package eq2net

// CRC16 table for CCITT polynomial (0x1021)
var crc16Table [256]uint16

func init() {
	// Initialize CRC16 lookup table
	for i := 0; i < 256; i++ {
		crc := uint16(i << 8)
		for j := 0; j < 8; j++ {
			if (crc & 0x8000) != 0 {
				crc = (crc << 1) ^ 0x1021
			} else {
				crc = crc << 1
			}
		}
		crc16Table[i] = crc
	}
}

// CRC16 calculates the CRC16-CCITT checksum with a key
func CRC16(data []byte, length int, key uint32) uint16 {
	if length <= 0 || len(data) < length {
		return 0
	}

	// Mix the key into initial CRC value
	crc := uint16(0xFFFF)
	keyBytes := []byte{
		byte(key),
		byte(key >> 8),
		byte(key >> 16),
		byte(key >> 24),
	}
	
	// Process key bytes first
	for _, b := range keyBytes {
		tableIndex := (uint8(crc>>8) ^ b) & 0xFF
		crc = (crc << 8) ^ crc16Table[tableIndex]
	}
	
	// Process data
	for i := 0; i < length; i++ {
		tableIndex := (uint8(crc>>8) ^ data[i]) & 0xFF
		crc = (crc << 8) ^ crc16Table[tableIndex]
	}
	
	return crc ^ 0xFFFF
}

// ValidateCRC checks if a packet has a valid CRC
func ValidateCRC(buffer []byte, key uint32) bool {
	if len(buffer) < 3 {
		return false
	}
	
	// Check for CRC-exempt packets
	if len(buffer) >= 2 && buffer[0] == 0x00 {
		switch buffer[1] {
		case byte(OPSessionRequest), byte(OPSessionResponse), byte(OPOutOfSession):
			return true // Session packets don't require CRC
		}
	}
	
	// Check for combined application packets (also CRC-exempt)
	if len(buffer) >= 4 && buffer[2] == 0x00 && buffer[3] == 0x19 {
		return true
	}
	
	// Calculate CRC for the packet (excluding last 2 CRC bytes)
	dataLen := len(buffer) - 2
	calculatedCRC := CRC16(buffer, dataLen, key)
	
	// Extract packet CRC (big-endian in last 2 bytes)
	packetCRC := uint16(buffer[dataLen])<<8 | uint16(buffer[dataLen+1])
	
	// Valid if no CRC present (0) or CRCs match
	return packetCRC == 0 || calculatedCRC == packetCRC
}

// AppendCRC adds CRC to the end of a packet
func AppendCRC(buffer []byte, key uint32) []byte {
	// Calculate CRC for current buffer
	crc := CRC16(buffer, len(buffer), key)
	
	// Append CRC in big-endian format
	result := make([]byte, len(buffer)+2)
	copy(result, buffer)
	result[len(buffer)] = byte(crc >> 8)
	result[len(buffer)+1] = byte(crc)
	
	return result
}