fix missing points and add tests

app/src/main/kotlin/dogacel/kotlinx/protobuf/gen/CodeGenerator.kt

@@ -166,16 +166,42 @@ class CodeGenerator {
     }
 
     /**
-     * Generate the code for the given [Descriptors.Descriptor]. Returns a Boolean determine whether the
-     * descriptor has a property field or not.
+     * Check if the given message descriptor contains any fields or not.
      *
-     * @param messageDescriptor [Descriptors.FileDescriptor] to generate code for.
-     * @return Boolean that represents whether the descriptor has a property field or not.
+     * This is used to avoid generating data classes with 0 properties because they are not allowed in Kotlin.
+     *
+     * @param messageDescriptor the descriptor to check if it has any fields.
+     * @return whether the given message descriptor contains any fields or not.
      */
-    private fun hasPropertyField(messageDescriptor: Descriptors.Descriptor): Boolean {
-        return messageDescriptor.fields.any {
-            it.isOptional || it.isRequired || it.isRepeated
-        }
+    fun hasNoField(messageDescriptor: Descriptors.Descriptor): Boolean {
+        return messageDescriptor.fields.isEmpty()
+    }
+
+    /**
+     * Get the overriden methods for a method with no fields.
+     *
+     * @param messageDescriptor descriptor of the message to generate empty override methods for.
+     * @return a list of [FunSpec]s that contain the overriden `toString`, `hashCode` and `equals`.
+     */
+    private fun getEmptyOverrideFunSpecs(messageDescriptor: Descriptors.Descriptor): List<FunSpec> {
+        return listOf(
+            FunSpec.builder("toString")
+                .addModifiers(KModifier.OVERRIDE)
+                .addStatement("return %S", messageDescriptor.name)
+                .returns(String::class)
+                .build(),
+            FunSpec.builder("hashCode")
+                .addModifiers(KModifier.OVERRIDE)
+                .addStatement("return %L", messageDescriptor.name.hashCode())
+                .returns(Int::class)
+                .build(),
+            FunSpec.builder("equals")
+                .addModifiers(KModifier.OVERRIDE)
+                .addParameter("other", Any::class.asTypeName().copy(nullable = true))
+                .addStatement("return other is ${messageDescriptor.name}")
+                .returns(Boolean::class)
+                .build()
+        )
     }
 
     /**
@@ -208,13 +234,13 @@ class CodeGenerator {
      * @return [TypeSpec.Builder] that contains the generated code.
      */
     private fun generateSingleClass(messageDescriptor: Descriptors.Descriptor): TypeSpec.Builder {
-        val typeSpec = if (hasPropertyField(messageDescriptor)) {
-            TypeSpec.classBuilder(messageDescriptor.name)
-                .addModifiers(KModifier.DATA)
-                .addAnnotation(Serializable::class)
+        val typeSpec = TypeSpec.classBuilder(messageDescriptor.name)
+            .addAnnotation(Serializable::class)
+
+        if (hasNoField(messageDescriptor)) {
+            typeSpec.addFunctions(getEmptyOverrideFunSpecs(messageDescriptor))
         } else {
-            TypeSpec.classBuilder(messageDescriptor.name)
-                .addAnnotation(Serializable::class)
+            typeSpec.addModifiers(KModifier.DATA)
         }
 
         // A Data class needs a primary constructor with all the parameters.

app/src/test/kotlin/dogacel/kotlinx/protobuf/gen/CodeGeneratorTest.kt

@@ -0,0 +1,23 @@
+package dogacel.kotlinx.protobuf.gen
+
+import messages.MessageNoFieldsOuterClass.MessageNoFields
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class CodeGeneratorTest {
+
+    @Test
+    fun hasAnyFieldShouldWork() {
+        val codeGenerator = CodeGenerator()
+
+        val messageNoFields = MessageNoFields.getDescriptor()
+        val messageSomeFields1 = MessageNoFields.SubMessageNoFields.getDescriptor()
+        val messageSomeFields2 = MessageNoFields.SubMessageNoFields.SubMessageNoFieldsExtend.getDescriptor()
+        val messageSomeFieldsOneof = MessageNoFields.SubMessageOneofFields.getDescriptor()
+
+        assertEquals(true, codeGenerator.hasNoField(messageNoFields))
+        assertEquals(false, codeGenerator.hasNoField(messageSomeFields1))
+        assertEquals(false, codeGenerator.hasNoField(messageSomeFields2))
+        assertEquals(false, codeGenerator.hasNoField(messageSomeFieldsOneof))
+    }
+}

