#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>

uint16_t MyCrc16(const unsigned char *data, int length, uint16_t poly, uint16_t crc)
{
        for(int byte=0; byte<length; ++byte)
        {
                for(int bit=7; bit>=0; --bit)
                {
                        bool doXor = false;
                        if(crc & 0x8000)
                        {
                                doXor = true;
                        }
                        crc <<= 1;
                        if(data[byte] & (1 << bit))
                        {
                                crc += 1;
                        }
                        if(doXor)
                        {
                                crc ^= poly;
                        }
                }
        }
        
        //augument the crc with 0s
        for(int i=0; i<16; i++)
        {
                bool doXor = false;
                if(crc & 0x8000)
                {
                        doXor = true;
                }
                
                crc = crc << 1;
                if(doXor)
                {
                        crc ^= poly;
                }
        }

        return crc;
}

uint16_t OtherCrc16(const unsigned char *data, int length, uint16_t poly, uint16_t crc)
{
        for(int i=0; i<length; i++)
        {
                crc = crc ^ (data[i] << 8);
                for (int bit = 0; bit< 8; bit++)
                {
                        bool doXor = false;
                        if(crc & 0x8000)
                        {
                                doXor = true;
                        }
                        crc <<=1;

                        if(doXor)
                        {
                                crc ^= poly;
                        }
                }
        }
        return crc;
}


int main(void) {
	// your code goes here
	uint16_t poly = 0x1021;
	
	unsigned char c[] = "123456789";
	printf("My CRC = %04x\n", MyCrc16(c, 9, poly, 0xffff));
	printf("Other CRC = %04x\n", OtherCrc16(c, 9, poly, 0xffff));
	return 0;
}