/*
 * 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.
 */
#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);
}