+ * Given two normalized paths (a trailing slash is ok), if subdir is
+ * outside dir, return -1. Otherwise return the offset in subdir that
+ * can be used as relative path to dir.
+ */
+int dir_inside_of(const char *subdir, const char *dir)
+{
+ int offset = 0;
+
+ assert(dir && subdir && *dir && *subdir);
+
+ while (*dir && *subdir && *dir == *subdir) {
+ dir++;
+ subdir++;
+ offset++;
+ }
+
+ /* hel[p]/me vs hel[l]/yeah */
+ if (*dir && *subdir)
+ return -1;
+
+ if (!*subdir)
+ return !*dir ? offset : -1; /* same dir */
+
+ /* foo/[b]ar vs foo/[] */
+ if (is_dir_sep(dir[-1]))
+ return is_dir_sep(subdir[-1]) ? offset : -1;
+
+ /* foo[/]bar vs foo[] */
+ return is_dir_sep(*subdir) ? offset + 1 : -1;
+}
+