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rcsid[] and sccsid[] and copyright[] are essentially unmaintained (and
unmaintainable).  these days, people use source.  these id's do not provide
any benefit, and do hurt the small install media
(the 33,000 line diff is essentially mechanical)
ok with the idea millert, ok dms

/*	$OpenBSD: strnsubst.c,v 1.5 2009/10/27 23:59:50 deraadt Exp $	*/
/*	$FreeBSD: strnsubst.c,v 1.6 2002/06/22 12:58:42 jmallett Exp $	*/

/*
 * Copyright (c) 2002 J. Mallett.  All rights reserved.
 * You may do whatever you want with this file as long as
 * the above copyright and this notice remain intact, along
 * with the following statement:
 * 	For the man who taught me vi, and who got too old, too young.
 */

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

void	strnsubst(char **, const char *, const char *, size_t);

/*
 * Replaces str with a string consisting of str with match replaced with
 * replstr as many times as can be done before the constructed string is
 * maxsize bytes large.  It does not free the string pointed to by str, it
 * is up to the calling program to be sure that the original contents of
 * str as well as the new contents are handled in an appropriate manner.
 * If replstr is NULL, then that internally is changed to a nil-string, so
 * that we can still pretend to do somewhat meaningful substitution.
 * No value is returned.
 */
void
strnsubst(char **str, const char *match, const char *replstr, size_t maxsize)
{
	char *s1, *s2, *this;
	size_t matchlen, repllen, s2len;
	int n;

	if ((s1 = *str) == NULL)
		return;
	if ((s2 = malloc(maxsize)) == NULL)
		err(1, NULL);

	if (replstr == NULL)
		replstr = "";

	if (match == NULL || *match == '\0' || strlen(s1) >= maxsize) {
		strlcpy(s2, s1, maxsize);
		goto done;
	}

	*s2 = '\0';
	s2len = 0;
	matchlen = strlen(match);
	repllen = strlen(replstr);
	for (;;) {
		if ((this = strstr(s1, match)) == NULL)
			break;
		n = snprintf(s2 + s2len, maxsize - s2len, "%.*s%s",
		    (int)(this - s1), s1, replstr);
		if (n == -1 || n + s2len + strlen(this + matchlen) >= maxsize)
			break;			/* out of room */
		s2len += n;
		s1 = this + matchlen;
	}
	strlcpy(s2 + s2len, s1, maxsize - s2len);
done:
	*str = s2;
	return;
}

#ifdef TEST
#include <stdio.h>

int
main(void)
{
	char *x, *y, *z, *za;

	x = "{}%$";
	strnsubst(&x, "%$", "{} enpury!", 255);
	y = x;
	strnsubst(&y, "}{}", "ybir", 255);
	z = y;
	strnsubst(&z, "{", "v ", 255);
	za = z;
	strnsubst(&z, NULL, za, 255);
	if (strcmp(z, "v ybir enpury!") == 0)
		printf("strnsubst() seems to work!\n");
	else
		printf("strnsubst() is broken.\n");
	printf("%s\n", z);
	free(x);
	free(y);
	free(z);
	free(za);
	return 0;
}
#endif