generated-code-tests/src/main/kotlin/testgen/messages/message_no_fields.proto.kt

@@ -0,0 +1,51 @@
+package testgen.messages
+
+import kotlin.Any
+import kotlin.Boolean
+import kotlin.Int
+import kotlin.String
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.protobuf.ProtoNumber
+
+@Serializable
+public class MessageNoFields() {
+  override fun toString(): String = "MessageNoFields"
+
+  override fun hashCode(): Int = 70_682_849
+
+  override fun equals(other: Any?): Boolean = other is MessageNoFields
+
+  @Serializable
+  public data class SubMessageNoFields(
+    @ProtoNumber(number = 1)
+    public val subHello: SubMessageNoFields? = null,
+  ) {
+    @Serializable
+    public data class SubMessageNoFieldsExtend(
+      @ProtoNumber(number = 1)
+      public val type: Type = testgen.messages.MessageNoFields.Type.UNKNOWN,
+    )
+  }
+
+  @Serializable
+  public data class SubMessageOneofFields(
+    @ProtoNumber(number = 1)
+    public val someValue: Int? = null,
+  ) {
+    init {
+      require(
+        listOfNotNull(
+          someValue,
+        ).size <= 1
+      ) { "Should only contain one of some_oneof." } 
+    }
+  }
+
+  @Serializable
+  public enum class Type {
+    @ProtoNumber(number = 0)
+    UNKNOWN,
+    @ProtoNumber(number = 1)
+    KNOWN,
+  }
+}

generated-code-tests/src/test/kotlin/dogacel/kotlinx/protobuf/gen/proto/MessageTest.kt

@@ -3,6 +3,7 @@ package dogacel.kotlinx.protobuf.gen.proto
 import kotlinx.serialization.decodeFromByteArray
 import kotlinx.serialization.encodeToByteArray
 import kotlinx.serialization.protobuf.ProtoBuf
+import testgen.messages.MessageNoFields
 import testgen.messages.MessagesMessage
 import kotlin.test.Test
 import kotlin.test.assertEquals
@@ -36,7 +37,10 @@ class MessageTest {
         assertEquals(message.id, result.id)
         assertEquals(message.optionalForeignMessage.c, result.optionalForeignMessage?.c)
         assertEquals(message.optionalNestedMessage.a, result.optionalNestedMessage?.a)
-        assertEquals(message.optionalNestedMessage.corecursive.id, result.optionalNestedMessage?.corecursive?.id)
+        assertEquals(
+            message.optionalNestedMessage.corecursive.id,
+            result.optionalNestedMessage?.corecursive?.id
+        )
         assertEquals(
             message.optionalNestedMessage.corecursive.optionalForeignMessage.c,
             result.optionalNestedMessage?.corecursive?.optionalForeignMessage?.c
@@ -60,4 +64,16 @@ class MessageTest {
         val deser = messages.Messages.MessagesMessage.parseFrom(ProtoBuf.encodeToByteArray(result))
         assertEquals(message, deser)
     }
+
+    @Test
+    fun shouldSerNoField() {
+        val message = messages.MessageNoFieldsOuterClass.MessageNoFields.newBuilder().build()
+
+        val messageBytes = message.toByteArray()
+        val result: MessageNoFields = ProtoBuf.decodeFromByteArray(messageBytes)
+
+        val deser =
+            messages.MessageNoFieldsOuterClass.MessageNoFields.parseFrom(ProtoBuf.encodeToByteArray(result))
+        assertEquals(message, deser)
+    }
 }

testProtos/message_no_fields.proto

@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package messages;
+
+message MessageNoFields {
+  enum Type {
+    UNKNOWN = 0;
+    KNOWN = 1;
+  }
+  message SubMessageNoFields {
+    message SubMessageNoFieldsExtend {
+      Type type = 1;
+    }
+    optional SubMessageNoFields subHello = 1;
+  }
+
+  message SubMessageOneofFields {
+    oneof some_oneof {
+      int32 some_value = 1;
+    }
+  }
+}