Fix Random mismatch between vanilla and FRAPI baked models (weighted and multipart)

fabric-renderer-api-v1/src/client/java/net/fabricmc/fabric/mixin/renderer/client/MultipartBakedModelMixin.java

@@ -81,15 +81,24 @@ public void emitBlockQuads(BlockRenderView blockView, BlockState state, BlockPos
 				Pair<Predicate<BlockState>, BakedModel> pair = components.get(i);
 
 				if (pair.getLeft().test(state)) {
-					pair.getRight().emitBlockQuads(blockView, state, pos, randomSupplier, context);
 					bitSet.set(i);
 				}
 			}
 
 			stateCache.put(state, bitSet);
-		} else {
-			for (int i = 0; i < this.components.size(); i++) {
-				if (bitSet.get(i)) components.get(i).getRight().emitBlockQuads(blockView, state, pos, randomSupplier, context);
+		}
+
+		Random random = randomSupplier.get();
+		// Imitate vanilla passing a new random to the submodels
+		long randomSeed = random.nextLong();
+		Supplier<Random> subModelRandomSupplier = () -> {
+			random.setSeed(randomSeed);
+			return random;
+		};
+
+		for (int i = 0; i < this.components.size(); i++) {
+			if (bitSet.get(i)) {
+				components.get(i).getRight().emitBlockQuads(blockView, state, pos, subModelRandomSupplier, context);
 			}
 		}
 	}

fabric-renderer-api-v1/src/client/java/net/fabricmc/fabric/mixin/renderer/client/WeightedBakedModelMixin.java

@@ -71,7 +71,11 @@ public void emitBlockQuads(BlockRenderView blockView, BlockState state, BlockPos
 		Weighted.Present<BakedModel> selected = Weighting.getAt(this.models, Math.abs((int) randomSupplier.get().nextLong()) % this.totalWeight).orElse(null);
 
 		if (selected != null) {
-			selected.getData().emitBlockQuads(blockView, state, pos, randomSupplier, context);
+			selected.getData().emitBlockQuads(blockView, state, pos, () -> {
+				Random random = randomSupplier.get();
+				random.nextLong(); // Imitate vanilla modifying the random before passing it to the submodel
+				return random;
+			}, context);
 		}
 	}
 
@@ -80,7 +84,11 @@ public void emitItemQuads(ItemStack stack, Supplier<Random> randomSupplier, Rend
 		Weighted.Present<BakedModel> selected = Weighting.getAt(this.models, Math.abs((int) randomSupplier.get().nextLong()) % this.totalWeight).orElse(null);
 
 		if (selected != null) {
-			selected.getData().emitItemQuads(stack, randomSupplier, context);
+			selected.getData().emitItemQuads(stack, () -> {
+				Random random = randomSupplier.get();
+				random.nextLong(); // Imitate vanilla modifying the random before passing it to the submodel
+				return random;
+			}, context);
 		}
 	}
 }

fabric-renderer-api-v1/src/testmod/resources/fabric.mod.json

@@ -15,6 +15,7 @@
       "net.fabricmc.fabric.test.renderer.RendererTest"
     ],
     "client": [
+      "net.fabricmc.fabric.test.renderer.client.RandomSupplierTest",
       "net.fabricmc.fabric.test.renderer.client.RendererClientTest"
     ]
   }

fabric-renderer-api-v1/src/testmodClient/java/net/fabricmc/fabric/test/renderer/client/RandomSupplierTest.java

@@ -0,0 +1,138 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.test.renderer.client;
+
+import java.util.List;
+import java.util.function.Supplier;
+
+import org.apache.commons.lang3.tuple.Pair;
+import org.jetbrains.annotations.Nullable;
+
+import net.minecraft.block.BlockState;
+import net.minecraft.block.Blocks;
+import net.minecraft.client.render.model.BakedModel;
+import net.minecraft.client.render.model.BakedQuad;
+import net.minecraft.client.render.model.MultipartBakedModel;
+import net.minecraft.client.render.model.WeightedBakedModel;
+import net.minecraft.client.render.model.json.ModelOverrideList;
+import net.minecraft.client.render.model.json.ModelTransformation;
+import net.minecraft.client.texture.Sprite;
+import net.minecraft.util.collection.Weighted;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.util.math.Direction;
+import net.minecraft.util.math.random.Random;
+import net.minecraft.world.BlockRenderView;
+
+import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.fabric.api.renderer.v1.render.RenderContext;
+
+/**
+ * Tests that vanilla and Fabric API give the same random results.
+ *
+ * <p>Never do this in a real mod, this is purely for testing!
+ */
+public class RandomSupplierTest implements ClientModInitializer {
+	private static long previousRandom = 0;
+	private static boolean hasPreviousRandom = false;
+
+	@Override
+	public void onInitializeClient() {
+		var checkingModel = new RandomCheckingBakedModel();
+		var weighted = new WeightedBakedModel(List.of(
+				Weighted.of(checkingModel, 1),
+				Weighted.of(checkingModel, 2)));
+		var multipart = new MultipartBakedModel(List.of(
+				Pair.of(state -> true, weighted),
+				Pair.of(state -> true, weighted)));
+		var weightedAgain = new WeightedBakedModel(List.of(
+				Weighted.of(multipart, 1),
+				Weighted.of(multipart, 2)));
+
+		long startingSeed = 42;
+		Random random = Random.create();
+
+		random.setSeed(startingSeed);
+		weightedAgain.getQuads(Blocks.STONE.getDefaultState(), null, random);
+
+		random.setSeed(startingSeed);
+		weightedAgain.getQuads(Blocks.STONE.getDefaultState(), null, random);
+
+		Supplier<Random> randomSupplier = () -> {
+			random.setSeed(startingSeed);
+			return random;
+		};
+		weightedAgain.emitBlockQuads(null, Blocks.STONE.getDefaultState(), BlockPos.ORIGIN, randomSupplier, null);
+	}
+
+	private static class RandomCheckingBakedModel implements BakedModel {
+		@Override
+		public List<BakedQuad> getQuads(@Nullable BlockState state, @Nullable Direction face, Random random) {
+			long value = random.nextLong();
+
+			if (hasPreviousRandom) {
+				if (value != previousRandom) {
+					throw new AssertionError("Random value is not the same as the previous one!");
+				}
+			} else {
+				hasPreviousRandom = true;
+				previousRandom = value;
+			}
+
+			return List.of();
+		}
+
+		@Override
+		public void emitBlockQuads(BlockRenderView blockView, BlockState state, BlockPos pos, Supplier<Random> randomSupplier, RenderContext context) {
+			getQuads(state, null, randomSupplier.get());
+		}
+
+		@Override
+		public boolean useAmbientOcclusion() {
+			return false;
+		}
+
+		@Override
+		public boolean hasDepth() {
+			return false;
+		}
+
+		@Override
+		public boolean isSideLit() {
+			return false;
+		}
+
+		@Override
+		public boolean isBuiltin() {
+			return false;
+		}
+
+		@Override
+		public Sprite getParticleSprite() {
+			return null;
+		}
+
+		@Override
+		public ModelTransformation getTransformation() {
+			return null;
+		}
+
+		@Override
+		public ModelOverrideList getOverrides() {
+			return null;
+		}
+	}
+}