/*
 * crc.c - reference CRC implementations and LUT generator
 */

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

/*
 * CRC-16-IBM -- reversed reciprocal representation
 * https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations_of_cyclic_redundancy_checks
 */
#define CRCPOLY (0xc002)

static uint16_t crc16_lut[256] = { 0x0000 };

uint16_t crc16(uint8_t *data, uint32_t len);
void crc16_gen();
uint16_t crc16_ref(uint8_t *Pkt, uint32_t PktCnt);

int main()
{
	uint8_t i;
	uint8_t golden_packet[32] = { 
		0xf2, 0x22, 0x22, 0x22,
		0x20, 0x00, 0x00, 0x00, 
		0x78, 0x56, 0x34, 0x12, 
		0xd2, 0xc3, 0xb4, 0xa5, 
		0x10, 0x00, 0x00, 0x00, 
		0x01, 0x5a, 0x00, 0x00, 
		0x42, 0x43, 0x44, 0x45, 
		0x46, 0x47, 0x00, 0x00
	};

	/* generate lookup table */
	crc16_gen();

	/* print out the table (for embedding into code data sections) */
	printf("CRC byte lookup table:\n0x0000");
	for(i = 0; i < 255; i++)
	{
		printf(",%s0x%04x",
		       (i && !((i + 1) % 8)) ? "\n" : " ",
		       crc16_lut[i + 1]);
	}
	printf("\n\n");

	/* compute the golden checksum using the LUT and a reference algorithm */
	printf("Golden Checksum: 0x%04x\n", crc16(golden_packet, 32));
	printf("Reference Golden Checksum: 0x%04x\n", crc16_ref(golden_packet, 32));

	return(0);
}

/* fast CRC calculation using LUT */
uint16_t crc16(uint8_t *data, uint32_t len)
{ 
	uint32_t i;
	uint16_t rem = 0;

	for (i = 0; i < len; i++)
	{
		rem = (rem >> 8) ^ crc16_lut[data[i] ^ (rem & 0xff)];
	}

	return(rem);
}

/* fast CRC LUT generation */
void crc16_gen()
{
	uint16_t crc = 1;
	uint8_t i;
	uint16_t j;

	for(i = 0x80; i > 0; i >>= 1)
	{
		if(crc & 1)
		{
			crc = (crc >> 1) ^ CRCPOLY;
		}
		else
		{
			crc >>= 1;
		}

		for(j = 0; j < 256; j += 2 * i)
		{
			crc16_lut[i + j] = crc ^ crc16_lut[j];
		}
	}
}

/* reference CRC implementation */
uint16_t crc16_ref(uint8_t *buf, uint32_t len)
{
	uint16_t rem = 0;
	uint16_t dat;
	uint16_t tmp;
	uint32_t i, j;

	for(i = 0; i < len; i++)
	{
		dat = buf[i];
		for(j = 0; j < 8; j++)
		{
			tmp = (rem ^ dat) & 0x0001;

			rem >>= 1;
			dat >>= 1;

			if(tmp)
			{
				rem ^= CRCPOLY;
			}
		}
	}

	return(rem);
}