Fix sort order of weighted transitions if weights are equal (#357)

use stable sort when sorting weighted transitions, respecting the original order

Author: hendrikmuhs

keyvi/include/keyvi/dictionary/fsa/traversal/weighted_traversal.h

@@ -69,8 +69,8 @@ inline void TraversalState<WeightedTransition>::Add(uint64_t s, uint32_t weight,
 template <>
 inline void TraversalState<WeightedTransition>::PostProcess(TraversalPayload<WeightedTransition>* payload) {
   if (traversal_state_payload.transitions.size() > 0) {
-    std::sort(traversal_state_payload.transitions.begin(), traversal_state_payload.transitions.end(),
-              WeightedTransitionCompare);
+    std::stable_sort(traversal_state_payload.transitions.begin(), traversal_state_payload.transitions.end(),
+                     WeightedTransitionCompare);
   }
 }
 

keyvi/tests/keyvi/dictionary/fsa/traversal/weighted_traversal_test.cpp

@@ -0,0 +1,66 @@
+//
+// keyvi - A key value store.
+//
+// Copyright 2015 Hendrik Muhs<[email protected]>
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+/*
+ * near_traversal.cpp
+ *
+ *  Created on: Sep 23, 2025
+ *      Author: hendrik
+ */
+
+#include <utility>
+
+#include <boost/test/unit_test.hpp>
+#include "keyvi/dictionary/fsa/traversal/weighted_traversal.h"
+
+namespace keyvi {
+namespace dictionary {
+namespace fsa {
+namespace traversal {
+
+BOOST_AUTO_TEST_SUITE(WeightedTraversalTests)
+
+BOOST_AUTO_TEST_CASE(PostProcessStableSortEqualWeights) {
+  TraversalStack<WeightedTransition> traversal_stack;
+
+  uint64_t s = 0;
+  unsigned char c = 97;
+
+  // add states with same weights
+  for (; c < 123; ++c, ++s) {
+    traversal_stack.traversal_states[0].Add(s, 24, c, &traversal_stack.traversal_stack_payload);
+  }
+
+  // sort by weight (all the same)
+  traversal_stack.traversal_states[0].PostProcess(&traversal_stack.traversal_stack_payload);
+
+  // assert that sort did not change the original order (== stable sort)
+  c = 97;
+  for (const auto state : traversal_stack.traversal_states[0].traversal_state_payload.transitions) {
+    BOOST_CHECK_EQUAL(c++, state.label);
+  }
+
+  BOOST_CHECK_EQUAL(c, 123);
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+
+} /* namespace traversal*/
+} /* namespace fsa */
+} /* namespace dictionary */
+} /* namespace keyvi */