/*
 * Copyright (c) 1997 Shigio Yamaguchi. All rights reserved.
 * Copyright (c) 1999 Tama Communications Corporation. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD$
 */
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include "pathconv.h" /* prototypes */
/*
 * abs2rel: convert an absolute path name into relative.
 *
 *	i)	path	absolute path
 *	i)	base	base directory (must be absolute path)
 *	o)	result	result buffer
 *	i)	size	size of result buffer
 *	r)		!= NULL: relative path
 *			== NULL: error
 */
char *
abs2rel(const char *path, const char *base, char *result, const size_t size)
{
	const char *pp, *bp, *branch;
	/*
	 * endp points the last position which is safe in the result buffer.
	 */
	const char *endp = result + size - 1;
	char *rp;

	if (*path != '/') {
		if (strlen(path) >= size)
			goto erange;
		strcpy(result, path);
		goto finish;
	} else if (*base != '/' || !size) {
		errno = EINVAL;
		return (NULL);
	} else if (size == 1)
		goto erange;
	/*
	 * seek to branched point.
	 */
	branch = path;
	for (pp = path, bp = base; *pp && *bp && *pp == *bp; pp++, bp++)
		if (*pp == '/')
			branch = pp;
	if ((*pp == 0 || (*pp == '/' && *(pp + 1) == 0)) &&
	    (*bp == 0 || (*bp == '/' && *(bp + 1) == 0))) {
		rp = result;
		*rp++ = '.';
		if (*pp == '/' || *(pp - 1) == '/')
			*rp++ = '/';
		if (rp > endp)
			goto erange;
		*rp = 0;
		goto finish;
	}
	if ((*pp == 0 && *bp == '/') || (*pp == '/' && *bp == 0))
		branch = pp;
	/*
	 * up to root.
	 */
	rp = result;
	for (bp = base + (branch - path); *bp; bp++)
		if (*bp == '/' && *(bp + 1) != 0) {
			if (rp + 3 > endp)
				goto erange;
			*rp++ = '.';
			*rp++ = '.';
			*rp++ = '/';
		}
	if (rp > endp)
		goto erange;
	*rp = 0;
	/*
	 * down to leaf.
	 */
	if (*branch) {
		if (rp + strlen(branch + 1) > endp)
			goto erange;
		strcpy(rp, branch + 1);
	} else
		*--rp = 0;
finish:
	return result;
erange:
	errno = ERANGE;
	return (NULL);
}