Add get_relative_path()

We can use this function to get the relative path between a base path
and a filename

Related to aseprite/aseprite#4389

base/fs.cpp

@@ -210,6 +210,40 @@ std::string get_file_title_with_path(const std::string& filename)
   return filename;
 }
 
+std::string get_relative_path(const std::string& filename, const std::string& base_path)
+{
+  std::vector<std::string> baseDirs;
+  split_string(base_path, baseDirs, "/\\");
+
+  std::vector<std::string> toParts;
+  split_string(filename, toParts, "/\\");
+
+  // Find the common prefix
+  auto itFrom = baseDirs.begin();
+  auto itTo = toParts.begin();
+
+  while (itFrom != baseDirs.end() && itTo != toParts.end() && *itFrom == *itTo) {
+    ++itFrom;
+    ++itTo;
+  }
+
+  if (itFrom == baseDirs.begin() && itTo == toParts.begin()) {
+    // No common prefix
+    return filename;
+  }
+
+  // Calculate the number of directories to go up from base path
+  std::string relativePath;
+  for (auto it = itFrom; it != baseDirs.end(); ++it)
+    relativePath = base::join_path(relativePath, "..");
+
+  // Append the remaining part of 'toPath'
+  for (auto it = itTo; it != toParts.end(); ++it)
+    relativePath = base::join_path(relativePath, *it);
+
+  return relativePath;
+}
+
 std::string join_path(const std::string& path, const std::string& file)
 {
   std::string result(path);

base/fs.h

@@ -77,6 +77,9 @@ namespace base {
   std::string get_file_title(const std::string& filename);
   std::string get_file_title_with_path(const std::string& filename);
 
+  // Returns the relative path of the given filename from the base_path.
+  std::string get_relative_path(const std::string& filename, const std::string& base_path);
+
   // Joins two paths or a path and a file name with a path-separator.
   std::string join_path(const std::string& path, const std::string& file);
 

base/fs_tests.cpp

@@ -174,6 +174,23 @@ TEST(FS, GetFileTitleWithPath)
   EXPECT_EQ("C:\\",            get_file_title_with_path("C:\\.cpp"));
 }
 
+TEST(FS, GetRelativePath)
+{
+  EXPECT_EQ("C:\\foo\\bar\\test.file", get_relative_path("C:\\foo\\bar\\test.file", "D:\\another\\disk"));
+
+#if LAF_WINDOWS
+  EXPECT_EQ("bar\\test.file",           get_relative_path("C:\\foo\\bar\\test.file", "C:\\foo"));
+  EXPECT_EQ("C:\\foo\\bar\\test.file",  get_relative_path("C:\\foo\\bar\\test.file", "D:\\another\\disk"));
+  EXPECT_EQ("..\\bar\\test.file",       get_relative_path("C:\\foo\\bar\\test.file", "C:\\foo\\another"));
+  EXPECT_EQ("..\\..\\bar\\test.file",   get_relative_path("C:\\foo\\bar\\test.file", "C:\\foo\\a\\b"));
+#else
+  EXPECT_EQ("bar/test.file",            get_relative_path("C:/foo/bar/test.file", "C:/foo"));
+  EXPECT_EQ("C:/foo/bar/test.file",     get_relative_path("C:/foo/bar/test.file", "D:/another/disk"));
+  EXPECT_EQ("../bar/test.file",         get_relative_path("/foo/bar/test.file", "/foo/another"));
+  EXPECT_EQ("../../bar/test.file",      get_relative_path("/foo/bar/test.file", "/foo/a/b"));
+#endif
+}
+
 TEST(FS, JoinPath)
 {
   std::string sep;