BUG: Read the extension from the final path component in get_ext

src/trx.cpp

@@ -849,12 +849,17 @@ std::string get_base(const std::string &delimiter, const std::string &str) {
 }
 
 std::string get_ext(const std::string &str) {
+  // Extract the extension from the final path component only. Scanning the
+  // whole path for '.' mistakes a dot in a parent directory (e.g. a
+  // version-numbered build path like ".../foo-5.4/out") for the extension.
+  const std::size_t sep = str.find_last_of("/\\");
+  const std::string name = (sep == std::string::npos) ? str : str.substr(sep + 1);
+
   std::string ext;
   constexpr char kDelimiter = '.';
-
-  const std::size_t pos = str.rfind(kDelimiter);
-  if (pos != std::string::npos && pos + 1 < str.length()) {
-    ext = str.substr(pos + 1);
+  const std::size_t pos = name.rfind(kDelimiter);
+  if (pos != std::string::npos && pos + 1 < name.length()) {
+    ext = name.substr(pos + 1);
   }
   return ext;
 }

tests/test_trx_io.cpp

@@ -839,3 +839,21 @@ TEST(TrxFileIo, export_dpv_to_tsf_errors) {
   trx::TrxFile<float> empty;
   EXPECT_THROW(empty.export_dpv_to_tsf("signal", output_path.string(), "1"), trx::TrxFormatError);
 }
+
+// get_ext must read the extension from the final path component only. A '.' in
+// a parent directory (e.g. a version-numbered build path like ".../foo-5.4/")
+// must not be treated as the extension. Regression for "Unsupported extension".
+TEST(TrxFileIo, get_ext_uses_final_path_component) {
+  // Dotted parent directory, file has no extension -> empty (the bug case).
+  EXPECT_EQ(trx::get_ext("/a/build-5.4/out/trx_test"), "");
+  EXPECT_EQ(trx::get_ext("/a/build-5.4/out/trx_test.trx"), "trx");
+  // No path separator: whole string is the name.
+  EXPECT_EQ(trx::get_ext("out.trx"), "trx");
+  EXPECT_EQ(trx::get_ext("trx_test"), "");
+  // dtype-style array filenames (name.dims.dtype) resolve to the dtype, even
+  // under a dotted parent directory.
+  EXPECT_EQ(trx::get_ext("/a/shard.1/positions.3.float32"), "float32");
+  EXPECT_EQ(trx::get_ext("offsets.uint64"), "uint64");
+  // Trailing dot -> empty.
+  EXPECT_EQ(trx::get_ext("/a/b.c/name."), "");
+}