type CRC16 struct {
	Tab      []uint16
	Constant uint16
}

// Init crc16
func (c *CRC16) Init() {
	c.Constant = 0xA001

	for i := 0; i < 256; i++ {
		crc := uint16(i)
		for j := 0; j < 8; j++ {
			if crc&0x0001 == 1 {
				crc = crc>>1 ^ c.Constant
			} else {
				crc = crc >> 1
			}
		}
		c.Tab = append(c.Tab, crc)
	}
}

// Calculate CRC16
func (c *CRC16) Calculate(data []byte) uint16 {
	var crcValue uint16 = 0x0000

	for _, d := range data {
		tmp := crcValue ^ uint16(d)
		rotated := crcValue >> 8
		crcValue = rotated ^ c.Tab[(tmp&0x00ff)]
	}
	return crcValue
}

c := CRC16{}
c.Init()
c.Calculate([]byte{})