function calculateIPv4Checksum(buffer) {
  const ihl = (buffer[0] & 0x0f) * 4;
  let sum = 0;

  for (let i = 0; i < ihl; i += 2) {
    if (i === 10) continue;
    sum += (buffer[i] << 8) | buffer[i + 1];
  }

  while (sum >> 16) {
    sum = (sum & 0xffff) + (sum >> 16);
  }

  return (~sum) & 0xffff;
}

export function updateIPv4Buffer(buffer) {
  const updated = new Uint8Array(buffer);

  updated[10] = 0x00;
  updated[11] = 0x00;

  const checksum = calculateIPv4Checksum(updated);
  updated[10] = (checksum >> 8) & 0xff;
  updated[11] = checksum & 0xff;

  return updated;
}

export function formatIPv4Address(bytes) {
  if (bytes.length < 4) return 'N/A';
  return `${bytes[0]}.${bytes[1]}.${bytes[2]}.${bytes[3]}`;
}

export function formatIPv4Protocol(proto) {
  const protocols = {
    1: 'ICMP',
    6: 'TCP',
    17: 'UDP',
    47: 'GRE',
    50: 'ESP',
    51: 'AH',
    89: 'OSPF',
    132: 'SCTP',
  };
  const name = protocols[proto] ?? 'Unknown';
  return `${name} (${proto} / 0x${proto.toString(16).padStart(2, '0').toUpperCase()})`;
}

export function formatVersionIHL(byte) {
  const version = (byte >> 4) & 0x0f;
  const ihl = (byte & 0x0f) * 4;
  return `IPv${version}, Header Length: ${ihl} bytes`;
}

export function formatFlagsFragment(bytes) {
  if (bytes.length < 2) return 'N/A';
  const raw = (bytes[0] << 8) | bytes[1];
  const df = (raw >> 14) & 1;
  const mf = (raw >> 13) & 1;
  const offset = raw & 0x1fff;
  return `DF=${df}, MF=${mf}, Offset=${offset}`;
}