package utils

import (
	"encoding/binary"
)

const WCPOLY uint16 = 0x8408

type CRC16CCITT struct {
	wCRCin uint16
}

func NewCRC16CCITT() *CRC16CCITT {
	return &CRC16CCITT{}
}

func (c *CRC16CCITT) Update(b byte) {
	c.wCRCin ^= uint16(b)
	for j := 0; j < 8; j++ {
		if (c.wCRCin & 0x0001) != 0 {
			c.wCRCin >>= 1
			c.wCRCin ^= WCPOLY
		} else {
			c.wCRCin >>= 1
		}
	}
}

func (c *CRC16CCITT) GetWCRCin(rawBytes []byte) []byte {
	for _, b := range rawBytes {
		c.Update(b)
	}
	//log.Printf("CRC16CCITT=0x%x", c.wCRCin)
	return uint16toBytes(c.wCRCin)
}

func uint16toBytes(v uint16) []byte {
	littleEndianBytes := make([]byte, 2)
	binary.BigEndian.PutUint16(littleEndianBytes, v)
	return littleEndianBytes

}