Add RedundantDstDataFilter

CHANGELOG.md

@@ -4,7 +4,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-- Added `EmptyElementFilter`, an adapter which filters out out elements that effectively don't contain any data
+- Added `EmptyElementFilter`, an adapter which filters out elements that effectively don't contain any data
+- Added `RedundantDstDataFilter`, an adapter which nulls out destination names and descriptors that are equal to their source counterparts
 - Added a simplified `MappingNsCompleter` constructor for completing all destination names with the source names
 - Added `MappingTree#propagateOuterClassNames` as a more efficient tree-API alternative to `OuterClassNamePropagator`
 - Promoted previously internal `NopMappingVisitor`, `VisitOrderVerifier` and `SubsetChecker` to experimental API

src/main/java/net/fabricmc/mappingio/adapter/RedundantDstDataFilter.java

@@ -0,0 +1,105 @@
+/*
+ * Copyright (c) 2025 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.mappingio.adapter;
+
+import java.io.IOException;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.fabricmc.mappingio.MappedElementKind;
+import net.fabricmc.mappingio.MappingVisitor;
+
+/**
+ * A mapping visitor that nulls out destination names and descriptors that are equal to their source counterparts.
+ *
+ * @apiNote Extending this class is allowed, but no guarantees are made regarding the stability of its protected members.
+ */
+public class RedundantDstDataFilter extends ForwardingMappingVisitor {
+	public RedundantDstDataFilter(MappingVisitor next) {
+		super(next);
+	}
+
+	protected void init() {
+		srcName = null;
+		srcDesc = null;
+	}
+
+	@Override
+	public void reset() {
+		init();
+		super.reset();
+	}
+
+	@Override
+	public boolean visitClass(String srcName) throws IOException {
+		this.srcName = srcName;
+		return super.visitClass(srcName);
+	}
+
+	@Override
+	public boolean visitField(String srcName, @Nullable String srcDesc) throws IOException {
+		this.srcName = srcName;
+		this.srcDesc = srcDesc;
+		return super.visitField(srcName, srcDesc);
+	}
+
+	@Override
+	public boolean visitMethod(String srcName, @Nullable String srcDesc) throws IOException {
+		this.srcName = srcName;
+		this.srcDesc = srcDesc;
+		return super.visitMethod(srcName, srcDesc);
+	}
+
+	@Override
+	public boolean visitMethodArg(int argPosition, int lvIndex, @Nullable String srcName) throws IOException {
+		this.srcName = srcName;
+		return super.visitMethodArg(argPosition, lvIndex, srcName);
+	}
+
+	@Override
+	public boolean visitMethodVar(int lvtRowIndex, int lvIndex, int startOpIdx, int endOpIdx, @Nullable String srcName) throws IOException {
+		this.srcName = srcName;
+		return super.visitMethodVar(lvtRowIndex, lvIndex, startOpIdx, endOpIdx, srcName);
+	}
+
+	@Override
+	public void visitDstName(MappedElementKind targetKind, int namespace, String name) throws IOException {
+		if (name != null && name.equals(srcName)) {
+			name = null;
+		}
+
+		super.visitDstName(targetKind, namespace, name);
+	}
+
+	@Override
+	public void visitDstDesc(MappedElementKind targetKind, int namespace, String desc) throws IOException {
+		if (desc != null && desc.equals(srcDesc)) {
+			desc = null;
+		}
+
+		super.visitDstDesc(targetKind, namespace, desc);
+	}
+
+	@Override
+	public boolean visitEnd() throws IOException {
+		init();
+		return super.visitEnd();
+	}
+
+	protected String srcName;
+	protected String srcDesc;
+}

src/test/java/net/fabricmc/mappingio/test/tests/filtering/RedundantDstDataFilterTest.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright (c) 2025 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.mappingio.test.tests.filtering;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+import java.io.IOException;
+import java.nio.file.Files;
+
+import org.jetbrains.annotations.Nullable;
+import org.junit.jupiter.api.Test;
+
+import net.fabricmc.mappingio.MappedElementKind;
+import net.fabricmc.mappingio.adapter.NopMappingVisitor;
+import net.fabricmc.mappingio.adapter.RedundantDstDataFilter;
+import net.fabricmc.mappingio.format.MappingFormat;
+import net.fabricmc.mappingio.test.TestMappings;
+import net.fabricmc.mappingio.test.TestMappings.MappingDir;
+
+public class RedundantDstDataFilterTest {
+	@Test
+	public void run() {
+		for (MappingDir dir : TestMappings.values()) {
+			for (MappingFormat format : MappingFormat.values()) {
+				assertDoesNotThrow(() -> check(dir, format), "Failed for " + dir + " with " + format);
+			}
+		}
+	}
+
+	private void check(MappingDir dir, MappingFormat format) throws Exception {
+		if (!Files.exists(dir.pathFor(format))) {
+			return;
+		}
+
+		dir.read(format, new RedundantDstDataFilter(new NoRedundantDstDataAsserter()));
+	}
+
+	private static class NoRedundantDstDataAsserter extends NopMappingVisitor {
+		private NoRedundantDstDataAsserter() {
+			super(true);
+		}
+
+		@Override
+		public boolean visitClass(String srcName) throws IOException {
+			this.srcName = srcName;
+			return super.visitClass(srcName);
+		}
+
+		@Override
+		public boolean visitField(String srcName, @Nullable String srcDesc) throws IOException {
+			this.srcName = srcName;
+			this.srcDesc = srcDesc;
+			return super.visitField(srcName, srcDesc);
+		}
+
+		@Override
+		public boolean visitMethod(String srcName, @Nullable String srcDesc) throws IOException {
+			this.srcName = srcName;
+			this.srcDesc = srcDesc;
+			return super.visitMethod(srcName, srcDesc);
+		}
+
+		@Override
+		public boolean visitMethodArg(int argPosition, int lvIndex, @Nullable String srcName) throws IOException {
+			this.srcName = srcName;
+			return super.visitMethodArg(argPosition, lvIndex, srcName);
+		}
+
+		@Override
+		public boolean visitMethodVar(int lvtRowIndex, int lvIndex, int startOpIdx, int endOpIdx, @Nullable String srcName) throws IOException {
+			this.srcName = srcName;
+			return super.visitMethodVar(lvtRowIndex, lvIndex, startOpIdx, endOpIdx, srcName);
+		}
+
+		@Override
+		public void visitDstName(MappedElementKind targetKind, int namespace, String name) {
+			assertNotEquals(srcName, name, "Redundant destination name for " + targetKind + " in destination namespace " + namespace + " (" + srcName + " -> " + name + ")");
+		}
+
+		@Override
+		public void visitDstDesc(MappedElementKind targetKind, int namespace, String desc) {
+			if (srcDesc == null) {
+				return;
+			}
+
+			assertNotEquals(srcDesc, desc, "Redundant destination descriptor for " + targetKind + " in destination namespace " + namespace + " (" + srcDesc + " -> " + desc + ")");
+		}
+
+		private String srcName;
+		private String srcDesc;
+	}
+}