fs/Traits: add function Relative()

Move code from Path::RelativeFS() and make it generic.

src/fs/Path.cxx

@@ -26,25 +26,3 @@ Path::ToUTF8() const
 {
 	return ::PathToUTF8(c_str());
 }
-
-const char *
-Path::RelativeFS(const char *other_fs) const
-{
-	const size_t l = length();
-	if (memcmp(data(), other_fs, l) != 0)
-		return nullptr;
-
-	other_fs += l;
-	if (*other_fs != 0) {
-		if (!PathTraitsFS::IsSeparator(*other_fs))
-			/* mismatch */
-			return nullptr;
-
-		/* skip remaining path separators */
-		do {
-			++other_fs;
-		} while (PathTraitsFS::IsSeparator(*other_fs));
-	}
-
-	return other_fs;
-}

src/fs/Path.hxx

@@ -135,7 +135,9 @@ public:
 	 * nullptr on mismatch.
 	 */
 	gcc_pure
-	const char *RelativeFS(const char *other_fs) const;
+	const char *RelativeFS(const char *other_fs) const {
+		return PathTraitsFS::Relative(value, other_fs);
+	}
 
 	gcc_pure
 	bool IsAbsolute() {

src/fs/Traits.cxx

@@ -78,6 +78,34 @@ GetParentPathImpl(typename Traits::const_pointer p)
 	return typename Traits::string(p, sep);
 }
 
+template<typename Traits>
+typename Traits::const_pointer
+RelativePathImpl(typename Traits::const_pointer base,
+		 typename Traits::const_pointer other)
+{
+	assert(base != nullptr);
+	assert(other != nullptr);
+
+	const auto base_length = Traits::GetLength(base);
+	if (memcmp(base, other, base_length * sizeof(*base)) != 0)
+		/* mismatch */
+		return nullptr;
+
+	other += base_length;
+	if (other != 0) {
+		if (!Traits::IsSeparator(*other))
+			/* mismatch */
+			return nullptr;
+
+		/* skip remaining path separators */
+		do {
+			++other;
+		} while (Traits::IsSeparator(*other));
+	}
+
+	return other;
+}
+
 PathTraitsFS::string
 PathTraitsFS::Build(PathTraitsFS::const_pointer a, size_t a_size,
 		    PathTraitsFS::const_pointer b, size_t b_size)
@@ -97,6 +125,12 @@ PathTraitsFS::GetParent(PathTraitsFS::const_pointer p)
 	return GetParentPathImpl<PathTraitsFS>(p);
 }
 
+PathTraitsFS::const_pointer
+PathTraitsFS::Relative(const_pointer base, const_pointer other)
+{
+	return RelativePathImpl<PathTraitsFS>(base, other);
+}
+
 PathTraitsUTF8::string
 PathTraitsUTF8::Build(PathTraitsUTF8::const_pointer a, size_t a_size,
 		      PathTraitsUTF8::const_pointer b, size_t b_size)
@@ -115,3 +149,9 @@ PathTraitsUTF8::GetParent(PathTraitsUTF8::const_pointer p)
 {
 	return GetParentPathImpl<PathTraitsUTF8>(p);
 }
+
+PathTraitsUTF8::const_pointer
+PathTraitsUTF8::Relative(const_pointer base, const_pointer other)
+{
+	return RelativePathImpl<PathTraitsUTF8>(base, other);
+}

src/fs/Traits.hxx

@@ -105,6 +105,15 @@ struct PathTraitsFS {
 	gcc_pure gcc_nonnull_all
 	static string GetParent(const_pointer p);
 
+	/**
+	 * Determine the relative part of the given path to this
+	 * object, not including the directory separator.  Returns an
+	 * empty string if the given path equals this object or
+	 * nullptr on mismatch.
+	 */
+	gcc_pure gcc_nonnull_all
+	static const_pointer Relative(const_pointer base, const_pointer other);
+
 	/**
 	 * Constructs the path from the given components.
 	 * If either of the components is empty string,
@@ -179,6 +188,15 @@ struct PathTraitsUTF8 {
 	gcc_pure gcc_nonnull_all
 	static string GetParent(const_pointer p);
 
+	/**
+	 * Determine the relative part of the given path to this
+	 * object, not including the directory separator.  Returns an
+	 * empty string if the given path equals this object or
+	 * nullptr on mismatch.
+	 */
+	gcc_pure gcc_nonnull_all
+	static const_pointer Relative(const_pointer base, const_pointer other);
+
 	/**
 	 * Constructs the path from the given components.
 	 * If either of the components is empty string,