feat(protogen): generate oneof case accessors and group clear

AGENTS.md

@@ -16,7 +16,9 @@ Maven project. The notable capabilities are:
   proto3: proto2 `required` fields are enforced by the builder chain, while
   proto3 (which has no required fields) generates all-optional builders with
   presence-aware `hasXxx()` accessors (only for message fields and explicit
-  `optional` fields).
+  `optional` fields). `oneof` groups get a `getXxxCase()` discriminator and a
+  `clearXxx()` for the whole group, so the selected member is inspectable and
+  resettable through the builder chain.
 - **Context engineering** (`code/context`) — a fast, regex-based finder that
   builds the tree of classes used by a given class, plus a `ProjectTreeBuilder`
   that scans a whole Java project into a tree of folders, files and

README.md

@@ -316,6 +316,12 @@ presence — every singular field in proto2, but in proto3 only message fields a
 those declared with the explicit `optional` keyword (implicit-presence proto3
 scalars, which have no `hasXxx()`, are left alone).
 
+A `oneof` group additionally gets a `getXxxCase()` accessor returning protobuf's
+generated `XxxCase` enum, so you can tell which member is set, plus a
+`clearXxx()` that resets the whole group — both reachable through the fluent
+builder chain. The synthetic oneofs that back proto3 `optional` fields are not
+treated as groups, so no spurious case accessor is generated for them.
+
 ## gRPC example
 
 An end-to-end gRPC example combining standard protobuf/gRPC code generation with the compile-time-safe builder generation from this project.

code/protogen-maven-plugin-test/src/main/proto/test4.proto

@@ -15,3 +15,11 @@ message Team {
   Profile lead = 1;
   repeated string members = 2;
 }
+
+message Payment {
+  string reference = 1;
+  oneof method {
+    string card = 2;
+    string cash = 3;
+  }
+}

code/protogen-maven-plugin-test/src/test/java/io/github/adamw7/tools/code/usecase/GeneretedCodeTest.java

@@ -17,6 +17,7 @@
 import org.output.generated.MeasurementBuilder;
 import org.output.generated.MyMessageBuilder;
 import org.output.generated.OneOptionalFieldOnlyBuilder;
+import org.output.generated.PaymentBuilder;
 import org.output.generated.ProfileBuilder;
 import org.output.generated.ServerBuilder;
 import org.output.generated.TeamBuilder;
@@ -35,6 +36,7 @@
 import io.github.adamw7.tools.code.test.MyMessage;
 import io.github.adamw7.tools.code.test.Server;
 import io.github.adamw7.tools.code.test.Wheel;
+import io.github.adamw7.tools.code.test4.Payment;
 import io.github.adamw7.tools.code.test4.Profile;
 import io.github.adamw7.tools.code.test4.Team;
 
@@ -206,4 +208,32 @@ public void proto3MessageFieldAndRepeatedFieldBuild() {
 		assertTrue(team.hasLead());
 	}
 
+	@Test
+	public void proto3OneofCaseReflectsSelectedMember() {
+		PaymentBuilder builder = new PaymentBuilder();
+		assertEquals(Payment.MethodCase.METHOD_NOT_SET, builder.getMethodCase());
+
+		builder.setCard("4111");
+		assertEquals(Payment.MethodCase.CARD, builder.getMethodCase());
+		assertTrue(builder.hasCard());
+		assertFalse(builder.hasCash());
+
+		builder.setCash("20");
+		assertEquals(Payment.MethodCase.CASH, builder.getMethodCase());
+		assertTrue(builder.hasCash());
+		assertFalse(builder.hasCard());
+	}
+
+	@Test
+	public void proto3OneofClearResetsSelection() {
+		PaymentBuilder builder = new PaymentBuilder();
+		builder.setReference("order-1").setCard("4111");
+		assertEquals(Payment.MethodCase.CARD, builder.getMethodCase());
+
+		Payment payment = builder.clearMethod().build();
+		assertEquals(Payment.MethodCase.METHOD_NOT_SET, payment.getMethodCase());
+		assertEquals("order-1", payment.getReference());
+		assertFalse(payment.hasCard());
+	}
+
 }

code/protogen-maven-plugin/src/main/java/io/github/adamw7/tools/code/gen/ClassInfo.java

@@ -7,6 +7,7 @@
 import com.google.protobuf.Descriptors;
 import com.google.protobuf.Descriptors.Descriptor;
 import com.google.protobuf.Descriptors.FieldDescriptor;
+import com.google.protobuf.Descriptors.OneofDescriptor;
 
 public class ClassInfo {
 
@@ -129,6 +130,10 @@ public String getInputPkg() {
 	public String fullName() {
 		return Utils.getClassName(descriptor.getFullName());
 	}
+
+	public List<OneofDescriptor> realOneofs() {
+		return descriptor.getRealOneofs();
+	}
 
 	public List<FieldDescriptor> getPureComplexFields() {
 		return getPureComplexFields(descriptor);

code/protogen-maven-plugin/src/main/java/io/github/adamw7/tools/code/gen/Implementations.java

