From 14b0bc7c09469ecb1348a1e3a3bf45a9b0ec2ea4 Mon Sep 17 00:00:00 2001
From: Alexander Gutkin <agutkin@google.com>
Date: Tue, 26 May 2026 12:25:11 -0700
Subject: [PATCH] Add `Dirname` and `Extension` API to path manipulation
 library.

PiperOrigin-RevId: 921615561
---
 openfst/compat/file_path.cc      | 22 ++++++++++++++++++++++
 openfst/compat/file_path.h       | 11 +++++++++++
 openfst/compat/file_path_test.cc | 24 ++++++++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/openfst/compat/file_path.cc b/openfst/compat/file_path.cc
index 9b60e107..a56bb514 100644
--- a/openfst/compat/file_path.cc
+++ b/openfst/compat/file_path.cc
@@ -46,6 +46,20 @@ std::pair<std::string_view, std::string_view> SplitPath(std::string_view path) {
                         absl::ClippedSubstr(path, pos + 1));
 }
 
+// Return the parts of the basename of path, split on the final ".".
+// If there is no "." in the basename or "." is the final character in the
+// basename, the second value will be empty.
+std::pair<std::string_view, std::string_view> SplitBasename(
+    std::string_view path) {
+  path = Basename(path);
+
+  const size_t pos = path.find_last_of('.');
+  if (pos == std::string_view::npos)
+    return std::make_pair(path, absl::ClippedSubstr(path, path.size(), 0));
+  return std::make_pair(path.substr(0, pos),
+                        absl::ClippedSubstr(path, pos + 1));
+}
+
 }  // namespace
 
 std::string JoinPath(std::string_view path1, std::string_view path2) {
@@ -81,4 +95,12 @@ std::string_view Basename(std::string_view path) {
   return SplitPath(path).second;
 }
 
+std::string_view Dirname(absl::string_view path) {
+  return SplitPath(path).first;
+}
+
+std::string_view Extension(std::string_view path) {
+  return SplitBasename(path).second;
+}
+
 }  // namespace fst
diff --git a/openfst/compat/file_path.h b/openfst/compat/file_path.h
index e4ae40a0..e9f7b6ce 100644
--- a/openfst/compat/file_path.h
+++ b/openfst/compat/file_path.h
@@ -36,6 +36,17 @@ std::string JoinPathRespectAbsolute(std::string_view path1,
 // "/" in the path, the result is the same as the input.
 std::string_view Basename(std::string_view path);
 
+// Returns the part of the path before the final "/", EXCEPT:
+// * If there is a single leading "/" in the path, the result will be the
+//   leading "/".
+// * If there is no "/" in the path, the result is the empty prefix of the
+//   input string.
+std::string_view Dirname(std::string_view path);
+
+// Returns the part of the basename of path after the final ".".  If
+// there is no "." in the basename, the result is empty.
+std::string_view Extension(std::string_view path);
+
 }  // namespace fst
 
 #endif  // OPENFST_COMPAT_FILE_PATH_H_
diff --git a/openfst/compat/file_path_test.cc b/openfst/compat/file_path_test.cc
index 2859afcd..afe1d11b 100644
--- a/openfst/compat/file_path_test.cc
+++ b/openfst/compat/file_path_test.cc
@@ -57,5 +57,29 @@ TEST(FilePathTest, CheckBasename) {
   EXPECT_EQ("hello", Basename("/a/b/c/hello"));
 }
 
+TEST(FilePathTest, CheckDirname) {
+  EXPECT_EQ("/hello", Dirname("/hello/"));
+  EXPECT_EQ("/", Dirname("/hello"));
+  EXPECT_EQ("/hello", Dirname("/hello/world"));
+  EXPECT_EQ("hello", Dirname("hello/world"));
+  EXPECT_EQ("hello", Dirname("hello/"));
+  EXPECT_EQ("", Dirname("world"));
+  EXPECT_EQ("/", Dirname("/"));
+  EXPECT_EQ("", Dirname(""));
+}
+
+TEST(FilePathTest, CheckExtension) {
+  EXPECT_EQ("gif", Extension("foo.gif"));
+  EXPECT_EQ("", Extension("foo."));
+  EXPECT_EQ("", Extension(""));
+  EXPECT_EQ("", Extension("/"));
+  EXPECT_EQ("", Extension("foo"));
+  EXPECT_EQ("", Extension("foo/"));
+  EXPECT_EQ("gif", Extension("/a/path/to/foo.gif"));
+  EXPECT_EQ("html", Extension("/a/path.bar/to/foo.html"));
+  EXPECT_EQ("", Extension("/a/path.bar/to/foo"));
+  EXPECT_EQ("baz", Extension("/a/path.bar/to/foo.bar.baz"));
+}
+
 }  // namespace
 }  // namespace fst