package udp

// EQ2EMu uses a specific CRC32 polynomial (reversed)
const crcPolynomial = 0xEDB88320

// Pre-computed CRC32 lookup table for fast calculation
var crcTable [256]uint32

// init builds the CRC lookup table at package initialization
func init() {
	for i := range crcTable {
		crc := uint32(i)
		for range 8 {
			if crc&1 == 1 {
				crc = (crc >> 1) ^ crcPolynomial
			} else {
				crc >>= 1
			}
		}
		crcTable[i] = crc
	}
}

// CalculateCRC32 computes CRC32 using EQ2EMu's algorithm
// Returns 16-bit value by truncating the upper bits
func CalculateCRC32(data []byte) uint16 {
	crc := uint32(0xFFFFFFFF)

	// Use lookup table for efficient calculation
	for _, b := range data {
		crc = crcTable[byte(crc)^b] ^ (crc >> 8)
	}

	// Return inverted result truncated to 16 bits
	return uint16(^crc)
}

// ValidateCRC checks if packet has valid CRC
// Expects CRC to be the last 2 bytes of data
func ValidateCRC(data []byte) bool {
	if len(data) < 2 {
		return false
	}

	// Split payload and CRC
	payload := data[:len(data)-2]
	expectedCRC := uint16(data[len(data)-2]) | (uint16(data[len(data)-1]) << 8)

	// Calculate and compare
	actualCRC := CalculateCRC32(payload)
	return expectedCRC == actualCRC
}

// AppendCRC adds 16-bit CRC to the end of data
func AppendCRC(data []byte) []byte {
	crc := CalculateCRC32(data)
	result := make([]byte, len(data)+2)
	copy(result, data)

	// Append CRC in little-endian format
	result[len(data)] = byte(crc)
	result[len(data)+1] = byte(crc >> 8)

	return result
}

// ValidateAndStrip validates CRC and returns data without CRC suffix
func ValidateAndStrip(data []byte) ([]byte, bool) {
	if !ValidateCRC(data) {
		return nil, false
	}
	return data[:len(data)-2], true
}