Understanding CRC16

crc16_comparison.c

//function 1

uint16_t crc16(uint8_t const *data, size_t size)
{
	/**
	 * Author: Mark Alder
	 * stackoverflow.com/users/1180620/mark-adler
	 */
	uint16_t crc = 0xFFFF;  // I also tried with 0x0000 too - Ganindu
	while (size--)
	{
		crc ^= *data++;
		for (unsigned k = 0; k < 8; k++)
			crc = crc & 1 ? (crc >> 1) ^ 0xa001 : crc >> 1;
	}
	return crc >> 8 | crc << 8; //Edit I made to swap bytes - Ganindu 

}

// function 2

uint16_t gen_crc16(const uint8_t *data, int16_t size)
{

	uint16_t out = 0;
	int bits_read = 0, bit_flag;

	uint16_t crc = 0;
	if(data == NULL)
		return 0;


	while(size > 0)
	{

//		printf("processing at top of loop %02X size %d \n", (uint8_t)((*data) & 0xFF), size);
		/**
		 * bit flag is the msb of out
		 */
		bit_flag = out >> 15;



		/**
		 * shift the out to the left by 1
		 * making lsb of out = 0
		 */
		out <<= 1;
		/**
		 * shift data by bits_read -> puts next bit to bit 0
		 * check if that is 1
		 * append to out
		 */
		out |= (*data >> bits_read) & 0x01;

		bits_read++;
		/**
		 * if a byte is read,
		 * 	- reset bits_read
		 * 	- increment the byte reference to the start of the next byte in the payload
		 * 	- decrement the remaining byte count to work on by 1
		 */
		if(bits_read > 7)
		{
			bits_read = 0;
			data++;
			size--;

		}

		/**
		 * for the condition to satisfy the msb of out must be 1
		 */
		if(bit_flag)
			out ^= CRC16;
	}


	/**
	 * now we have at most two bytes that were not XORed
	 * so we push the last 16 bits
	 *
	 */

	int i;
	for(i = 0; i < 16; ++i){
		bit_flag = out >> 15;
		out <<= 1;
		if(bit_flag)
			out ^= CRC16;
	}

	/**
	 * now we reverse the bits to match the protocol
	 */


	i = 0x8000;
	int j = 0x0001;

	for(; i != 0; i >>=1, j<<=1){
		if(i & out)
			crc |= j;
	}

	return crc;

}

//Test patterns 
  unsigned char pattern1[8] = {0x01, 0x03, 0x00, 0x32, 0x00, 0x24, 0xE4, 0x1E};
  unsigned char pattern2[8] = {0x01, 0x03, 0x00, 0x01, 0x00, 0x0A, 0x94, 0x0D};

  uint8_t crc_1 = gen_crc16(pattern1, 8);
  printf("crc_ =  0x%02X 0x%02X\n", (uint8_t)(crc_1 >> 8) , (uint8_t)(crc_1 & 0X00FF));


// this one gives 0x00 0x00 (works)

  uint16_t crc = crc16(pattern1, 8);
  printf("crc2 =  0x%02X 0x%02X\n", (uint8_t)(crc >> 8) , (uint8_t)(crc & 0X00FF));