Add functions get_relative_cwd() and is_inside_dir()

The function get_relative_cwd() works just as getcwd(), only that it
takes an absolute path as additional parameter, returning the prefix
of the current working directory relative to the given path.  If the
cwd is no subdirectory of the given path, it returns NULL.

is_inside_dir() is just a trivial wrapper over get_relative_cwd().

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 41 insertions, 0 deletions

diff --git a/dir.c b/dir.c
index 8d8faf5d7..b3329f41b 100644
--- a/dir.c
+++ b/dir.c
@@ -642,3 +642,41 @@ file_exists(const char *f)
   struct stat sb;
   return stat(f, &sb) == 0;
 }
+
+/*
+ * get_relative_cwd() gets the prefix of the current working directory
+ * relative to 'dir'.  If we are not inside 'dir', it returns NULL.
+ * As a convenience, it also returns NULL if 'dir' is already NULL.
+ */
+char *get_relative_cwd(char *buffer, int size, const char *dir)
+{
+	char *cwd = buffer;
+
+	/*
+	 * a lazy caller can pass a NULL returned from get_git_work_tree()
+	 * and rely on this function to return NULL.
+	 */
+	if (!dir)
+		return NULL;
+	if (!getcwd(buffer, size))
+		die("can't find the current directory: %s", strerror(errno));
+
+	if (!is_absolute_path(dir))
+		dir = make_absolute_path(dir);
+
+	while (*dir && *dir == *cwd) {
+		dir++;
+		cwd++;
+	}
+	if (*dir)
+		return NULL;
+	if (*cwd == '/')
+		return cwd + 1;
+	return cwd;
+}
+
+int is_inside_dir(const char *dir)
+{
+	char buffer[PATH_MAX];
+	return get_relative_cwd(buffer, sizeof(buffer), dir) != NULL;
+}
diff --git a/dir.h b/dir.h
index ec0e8abab..f55a87b2c 100644
--- a/dir.h
+++ b/dir.h
@@ -61,4 +61,7 @@ extern void add_exclude(const char *string, const char *base,
 extern int file_exists(const char *);
 extern struct dir_entry *dir_add_name(struct dir_struct *dir, const char *pathname, int len);
 
+extern char *get_relative_cwd(char *buffer, int size, const char *dir);
+extern int is_inside_dir(const char *dir);
+
 #endif