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

#ifndef BUFFER
#define BUFFER 4096
#endif
uint8_t msg[BUFFER];

/* CRC-32, eg. ethernet */
const uint32_t crc32_tbl[] = {
	0x00000000, 0x1db71064, 0x3b6e20c8, 0x26d930ac,
	0x76dc4190, 0x6b6b51f4, 0x4db26158, 0x5005713c,
	0xedb88320, 0xf00f9344, 0xd6d6a3e8, 0xcb61b38c,
	0x9b64c2b0, 0x86d3d2d4, 0xa00ae278, 0xbdbdf21c
};

/* the idea is to act on nybbles instead of bytes to require less CPU cache */
uint32_t crc32_calc(uint8_t *ptr, int cnt, uint32_t crc)
{
	while (cnt--) {
		crc = (crc >> 4) ^ crc32_tbl[(crc & 0xf) ^ (*ptr & 0xf)];
		crc = (crc >> 4) ^ crc32_tbl[(crc & 0xf) ^ (*(ptr++) >> 4)];
	}
	return crc;
}

int main(int argc, char **argv)
{
	int len;
	uint32_t v32 = ~0;
	FILE *f;

	if (argc == 1) {
		freopen(NULL, "rb", stdin);
		f = stdin;
	} else if (argc == 2) {
		f = fopen(argv[1], "rb");
		if (f == NULL) {
			perror(argv[1]);
			exit(1);
		}
	} else {
		fputs("usage: crc32 [filename]", stderr);
		exit(1);
	}

	do {
		len = fread(msg, 1, BUFFER, f);
		if (ferror(f)) {
			perror(NULL);
			exit(1);
		}
		v32 = crc32_calc(msg, len, v32);
	} while (!feof(f));

	fclose(f);

	printf("%08lX\n", (unsigned long) ~v32);
}