const zlib = require('zlib');
const fs = require('fs');

function createTar(files) {
  const chunks = [];

  for (const file of files) {
    const name = file.name;
    const content = Buffer.isBuffer(file.content) ? file.content : Buffer.from(file.content, 'utf8');
    const size = content.length;

    const header = Buffer.alloc(512);
    // name (0..99)
    header.write(name, 0, 100, 'utf8');
    // mode (100..107)
    header.write('0000644\0', 100, 8, 'utf8');
    // uid (108..115)
    header.write('0000000\0', 108, 8, 'utf8');
    // gid (116..123)
    header.write('0000000\0', 116, 8, 'utf8');
    // size (124..135)
    header.write(size.toString(8).padStart(11, '0') + '\0', 124, 12, 'utf8');
    // mtime (136..147)
    const mtime = Math.floor(Date.now() / 1000);
    header.write(mtime.toString(8).padStart(11, '0') + '\0', 136, 12, 'utf8');
    // chksum placeholder (148..155) - 8 spaces
    header.write('        ', 148, 8, 'utf8');
    // typeflag (156) - '0' regular file
    header.write('0', 156, 1, 'utf8');
    // magic (257..262) - 'ustar\0'
    header.write('ustar\0', 257, 6, 'utf8');
    // version (263..264) - '00'
    header.write('00', 263, 2, 'utf8');

    // calculate checksum
    let sum = 0;
    for (let i = 0; i < 512; i++) {
      sum += header[i];
    }
    header.write(sum.toString(8).padStart(6, '0') + '\0 ', 148, 8, 'utf8');

    chunks.push(header);
    chunks.push(content);

    // pad content to 512 bytes
    const remainder = size % 512;
    if (remainder > 0) {
      chunks.push(Buffer.alloc(512 - remainder));
    }
  }

  // 2 blocks of 512 zeros at end of tar
  chunks.push(Buffer.alloc(1024));

  const tarBuffer = Buffer.concat(chunks);
  return zlib.gzipSync(tarBuffer);
}

const testTarGz = createTar([
  { name: 'Dockerfile', content: 'FROM alpine:latest\nCMD ["echo", "hello"]\n' },
  { name: 'test.txt', content: 'hello world\n' }
]);

console.log('Tar.gz created successfully, size:', testTarGz.length);
// verify decompression
const uncompressed = zlib.gunzipSync(testTarGz);
console.log('Uncompressed tar size:', uncompressed.length);