fs/Traits.hxx: implement BuildFS() method

src/fs/Traits.cxx

@@ -22,6 +22,31 @@
 
 #include <string.h>
 
+PathTraits::string
+PathTraits::BuildFS(PathTraits::const_pointer a, size_t a_size,
+		    PathTraits::const_pointer b, size_t b_size)
+{
+	assert(a != nullptr);
+	assert(b != nullptr);
+
+	if (a_size == 0)
+		return string(b, b_size);
+	if (b_size == 0)
+		return string(a, a_size);
+
+	string result(a, a_size);
+
+	if (!IsSeparatorFS(a[a_size - 1]))
+		result.push_back(SEPARATOR_FS);
+
+	if (IsSeparatorFS(b[0]))
+		result.append(b + 1, b_size - 1);
+	else
+		result.append(b, b_size);
+
+	return result;
+}
+
 const char *
 PathTraits::GetBaseUTF8(const char *p)
 {

src/fs/Traits.hxx

@@ -87,6 +87,16 @@ struct PathTraits {
 	}
 
 	/**
+	 * Constructs the path from the given components.
+	 * If either of the components is empty string,
+	 * remaining component is returned unchanged.
+	 * If both components are empty strings, empty string is returned.
+	 */
+	gcc_pure gcc_nonnull_all
+	static string BuildFS(const_pointer a, size_t a_size,
+			      const_pointer b, size_t b_size);
+
+	/**
 	 * Determine the "base" file name of the given UTF-8 path.
 	 * The return value points inside the given string.
 	 */