, 30);
    chunks.push(local, data);

    const cd = new Uint8Array(46 + nameBytes.length);
    const cv = new DataView(cd.buffer);
    cv.setUint32(0, 0x02014b50, true); // central directory header signature
    cv.setUint16(4, 20, true);   // version made by
    cv.setUint16(6, 20, true);   // version needed
    cv.setUint16(8, 0x0800, true);
    cv.setUint16(10, 0, true);   // method
    cv.setUint16(12, DOS_TIME, true);
    cv.setUint16(14, DOS_DATE, true);
    cv.setUint32(16, crc, true);
    cv.setUint32(20, data.length, true);
    cv.setUint32(24, data.length, true);
    cv.setUint16(28, nameBytes.length, true);
    cv.setUint16(30, 0, true);   // extra length
    cv.setUint16(32, 0, true);   // comment length
    cv.setUint16(34, 0, true);   // disk number start
    cv.setUint16(36, 0, true);   // internal attributes
    cv.setUint32(38, 0, true);   // external attributes
    cv.setUint32(42, offset, true); // relative offset of local header
    cd.set(nameBytes, 46);
    central.push(cd);

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

  const centralStart = offset;
  let centralSize = 0;
  for (const cd of central) { chunks.push(cd); centralSize += cd.length; }

  const eocd = new Uint8Array(22);
  const ev = new DataView(eocd.buffer);
  ev.setUint32(0, 0x06054b50, true); // end of central directory signature
  ev.setUint16(8, central.length, true);  // entries on this disk
  ev.setUint16(10, central.length, true); // total entries
  ev.setUint32(12, centralSize, true);
  ev.setUint32(16, centralStart, true);
  ev.setUint16(20, 0, true); // comment length
  chunks.push(eocd);

  let total = 0;
  for (const c of chunks) total += c.length;
  const out = new Uint8Array(total);
  let p = 0;
  for (const c of chunks) { out.set(c, p); p += c.length; }
  return out;
}