writetag.cpp

//	$Id: writetag.cpp,v 1.1 2000/02/18 02:11:56 cisc Exp $

#include <stdio.h>

typedef unsigned char uint8;
typedef unsigned int  uint32;
typedef unsigned int  uint;

int main(int ac, char** av)
{
	if (ac == 2)
	{
		FILE* fp = fopen(av[1], "r+b");
		if (fp)
		{
			int len;
			
			fseek(fp, 0, SEEK_END);
			len = ftell(fp);
			fseek(fp, 0, SEEK_SET);
			
			uint8* mod = new uint8[len];
			if (mod)
			{
				fread(mod, 1, len, fp);
				
				const int tagpos = 0x7c;
				uint32 crc = 0xffffffff;
				*(uint32*)(mod + tagpos) = 0;

				uint crctable[256];
				uint i;
				for (i=0; i<256; i++)
				{
					uint r = i << 24;
					for (int j=0; j<8; j++)
						r = (r << 1) ^ (r & 0x80000000 ? 0x04c11db7 : 0);
					crctable[i] = r;
				}

				// CRC ŚvŽZ

				for (i=0; i<len; i++)
					crc = (crc << 8) ^ crctable[((crc >> 24) ^ mod[i]) & 0xff];
				delete[] mod;
				
				printf("crc = %.8x\n", crc);
				crc = ~crc;
				fseek(fp, tagpos, SEEK_SET);
				fwrite(&crc, sizeof(crc), 1, fp);
			}
			fclose(fp);
		}
	}
	return 0;
}