#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <inttypes.h>

#define lo8(x) (x & 0xFF)
#define hi8(x) ((x >> 8) & 0xFF)

unsigned short crc_ccitt_update (uint16_t crc, uint8_t data)
{
  data ^= lo8 (crc);
  data ^= data << 4;
  return ((((uint16_t)data << 8) | hi8 (crc)) ^ (uint8_t)(data >> 4) ^ ((uint16_t)data << 3));
}	

int main(int argc, char *argv[])
{
  if ((argc <2) || (argc >3))
  {
  	printf("Usage: crcgenfill <filename> [fillstart]\r\n");
	return 1;
  }

	
	FILE *f;
	
	f = fopen(argv[1], "rb+");
	
	if (f == 0)
	{
		printf("Unable to open file %s\r\n", argv[1]);
		return 1;
	}

	struct stat buf;
	stat(argv[1], &buf);
	unsigned long l;
	unsigned char ff[1]={0xff};

        unsigned long start=0;
        if(argc==3)
        {
		start=atoi(argv[2]);

		fseek(f,start,SEEK_SET);
		for(l=start; l<buf.st_size - 8; l++)
		{
			fwrite(ff,1,1,f);
		}
		rewind(f);
		printf("Filled with 0xff from Addr: %lx\r\n",start);
	}

	unsigned short crc = 0xFFFF;
	unsigned char c;
	
	for (l=0; l < buf.st_size - 2; l++)
		crc = crc_ccitt_update(crc, fgetc(f));
	
	fseek(f, -2, SEEK_END);
	fwrite (&crc, 2, 1, f);
	
	fclose(f);
	
	printf("\r\nWritten CRC 0x%.4X to file %s", crc, argv[1]);

  return 0;
}