From 6eabebff0085fe80f41aef4ac2346cf32436536e Mon Sep 17 00:00:00 2001
From: Jean-Luc Makiola <business@jeanlucmakiola.de>
Date: Mon, 1 Jun 2026 17:28:32 +0200
Subject: [PATCH] fix(combinations): guard null response; reject
 self-combination at load
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- Combinations.tryUse: only write response when non-null and non-blank,
  preventing NPE/IllegalArgument for effects-only recipes.
- CombinationFactory.fromDto: throw WorldLoadException when a == b,
  since item singletons make a self-combination impossible at runtime.
- Tests: +effectsOnlyRecipeAppliesEffectsWithoutProduceOrResponse
  (CombinationsTest 7→8), +rejectsSelfCombination
  (CombinationFactoryTest 4→5).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 .../thb/jeanluc/adventure/game/Combinations.java   |  4 +++-
 .../adventure/loader/CombinationFactory.java       |  4 ++++
 .../jeanluc/adventure/game/CombinationsTest.java   | 14 ++++++++++++++
 .../adventure/loader/CombinationFactoryTest.java   |  8 ++++++++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/Semesterprojekt/src/main/java/thb/jeanluc/adventure/game/Combinations.java b/Semesterprojekt/src/main/java/thb/jeanluc/adventure/game/Combinations.java
index d34108a..01f5ed1 100644
--- a/Semesterprojekt/src/main/java/thb/jeanluc/adventure/game/Combinations.java
+++ b/Semesterprojekt/src/main/java/thb/jeanluc/adventure/game/Combinations.java
@@ -47,7 +47,9 @@ public final class Combinations {
             }
         }
         Effects.applyAll(c.effects(), ctx);
-        ctx.getIo().write(c.response());
+        if (c.response() != null && !c.response().isBlank()) {
+            ctx.getIo().write(c.response());
+        }
     }
 
     private static boolean present(GameContext ctx, String id) {
diff --git a/Semesterprojekt/src/main/java/thb/jeanluc/adventure/loader/CombinationFactory.java b/Semesterprojekt/src/main/java/thb/jeanluc/adventure/loader/CombinationFactory.java
index 81234b4..296b4bd 100644
--- a/Semesterprojekt/src/main/java/thb/jeanluc/adventure/loader/CombinationFactory.java
+++ b/Semesterprojekt/src/main/java/thb/jeanluc/adventure/loader/CombinationFactory.java
@@ -18,6 +18,10 @@ public final class CombinationFactory {
     public static Combination fromDto(CombinationDto dto, Map<String, Item> items) {
         requireItem(items, dto.a(), "a");
         requireItem(items, dto.b(), "b");
+        if (dto.a().equals(dto.b())) {
+            throw new WorldLoadException(
+                    "Combination cannot combine an item with itself: '" + dto.a() + "'");
+        }
         if (dto.consume() != null) {
             for (String id : dto.consume()) {
                 requireItem(items, id, "consume");
diff --git a/Semesterprojekt/src/test/java/thb/jeanluc/adventure/game/CombinationsTest.java b/Semesterprojekt/src/test/java/thb/jeanluc/adventure/game/CombinationsTest.java
index 16fb1e6..910446e 100644
--- a/Semesterprojekt/src/test/java/thb/jeanluc/adventure/game/CombinationsTest.java
+++ b/Semesterprojekt/src/test/java/thb/jeanluc/adventure/game/CombinationsTest.java
@@ -123,6 +123,20 @@ class CombinationsTest {
         assertThat(ctx.getPlayer().hasItem("matches")).isTrue(); // not consumed
     }
 
+    @Test
+    void effectsOnlyRecipeAppliesEffectsWithoutProduceOrResponse() {
+        Combination effectsOnly = new Combination("matches", "shovel",
+                java.util.List.of(), java.util.List.of(), null,
+                java.util.List.of(new Effect(Effect.Type.SET_FLAG, "rite_done")),
+                null, null);
+        GameContext ctx = ctx(effectsOnly);
+        give(ctx, "matches");
+        give(ctx, "shovel");
+        Combinations.tryUse(ctx, "matches", "shovel");
+        assertThat(ctx.getState().isSet("rite_done")).isTrue();
+        assertThat(out(ctx)).doesNotContain("null");
+    }
+
     @Test
     void metRequiresAppliesEffectsAndProduces() {
         Combination gated = new Combination("matches", "torch",
diff --git a/Semesterprojekt/src/test/java/thb/jeanluc/adventure/loader/CombinationFactoryTest.java b/Semesterprojekt/src/test/java/thb/jeanluc/adventure/loader/CombinationFactoryTest.java
index 61cd55c..afa1723 100644
--- a/Semesterprojekt/src/test/java/thb/jeanluc/adventure/loader/CombinationFactoryTest.java
+++ b/Semesterprojekt/src/test/java/thb/jeanluc/adventure/loader/CombinationFactoryTest.java
@@ -58,4 +58,12 @@ class CombinationFactoryTest {
                 .isInstanceOf(WorldLoadException.class)
                 .hasMessageContaining("wraith");
     }
+
+    @Test
+    void rejectsSelfCombination() {
+        CombinationDto dto = new CombinationDto("torch", "torch", null, null, null, null, "x", null);
+        assertThatThrownBy(() -> CombinationFactory.fromDto(dto, items()))
+                .isInstanceOf(WorldLoadException.class)
+                .hasMessageContaining("itself");
+    }
 }