#include <stdio.h>
#include <string.h>

#define byte(x, i)	((x >> (8 * (3 - i))) & 0xff)

const unsigned long crc_table[256] = {
	0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L,
	0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L,
	0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L,
	0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
	0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L,
	0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L,
	0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L,
	0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
	0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L,
	0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL,
	0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L,
	0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
	0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L,
	0x01db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x06b6b51fL,
	0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL,
	0xe10e9818L, 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
	0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL,
	0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L,
	0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L,
	0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
	0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL,
	0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L,
	0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L,
	0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
	0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L,
	0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L,
	0x03b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x04db2615L,
	0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
	0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 0xf00f9344L,
	0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL,
	0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL,
	0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
	0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L,
	0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL,
	0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL,
	0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
	0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL,
	0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L,
	0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL,
	0x026d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
	0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 0x92d28e9bL,
	0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 0x86d3d2d4L, 0xf1d4e242L,
	0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L,
	0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
	0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L,
	0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L,
	0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L,
	0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
	0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L,
	0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L,
	0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL,
	0x2d02ef8dL
};

const unsigned long lookup[256] = {
	0, 65, 195, 130, 134, 199, 69, 4, 77, 12, 142, 207, 203, 138, 8, 73,
	154, 219, 89, 24, 28, 93, 223, 158, 215, 150, 20, 85, 81, 16, 146, 211,
	117, 52, 182, 247, 243, 178, 48, 113, 56, 121, 251, 186, 190, 255, 125, 60,
	239, 174, 44, 109, 105, 40, 170, 235, 162, 227, 97, 32, 36, 101, 231, 166,
	234, 171, 41, 104, 108, 45, 175, 238, 167, 230, 100, 37, 33, 96, 226, 163,
	112, 49, 179, 242, 246, 183, 53, 116, 61, 124, 254, 191, 187, 250, 120, 57,
	159, 222, 92, 29, 25, 88, 218, 155, 210, 147, 17, 80, 84, 21, 151, 214,
	5, 68, 198, 135, 131, 194, 64, 1, 72, 9, 139, 202, 206, 143, 13, 76,
	149, 212, 86, 23, 19, 82, 208, 145, 216, 153, 27, 90, 94, 31, 157, 220,
	15, 78, 204, 141, 137, 200, 74, 11, 66, 3, 129, 192, 196, 133, 7, 70,
	224, 161, 35, 98, 102, 39, 165, 228, 173, 236, 110, 47, 43, 106, 232, 169,
	122, 59, 185, 248, 252, 189, 63, 126, 55, 118, 244, 181, 177, 240, 114, 51,
	127, 62, 188, 253, 249, 184, 58, 123, 50, 115, 241, 176, 180, 245, 119, 54,
	229, 164, 38, 103, 99, 34, 160, 225, 168, 233, 107, 42, 46, 111, 237, 172,
	10, 75, 201, 136, 140, 205, 79, 14, 71, 6, 132, 197, 193, 128, 2, 67,
	144, 209, 83, 18, 22, 87, 213, 148, 221, 156, 30, 95, 91, 26, 152, 217,
};

unsigned long crc32(const char *buf, unsigned int len) {
	unsigned long crc = 0xffffffff;
	unsigned int i = 0;
	for(i = 0; i < len; i ++) {
		crc = crc_table[(crc ^ buf[i]) & 0xff] ^ (crc >> 8);
	}
	return crc ^ 0xffffffff;
}

void setcrc32(char *buf, unsigned long *len, unsigned long tcrc) {
	// We start with -1 and then flip the bits at the end
	unsigned long crc = 0xffffffff;
	tcrc ^= 0xffffffff;
	
	// Calculate the CRC so far
	unsigned long i, j;
	for(i = 0; i < *len; i ++) {
		crc = crc_table[(crc ^ buf[i]) & 0xff] ^ (crc >> 8);
	}
	
	// Generate a lookup table
	/*
	unsigned long lookup[256];
	for(i = 0; i < 256; i ++) {
		for(j = 0; j < 256; j ++) {
			if(byte(crc_table[j], 0) == i) {
				lookup[i] = j;
				break;
			}
		}
	}
	*/
	
	// Get the indices of values in the CRC table
	unsigned long i4 = lookup[byte(tcrc, 0)];
	unsigned long i3 = lookup[byte(tcrc, 1) ^ byte(crc_table[i4], 1)];
	unsigned long i2 = lookup[byte(tcrc, 2) ^ byte(crc_table[i4], 2) ^ byte(crc_table[i3], 1)];
	unsigned long i1 = lookup[byte(tcrc, 3) ^ byte(crc_table[i4], 3) ^ byte(crc_table[i3], 2) ^ byte(crc_table[i2], 1)];
	
	// Use these to find the bytes we need to append
	char b1 = crc ^ i1;
	crc = crc_table[(crc ^ b1) & 0xff] ^ (crc >> 8);
	char b2 = crc ^ i2;
	crc = crc_table[(crc ^ b2) & 0xff] ^ (crc >> 8);
	char b3 = crc ^ i3;
	crc = crc_table[(crc ^ b3) & 0xff] ^ (crc >> 8);
	char b4 = crc ^ i4;
	
	// Stick these on the end of our buffer
	buf[(*len) ++] = b1;
	buf[(*len) ++] = b2;
	buf[(*len) ++] = b3;
	buf[(*len) ++] = b4;
}

int main() {
	char msg[256] = "Good evening!";
	unsigned int msglen = strlen(msg);
	
	// Print out the bytes
	unsigned int i;
	
	printf("bytes: ");
	for(i = 0; i < msglen; i ++) {
		printf("%02x ", (unsigned char)msg[i]);
	}
	printf("\n");
	
	// Print out the current CRC32
	printf("CRC32: %08x\n\n", crc32(msg, msglen));
	
	// Set the CRC32
	setcrc32(msg, &msglen, 0xdeadbeef);
	
	// Print out the bytes again
	printf("bytes: ");
	for(i = 0; i < msglen; i ++) {
		printf("%02x ", (unsigned char)msg[i]);
	}
	printf("\n");
	
	// Print out the new CRC32
	printf("CRC32: %08x\n", crc32(msg, msglen));
}