#include <stdio.h>
#include <math.h>

#define NXIMG 10800
#define NYIMG 6336
#define NCHUNK 2

/* NYIMG/NCHUNK must divide evenly.  
	Choose NCHUNK so that 2*NXIMG*(NYIMG/NCHUNK) bytes fits easily in core  */

main(argc, argv) 
int argc;
char **argv;
{
	FILE	*fp1, *fp2, *fp3;
	int	i, j, npixels, ichunk, ipixel;
	short	*a, dummy;

	if (argc != 4) {
		fprintf(stderr,"usage:  imgdiff <filex> <filey> <filez> where z = y - x are img.7.2 files.\n");
		exit(-1);
	}

	fp1 = fopen(argv[1], "r");
	fp2 = fopen(argv[2], "r");
	fp3 = fopen(argv[3], "w");

	npixels = NXIMG*(NYIMG/NCHUNK);

	if ( (a = (short *)malloc(2*npixels)) == NULL) {
		fprintf(stderr,"imgdiff:  ERROR.  Cannot allocate an array of %d bytes.\n", 2*npixels);
		exit(-1);
	}

	for (ichunk = 0; ichunk < NCHUNK; ichunk++) {
		if ( (fread((char *)a, 2, npixels, fp1)) != npixels) {
			fprintf(stderr,"imgdiff:  ERROR reading ichunk %d from %s\n", ichunk, argv[1]);
			exit(-1);
		}
		for (ipixel = 0; ipixel < npixels; ipixel++) {
			if ( (fread((char *)&dummy, 2, 1, fp2)) != 1) {
				fprintf(stderr,"imgdiff:  ERROR reading from %s\n", argv[2]);
				exit(-1);
			}
			a[ipixel] = dummy - a[ipixel];
		}
		if ( (fwrite((char *)a, 2, npixels, fp3)) != npixels) {
			fprintf(stderr,"imgdiff:  ERROR writing ichunk %d to %s\n", ichunk, argv[3]);
			exit(-1);
		}
	}

	free((char *)a);

	fclose(fp1);
	fclose(fp2);
	fclose(fp3);

	exit(0);
}