#include <stdio.h>

static unsigned long crctable[256];
static unsigned char crcinvtable[256];

void calc_crc_table(void)
{
	unsigned long poly = 0xedb88320, value, idx, i;

	for (idx = 0 ; idx < 256 ; idx++) {
		for (value = idx, i = 0 ; i < 8 ; i++) {
			if (value & 1) {
				value >>= 1;
				value ^= poly;
			} else {
				value >>= 1;
			}
		}
		crctable[idx] = value;
		crcinvtable[value >> 24] = idx & 0xff;
	}
}

unsigned long calc_crc(char *buf, unsigned long buflen)
{
	unsigned long crc = 0xffffffff;
	while (buflen--)
		crc = (crc >> 8) ^ crctable[*buf++ ^ crc & 0xff];
	return ~crc;
}

static inline unsigned char l2c(unsigned long l, int nr) { return (l >> (8*(nr-1))) & 0xff; }

void solve(char *buf, unsigned long buflen, unsigned long wcrc, unsigned char *a, unsigned char *b, unsigned char *c, unsigned char *d)
{
	unsigned long crc = 0xffffffff;
	while (buflen--)
		crc = (crc >> 8) ^ crctable[*buf++ ^ crc & 0xff];

	wcrc = ~wcrc;

	// sequence of crc's we must have to finish with wcrc
	unsigned long c1, c2, c3, c4;
	c4 = crctable[ crcinvtable[ l2c(wcrc, 4) ] ];
	c3 = crctable[ crcinvtable[ l2c(wcrc, 3) ^ l2c(c4, 3) ] ];
	c2 = crctable[ crcinvtable[ l2c(wcrc, 2) ^ l2c(c4, 2) ^ l2c(c3, 3) ] ];
	c1 = crctable[ crcinvtable[ l2c(wcrc, 1) ^ l2c(c4, 1) ^ l2c(c3, 2) ^ l2c(c2, 3) ] ];

	*a = crcinvtable[l2c(c1, 4)] ^ l2c(crc, 1);
	crc = (crc >> 8) ^ crctable[*a ^ crc & 0xff];
	
	*b = crcinvtable[l2c(c2, 4)] ^ l2c(crc, 1);
	crc = (crc >> 8) ^ crctable[*b ^ crc & 0xff];
	
	*c = crcinvtable[l2c(c3, 4)] ^ l2c(crc, 1);
	crc = (crc >> 8) ^ crctable[*c ^ crc & 0xff];
	
	*d = crcinvtable[l2c(c4, 4)] ^ l2c(crc, 1);
	crc = (crc >> 8) ^ crctable[*d ^ crc & 0xff];
}

static unsigned char mkhex(char c)
{
	char oc = c;
	
	if (c < '0')
		goto err;
	c -= '0';
	if (c <= 9)
		return c;
	if (c < 'A'-'0')
		goto err;
	c -= 'A'-'0';
	if (c <= 6)
		return 10+c;
	if (c < 'a'-'A')
		goto err;
	c -= 'a' - 'A';
	if (c <= 6)
		return 10+c;
err:
	fprintf(stderr, "invalid character %c (%hhx)\n", oc, oc);
	exit(1);
}

static unsigned long interpol(char *src, char *dst, const unsigned long dstlen)
{
	unsigned long i=0;
	register char c;
	
	while (*src && i < dstlen) {
		if ((c = *src++) == '\\')
			if ((c = *src++) == 'x') {
				c = mkhex(*src++) << 4;
				c |= mkhex(*src++);
			} else if (!c)
				break;
		dst[i++] = c;
	}
	return i;
}

static void test(void)
{
	printf("crcinv for 0x28: %.8lX\n", crctable[crcinvtable[0x28]]);
	printf("decomp of 0x12345678: %.2X, %.2X, %.2X, %.2X\n", l2c(0x12345678, 4), l2c(0x12345678, 3), l2c(0x12345678, 2), l2c(0x12345678, 1));
	printf("\n");
}

int main(int argc, char **argv)
{
	int i;
	char buf[256];
	unsigned long buflen = 256;
	unsigned long wishcrc = 0;
	
	calc_crc_table();
	
	if (argc < 2) {
		printf("usage: calc <str> <crc>\n");
		exit(0);
	}
	
	buflen = interpol(argv[1], buf, buflen);
	for (i=0 ; i<8 ; i++)
		wishcrc = wishcrc << 4 | mkhex(argv[2][i]);

	unsigned char a, b, c, d;
	solve(buf, buflen, wishcrc, &a, &b, &c, &d);
	printf("padding: %.2X %.2X %.2X %.2X\n", a, b, c, d);
	
	return 0;
}