Modify mjListKeyMap to be a fixed-size array, and add internal object_lists variable to mjCModel to remove redundant code.

PiperOrigin-RevId: 590997348
Change-Id: I5f4d7de605f44d40a75eb1e9ceec208325fa3be5

test/user/user_model_test.cc

@@ -30,16 +30,41 @@ namespace {
 
 using ::testing::DoubleNear;
 using ::testing::ElementsAre;
+using ::testing::HasSubstr;
+using ::testing::IsNull;
 using ::testing::NotNull;
-using UserDataTest = MujocoTest;
 
 static std::vector<mjtNum> GetRow(const mjtNum* array, int ncolumn, int row) {
   return std::vector<mjtNum>(array + ncolumn * row,
                              array + ncolumn * (row + 1));
 }
 
+// ----------------------------- test mjCModel  --------------------------------
+
+using UserCModelTest = MujocoTest;
+
+TEST_F(UserCModelTest, RepeatedNames) {
+  static constexpr char xml[] = R"(
+   <mujoco>
+     <worldbody>
+       <body name="body1">
+         <joint axis="0 1 0" name="joint1"/>
+         <geom size="1" name="geom1"/>
+         <geom size="1" name="geom1"/>
+        </body>
+      </worldbody>
+    </mujoco>)";
+
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(model, IsNull());
+  EXPECT_THAT(error.data(), HasSubstr("repeated name 'geom1' in geom"));
+}
+
 // ------------- test automatic inference of nuser_xxx -------------------------
 
+using UserDataTest = MujocoTest;
+
 TEST_F(UserDataTest, AutoNUserBody) {
   static constexpr char xml[] = R"(
   <mujoco>