@@ -4,6 +4,7 @@
 import java.util.List;
 
 import com.google.protobuf.Descriptors.FieldDescriptor;
+import com.google.protobuf.Descriptors.OneofDescriptor;
 
 public class Implementations extends AbstractStatements {
 
@@ -76,7 +77,11 @@ public StringBuilder generateMethods() {
 		for (FieldDescriptor field : info.optional()) {
 			builder.append(methods.setter(field, optionalIfcName));
 			builder.append(methods.has("builder", field));
-			builder.append(methods.clear("builder", field, optionalIfcName));	
+			builder.append(methods.clear("builder", field, optionalIfcName));
+		}
+		for (OneofDescriptor oneof : info.realOneofs()) {
+			builder.append(methods.oneofCaseGetter("builder", oneof));
+			builder.append(methods.oneofClear("builder", oneof, optionalIfcName));
 		}
 		return builder;
 	}

code/protogen-maven-plugin/src/main/java/io/github/adamw7/tools/code/gen/Interfaces.java

@@ -4,6 +4,7 @@
 import java.util.List;
 
 import com.google.protobuf.Descriptors.FieldDescriptor;
+import com.google.protobuf.Descriptors.OneofDescriptor;
 
 public class Interfaces extends AbstractStatements {
 
@@ -26,11 +27,16 @@ public ClassContainer generateOptional() {
 
 		for (FieldDescriptor optionalField : info.optional()) {
 			builder.append(methods.declareSetter(optionalField, optionalIfcName));
-			builder.append(methods.declareHas(optionalField));	
+			builder.append(methods.declareHas(optionalField));
 			String clearReturnType = Utils.getNextIfc(info.name(), info.nonOptional(), optionalField);
 			builder.append(methods.declareClear(optionalField, clearReturnType));
 		}
-
+
+		for (OneofDescriptor oneof : info.realOneofs()) {
+			builder.append(methods.declareOneofCaseGetter(oneof));
+			builder.append(methods.declareOneofClear(oneof, optionalIfcName));
+		}
+
 		builder.append(info.name()).append(" build();}");
 
 		return new ClassContainer(optionalIfcName, builder);

code/protogen-maven-plugin/src/main/java/io/github/adamw7/tools/code/gen/Methods.java

@@ -3,6 +3,7 @@
 import java.util.List;
 
 import com.google.protobuf.Descriptors.FieldDescriptor;
+import com.google.protobuf.Descriptors.OneofDescriptor;
 
 public class Methods {
 
@@ -65,10 +66,25 @@ private boolean needsHas(FieldDescriptor field) {
 	}
 
 	public StringBuilder clear(String classOrBuilder, FieldDescriptor field, String returnType) {
-		String fieldName = Utils.toUpperCamelCase(field.getName());
+		return clearByName(classOrBuilder, field.getName(), returnType);
+	}
+
+	private StringBuilder clearByName(String classOrBuilder, String rawName, String returnType) {
+		String name = Utils.toUpperCamelCase(rawName);
 		String impl = returnType.replace(Utils.IFC_SUFFIX, Utils.IMPL_SUFFIX);
 		return override("public %s clear%s() {%s.clear%s();return new %s(%s);}"
-				.formatted(returnType, fieldName, classOrBuilder, fieldName, impl, classOrBuilder));
+				.formatted(returnType, name, classOrBuilder, name, impl, classOrBuilder));
+	}
+
+	public StringBuilder oneofCaseGetter(String classOrBuilder, OneofDescriptor oneof) {
+		String name = Utils.toUpperCamelCase(oneof.getName());
+		String caseType = "%s.%sCase".formatted(className, name);
+		return override("public %s get%sCase() {return %s.get%sCase();}"
+				.formatted(caseType, name, classOrBuilder, name));
+	}
+
+	public StringBuilder oneofClear(String classOrBuilder, OneofDescriptor oneof, String returnType) {
+		return clearByName(classOrBuilder, oneof.getName(), returnType);
 	}
 
 	private StringBuilder override(String body) {
@@ -93,8 +109,22 @@ public StringBuilder declareHas(FieldDescriptor field) {
 	}
 
 	public StringBuilder declareClear(FieldDescriptor field, String returnType) {
+		return declareClearByName(field.getName(), returnType);
+	}
+
+	private StringBuilder declareClearByName(String rawName, String returnType) {
 		return new StringBuilder(
-				"%s clear%s();".formatted(returnType, Utils.toUpperCamelCase(field.getName())));
+				"%s clear%s();".formatted(returnType, Utils.toUpperCamelCase(rawName)));
+	}
+
+	public StringBuilder declareOneofCaseGetter(OneofDescriptor oneof) {
+		String name = Utils.toUpperCamelCase(oneof.getName());
+		return new StringBuilder(
+				"%s.%sCase get%sCase();".formatted(className, name, name));
+	}
+
+	public StringBuilder declareOneofClear(OneofDescriptor oneof, String returnType) {
+		return declareClearByName(oneof.getName(), returnType);
 	}
 
 	private String generateSetter(FieldDescriptor field) {