Replace make_relative() with relpath()

Also improve the documentation for this function, and add a few
examples to clarify what it does (and what not).

3 files changed, 25 insertions, 19 deletions

diff --git a/src/build.c b/src/build.c
index c1dd602..71f10f9 100644
--- a/src/build.c
+++ b/src/build.c
@@ -322,9 +322,9 @@ static char *get_relpath(const char *target) {
 	if (!abstarget)
 		fatal("redo: failed to get realpath() of %s", target);
 
-	char *relpath = xstrdup(make_relative(root, abstarget));
+	char *path = xstrdup(relpath(abstarget, root));
 	free(abstarget);
-	return relpath;
+	return path;
 }
 
 /* Return the dependency file path of target. */
diff --git a/src/filepath.c b/src/filepath.c
index d0b68de..e4d8e62 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -53,27 +53,33 @@ char *take_extension(const char *target) {
 		return "";
 }
 
-/* Make one path relative to another i.e. some/path to some/path/a/b/c would
-   yield a/b/c as result. Prints '.' if the 2 paths match. */
-// TODO: nameing, requires absolute paths, doesn't MALLOC
-const char *make_relative(const char *target, const char *to) {
+/* Returns a pointer to a relative path to `path` from `start`. This pointer may
+   be pointing to either of it's arguments, or to the statically allocated
+   string "." should both paths match. Both paths must be canonicalized.
+   A few examples:
+    relpath("/abc/de", "/abc/de") => "."
+    relpath("some/path/a/b/c", "some/path") => "a/b/c"
+    relpath("some/path", "some/path/a/b/c") => "some/path/a/b/c"
+    relpath("/", "/") => "."
+    relpath("/abc", "/") => "abc"
+ */
+char *relpath(char *path, char *start) {
 	int i;
-	for (i = 0; target[i] && to[i]; ++i)
-		if (target[i] != to[i]) {
-			/* the paths do not match */
-			return to;
-		}
-
-	if (!target[i] && !to[i]) {
-		/* both paths match completely */
-		return ".";
+	for (i = 0; path[i] && start[i]; ++i) {
+		if (path[i] != start[i])
+			return path; /* paths share nothing */
 	}
 
-	/* skip any leading seperators */
-	while (to[i] == '/')
+	if (!path[i] && start[i])
+		return start; /* path is above start */
+
+	if (!path[i] && !start[i])
+		return "."; /* paths match completely */
+
+	if (path[i] == '/')
 		++i;
 
-	return &to[i];
+	return &path[i];
 }
 
 /* Transforms target into a safe filename, replacing all '/' with '!'. */
diff --git a/src/filepath.h b/src/filepath.h
index 3eb68ba..68a4ecf 100644
--- a/src/filepath.h
+++ b/src/filepath.h
@@ -14,7 +14,7 @@
 extern bool is_absolute(const char* path);
 extern char *remove_ext(const char *str);
 extern char *take_extension(const char *target);
-extern const char *make_relative(const char *target, const char *to);
+extern char *relpath(char *path, char *start);
 extern char *transform_path(const char *target);
 extern char *xbasename(const char *path);
 extern bool fexists(const char *target);