Zip files on web platform

zip.mjs

const GZIP_HEADER = Uint8Array.from([
  31, 139, // gzip magic
  8, // deflate
  0, // no extra fields
  0, 0, 0, 0, // mtime (n/a)
  0, 0, // extra flags, OS
]);

class ZipEntry {
  constructor(f, meta) {
    this.f = f;
    this.meta = meta;
  }

  get size() {
    return this.meta.uncompressed;
  }
  
  fetch() {
    return new Response(this.stream());
  }

  stream() {
    let s = this.f.slice(this.meta.offset, this.meta.offset+this.meta.compressed).stream()
    if (this.meta.method === 8) {
      // deflate uses adler32, but gzip uses the same crc32 as zip!
      s = s.pipeThrough(new TransformStream({
        meta: this.meta,
        start(controller) {
          controller.enqueue(GZIP_HEADER);
        },
        flush(controller) {
          const tmp = new DataView(new ArrayBuffer(8));
          tmp.setUint32(0, this.meta.crc, true);
          tmp.setUint32(4, this.meta.uncompressed, true);
          controller.enqueue(new Uint8Array(tmp.buffer));
        }
      }));
      s = s.pipeThrough(new DecompressionStream('gzip'));
    }
    return s;
  }
}

export async function openZip(f) {
  let eocd;
  const b = new DataView(await f.slice(-(22+65535)).arrayBuffer());
  for (let i = b.byteLength - 22; i >= 0; i--) {
    if (b.getUint32(i, true) == 0x06054b50 && i + 22 + b.getUint16(i+20, true) <= b.byteLength) {
      eocd = new DataView(b.buffer.slice(i));
      break;
    }
  }
  if (eocd == null) {
    throw new Error('Unable to locate end of central directory')
  }
  const nrecs = eocd.getUint16(10, true);
  if (eocd.getUint16(4, true) !== 0 || eocd.getUint16(6, true) !== 0 || eocd.getUint16(8, true) !== nrecs) {
    throw new Error('Split archives not supported');
  }
  const cdlen = eocd.getUint32(12, true);
  const cdstart = f.size - eocd.byteLength - cdlen;
  const pfx = cdstart - eocd.getUint32(16, true);
  if (pfx < 0) {
    throw new Error('ZIP file corrupted: CD length does not match up')
  }
  const cd = new DataView(await f.slice(cdstart, cdstart+cdlen).arrayBuffer());
  const files = new Map;
  const decoder = new TextDecoder();
  for (let i = 0; i < cdlen;) {
    if (cd.getUint32(i, true) !== 0x02014b50) {
      throw new Error('Central directory corrupted');
    }
    const fnlen = cd.getUint16(i+28, true);
    const eflen = cd.getUint16(i+30, true);
    const cmtlen = cd.getUint16(i+32, true);
    const nexti = i + 46 + fnlen + eflen + cmtlen;
    if (nexti > cdlen) {
      throw new Error('Central directory corrupted');
    }
    const minver = cd.getUint8(i+6);
    const flags = cd.getUint8(i+8);
    const encrypted = flags & 1 !== 0;
    const delayedSizes = flags & 8 !== 0;
    const method = cd.getUint16(i+10, true);
    const disk = cd.getUint16(i+34, true);
    if (minver <= 20 && (method === 0 || method === 8) && !encrypted && !delayedSizes && disk === 0) {
      const fn = decoder.decode(new Uint8Array(cd.buffer, i+46, fnlen));
      if (!fn.endsWith('/')) {
        files.set(fn, new ZipEntry(f, {
          method: method,
          offset: pfx+cd.getUint32(i+42, true)+30+fnlen+eflen,
          compressed: cd.getUint32(i+20, true),
          uncompressed: cd.getUint32(i+24, true),
          crc: cd.getUint32(i+16, true),
        }));
      }
    }
    i = nexti;
  }
  return files;
}