// tcpUtils.js
/**
 * 解析TCP头部信息
 * @param {Buffer} packet - IP数据包（包含TCP负载）
 * @param {number} ipHeaderLength - IP头部长度
 * @returns {Object} TCP头部信息
 */
function parseTCPHeader(packet, ipHeaderLength) {
  if (packet.length < ipHeaderLength + 20) {
    throw new Error('TCP数据包太短，无法解析TCP头部');
  }

  const tcpStart = ipHeaderLength;
  const sourcePort = (packet[tcpStart] << 8) + packet[tcpStart + 1];
  const destPort = (packet[tcpStart + 2] << 8) + packet[tcpStart + 3];
  const seqNum = (
    (packet[tcpStart + 4] << 24) +
    (packet[tcpStart + 5] << 16) +
    (packet[tcpStart + 6] << 8) +
    packet[tcpStart + 7]
  ) >>> 0; // 使用无符号右移确保得到正数
  const ackNum = (
    (packet[tcpStart + 8] << 24) +
    (packet[tcpStart + 9] << 16) +
    (packet[tcpStart + 10] << 8) +
    packet[tcpStart + 11]
  ) >>> 0; // 使用无符号右移确保得到正数
  
  const dataOffset = (packet[tcpStart + 12] >> 4) & 0x0F; // TCP头部长度（以4字节为单位）
  const tcpHeaderLength = dataOffset * 4;
  
  // 控制位
  const flags = packet[tcpStart + 13];
  const isURG = (flags & 0x20) !== 0;
  const isACK = (flags & 0x10) !== 0;
  const isPSH = (flags & 0x08) !== 0;
  const isRST = (flags & 0x04) !== 0;
  const isSYN = (flags & 0x02) !== 0;
  const isFIN = (flags & 0x01) !== 0;
  
  const windowSize = (packet[tcpStart + 14] << 8) + packet[tcpStart + 15];
  const checksum = (packet[tcpStart + 16] << 8) + packet[tcpStart + 17];
  const urgentPointer = (packet[tcpStart + 18] << 8) + packet[tcpStart + 19];
  
  // 计算数据偏移量
  const dataOffsetBytes = tcpHeaderLength;
  
  return {
    sourcePort,
    destPort,
    seqNum,
    ackNum,
    tcpHeaderLength,
    isURG,
    isACK,
    isPSH,
    isRST,
    isSYN,
    isFIN,
    windowSize,
    checksum,
    urgentPointer,
    dataOffsetBytes,
    flags
  };
}

/**
 * 构造TCP数据包
 * @param {Object} options - TCP数据包选项
 * @returns {Buffer} 构造的TCP数据包
 */
function constructTCPPacket(options) {
  const {
    sourcePort,
    destPort,
    seqNum,
    ackNum,
    isACK = false,
    isSYN = false,
    isFIN = false,
    isRST = false,
    windowSize = 65535,
    data = Buffer.alloc(0)
  } = options;
  
  // TCP头部最小长度为20字节
  const tcpHeaderLength = 20;
  const packetLength = tcpHeaderLength + data.length;
  const packet = Buffer.alloc(packetLength);
  
  // 设置端口
  packet.writeUInt16BE(sourcePort, 0);
  packet.writeUInt16BE(destPort, 2);
  
  // 设置序列号和确认号
  packet.writeUInt32BE(seqNum, 4);
  packet.writeUInt32BE(ackNum, 8);
  
  // 设置数据偏移和保留位（数据偏移为5，表示20字节头部）
  packet[12] = (5 << 4) | 0; // 数据偏移=5，保留位=0
  
  // 设置标志位
  let flags = 0;
  if (isACK) flags |= 0x10;
  if (isSYN) flags |= 0x02;
  if (isFIN) flags |= 0x01;
  if (isRST) flags |= 0x04;
  packet[13] = flags;
  
  // 设置窗口大小
  packet.writeUInt16BE(windowSize, 14);
  
  // 校验和和紧急指针设为0（简化处理）
  packet.writeUInt16BE(0, 16); // 校验和
  packet.writeUInt16BE(0, 18); // 紧急指针
  
  // 添加数据
  if (data.length > 0) {
    data.copy(packet, tcpHeaderLength);
  }
  
  return packet;
}

/**
 * 检查是否为TCP SYN包（连接请求）
 * @param {Object} tcpHeader - TCP头部信息
 * @returns {boolean} 是否为SYN包
 */
function isSYNPacket(tcpHeader) {
  return tcpHeader.isSYN && !tcpHeader.isACK;
}

/**
 * 检查是否为TCP FIN包（连接终止）
 * @param {Object} tcpHeader - TCP头部信息
 * @returns {boolean} 是否为FIN包
 */
function isFINPacket(tcpHeader) {
  return tcpHeader.isFIN;
}

/**
 * 检查是否为TCP RST包（连接重置）
 * @param {Object} tcpHeader - TCP头部信息
 * @returns {boolean} 是否为RST包
 */
function isRSTPacket(tcpHeader) {
  return tcpHeader.isRST;
}

module.exports = {
  parseTCPHeader,
  constructTCPPacket,
  isSYNPacket,
  isFINPacket,
  isRSTPacket
};