// CRC16 implements the CRC-16-CCITT algorithm.
// The polynomial is 0x1021.
// It accepts a byte slice and returns a 16-bit checksum.
package crc16

var Crc16Table = make([]uint16, 256)

// GeneratorPolynomial is the polynomial used in the CRC-16-CCITT algorithm.
const GeneratorPolynomial = 0x1021

func init() {
	calculateTable()
}

// calculateTable computes the CRC-16 lookup table.
// The table is used to speed up the CRC calculation.
func calculateTable() {
	for dividend := 0; dividend < 256; dividend++ {
		crc := uint16(dividend << 8)
		for bit := 0; bit < 8; bit++ {
			if (crc & 0x8000) != 0 {
				crc = (crc << 1) ^ GeneratorPolynomial
			} else {
				crc <<= 1
			}
		}
		Crc16Table[dividend] = crc
	}
}

// Calculates generates a CRC-16 checksum for the given data.
// It accepts a byte slice and returns a 16-bit checksum.
func Calculate(data []byte) uint16 {
	crc := uint16(0)
	for _, b := range data {
		pos := byte(crc>>8) ^ b
		crc = (crc << 8) ^ Crc16Table[pos]
	}
	return crc
}