package fitparser

// CRC lookup table for FIT files
var crcTable = [16]uint16{
	0x0000, 0xCC01, 0xD801, 0x1400, 0xF001, 0x3C00, 0x2800, 0xE401,
	0xA001, 0x6C00, 0x7800, 0xB401, 0x5000, 0x9C01, 0x8801, 0x4400,
}

// CalculateCRC calculates the CRC-16 checksum for FIT files
// This uses the CRC-16-ANSI algorithm with polynomial 0x8005
func CalculateCRC(data []byte) uint16 {
	crc := uint16(0)
	for _, b := range data {
		// Compute CRC for lower nibble (bits 0-3)
		tmp := crcTable[crc&0xF]
		crc = (crc >> 4) & 0x0FFF
		crc = crc ^ tmp ^ crcTable[b&0xF]

		// Compute CRC for upper nibble (bits 4-7)
		tmp = crcTable[crc&0xF]
		crc = (crc >> 4) & 0x0FFF
		crc = crc ^ tmp ^ crcTable[(b>>4)&0xF]
	}

	return crc
}

// UpdateCRC updates an existing CRC with new data
func UpdateCRC(crc uint16, data []byte) uint16 {
	for _, b := range data {
		// Compute CRC for lower nibble (bits 0-3)
		tmp := crcTable[crc&0xF]
		crc = (crc >> 4) & 0x0FFF
		crc = crc ^ tmp ^ crcTable[b&0xF]

		// Compute CRC for upper nibble (bits 4-7)
		tmp = crcTable[crc&0xF]
		crc = (crc >> 4) & 0x0FFF
		crc = crc ^ tmp ^ crcTable[(b>>4)&0xF]
	}

	return crc
}