package crypto

import (
	"encoding/binary"
	"hash/crc32"
)

// Standard CRC32 polynomial
var crcTable = crc32.MakeTable(0xEDB88320)

// Fixed key used by EQ2
var EQ2CRCKey = 0x33624702

// CalculateCRC gets the CRC for a given byte slice using a custom key
func CalculateCRC(data []byte, key uint32) uint16 {
	// Pre-process the key (4 rounds of CRC on key bytes)
	crc := uint32(0xFFFFFFFF)
	keyBytes := make([]byte, 4)
	binary.LittleEndian.PutUint32(keyBytes, key)

	for _, b := range keyBytes {
		crc = crcTable[(crc^uint32(b))&0xFF] ^ (crc >> 8)
	}

	// Process actual data
	for _, b := range data {
		crc = crcTable[(crc^uint32(b))&0xFF] ^ (crc >> 8)
	}

	// Return lower 16 bits of inverted result
	return uint16(^crc & 0xFFFF)
}