/*
 * applybp - Apply binary patch
 *	Brian Katzung	15 August 1993
 *
 * Copyright 1993 by Brian Katzung.  All rights reserved.
 * Permission is granted to use or modify this code as long as this
 * notice is preserved and modified versions are clearly marked.
 * This software may not be sold without written permission from the
 * author.
 *
 * This software is provided strictly on an "as-is" basis.
 * Use it at your own risk.
 *
 * usage:	applybp old patch new
 */

#include <stdio.h>

int
GetOldChar (OldFile)
FILE	*OldFile;
{
	int		OldChar;

	while ((OldChar = getc(OldFile)) == 0)
		;
	if (OldChar == EOF)
	{
		rewind(OldFile);
		while ((OldChar = getc(OldFile)) == 0)
			;
		if (OldChar == EOF)
		{
			fputs("Old file must have non-zero contents\n",
			  stderr);
			exit(1);
		}
	}
	return OldChar;
}

int
main (ac, av)
char	**av;
{
	int		PatchChar;
	int		OldChar;
	int		XORChar;
	int		i;
	int		j;
	FILE		*OldFile;
	FILE		*NewFile;
	FILE		*PatchFile;

	if (ac != 4)
	{
		fputs("usage:  applybp old patch new\n", stderr);
		exit(1);
	}

	if ((OldFile = fopen(av[1], "r")) == NULL)
	{
		perror(av[1]);
		return 1;
	}

	if ((PatchFile = fopen(av[2], "r")) == NULL)
	{
		perror(av[2]);
		return 1;
	}

	if (access(av[3], 0) == 0)
	{
		fprintf(stderr, "A file called %s already exists\n", av[3]);
		return 1;
	}
	if ((NewFile = fopen(av[3], "w")) == NULL)
	{
		perror(av[3]);
		return 1;
	}

	while ((i = getc(PatchFile)) != EOF)
	{
		/*
		 * Non-zero characters
		 */
		if (i != 0)
		{
			if (i > 1)
			{
				if ((XORChar = getc(PatchFile)) == EOF)
				{
					fputs(
"Premature EOF encountered in patch file\n",
					  stderr);
					return 1;
				}
			}
			for (j = i & 255; --j >= 0; )
			{
				if ((PatchChar = getc(PatchFile)) == EOF)
				{
					fputs(
"Premature EOF encountered in patch file\n",
					  stderr);
					return 1;
				}
				PatchChar ^= GetOldChar(OldFile);
				putc(PatchChar, NewFile);
				XORChar ^= PatchChar;
			}
			if (i > 1 && XORChar != 0)
			{
				fputs(
"Patch file does not correspond to old file\n",
				  stderr);
				return 1;
			}
		}

		/*
		 * Zero characters
		 */
		if ((i = getc(PatchFile)) != EOF && i != 0)
		{
			for (i &= 255; --i >= 0; putc(0, NewFile))
				;
		}
	}

	return 0;
}