perf(gb): reuse adjacent discard preference on memo hit

src/main/java/top/ellan/mahjong/table/core/round/GbBotDecisionService.java

@@ -187,9 +187,12 @@ private DiscardChoice bestDiscardChoice(
         }
         DiscardChoice best = null;
         EnumMap<MahjongTile, GbTingResponse> tingMemo = new EnumMap<>(MahjongTile.class);
+        MahjongTile previousDiscarded = null;
+        int previousDiscardPreference = 0;
         for (int i = 0; i < hand.size(); i++) {
             MahjongTile discarded = hand.get(i);
             GbTingResponse ting = tingMemo.get(discarded);
+            boolean tingMemoHit = ting != null;
             if (ting == null) {
                 List<MahjongTile> remaining = new ArrayList<>(hand);
                 remaining.remove(i);
@@ -198,7 +201,13 @@ private DiscardChoice bestDiscardChoice(
                     tingMemo.put(discarded, ting);
                 }
             }
-            DiscardChoice candidate = new DiscardChoice(i, readyScore(ting), discardPreference(hand, discarded));
+            long candidateReadyScore = readyScore(ting);
+            int candidateDiscardPreference = tingMemoHit && discarded == previousDiscarded
+                ? previousDiscardPreference
+                : discardPreference(hand, discarded);
+            DiscardChoice candidate = new DiscardChoice(i, candidateReadyScore, candidateDiscardPreference);
+            previousDiscarded = discarded;
+            previousDiscardPreference = candidateDiscardPreference;
             if (best == null || candidate.compareTo(best) > 0) {
                 best = candidate;
             }

src/test/kotlin/top/ellan/mahjong/table/core/round/GbBotDecisionServiceTest.kt

@@ -65,6 +65,56 @@ class GbBotDecisionServiceTest {
         )
     }
 
+    @Test
+    fun `discard suggestion keeps the earliest index for equal duplicate candidates`() {
+        val service = GbBotDecisionService(8)
+        val hand = listOf(MahjongTile.M1, MahjongTile.M1, MahjongTile.M1)
+        var evaluations = 0
+
+        val suggestedIndex =
+            service.suggestedDiscardIndex(hand, emptyList()) { _, _ ->
+                evaluations++
+                GbTingResponse(true, emptyList(), null)
+            }
+
+        assertEquals(0, suggestedIndex)
+        assertEquals(1, evaluations)
+    }
+
+    @Test
+    fun `discard suggestion recomputes null before memoizing a later duplicate response`() {
+        val service = GbBotDecisionService(8)
+        val hand = listOf(MahjongTile.M1, MahjongTile.M1, MahjongTile.M1)
+        val ready = GbTingResponse(true, listOf(GbTingCandidate("W1", 8)), null)
+        var evaluations = 0
+
+        val suggestedIndex =
+            service.suggestedDiscardIndex(hand, emptyList()) { _, _ ->
+                evaluations++
+                if (evaluations == 1) null else ready
+            }
+
+        assertEquals(1, suggestedIndex)
+        assertEquals(2, evaluations)
+    }
+
+    @Test
+    fun `discard suggestion does not reuse red five preference for the exact regular tile`() {
+        val service = GbBotDecisionService(8)
+        val hand = listOf(MahjongTile.M5, MahjongTile.M5_RED, MahjongTile.M5)
+        val ready = GbTingResponse(true, listOf(GbTingCandidate("W1", 8)), null)
+        var evaluations = 0
+
+        val suggestedIndex =
+            service.suggestedDiscardIndex(hand, emptyList()) { remaining, _ ->
+                evaluations++
+                if (MahjongTile.M5_RED in remaining) ready else GbTingResponse(true, emptyList(), null)
+            }
+
+        assertEquals(0, suggestedIndex)
+        assertEquals(2, evaluations)
+    }
+
     @Test
     fun `discard suggestion preserves null evaluator recomputation`() {
         val service = GbBotDecisionService(8)