[transcoding] Better message merging syntax/implementation

core/src/main/scala/org/ivovk/connect_rpc_scala/ConnectRouteBuilder.scala

@@ -8,10 +8,10 @@ import io.grpc.{ManagedChannelBuilder, ServerBuilder, ServerServiceDefinition}
 import org.http4s.dsl.Http4sDsl
 import org.http4s.{HttpApp, HttpRoutes, MediaType, Method, Response, Uri}
 import org.ivovk.connect_rpc_scala.grpc.*
+import org.ivovk.connect_rpc_scala.grpc.MergingBuilder.*
 import org.ivovk.connect_rpc_scala.http.*
 import org.ivovk.connect_rpc_scala.http.QueryParams.*
 import org.ivovk.connect_rpc_scala.http.codec.*
-import org.ivovk.connect_rpc_scala.syntax.all.*
 import scalapb.{GeneratedMessage as Message, GeneratedMessageCompanion as Companion}
 
 import java.util.concurrent.Executor
@@ -182,7 +182,7 @@ final class ConnectRouteBuilder[F[_] : Async] private(
                 val queryMessage = jsonCodec.parser.fromJson[Message](queryJson)
 
                 transcodingHandler.handleUnary(
-                  bodyMessage.concat(pathMessage, queryMessage),
+                  bodyMessage.merge(pathMessage).merge(queryMessage).build,
                   req.headers,
                   method
                 )

core/src/main/scala/org/ivovk/connect_rpc_scala/grpc/MergingBuilder.scala

@@ -0,0 +1,45 @@
+package org.ivovk.connect_rpc_scala.grpc
+
+import com.google.protobuf.ByteString
+import scalapb.{GeneratedMessage as Message, GeneratedMessageCompanion as Companion}
+
+object MergingBuilder {
+  extension [T <: Message](t: T) {
+    def merge(other: T)(using Companion[T]): MergingBuilder[T] =
+      SingleMergingBuilder(t).merge(other)
+  }
+}
+
+sealed trait MergingBuilder[T <: Message] {
+  def merge(other: T): MergingBuilder[T]
+
+  def build: T
+}
+
+private class SingleMergingBuilder[T <: Message](t: T)(using cmp: Companion[T]) extends MergingBuilder[T] {
+  override def merge(other: T): MergingBuilder[T] = {
+    val empty = cmp.defaultInstance
+
+    if other == empty then this
+    else if t == empty then SingleMergingBuilder(other)
+    else ListMergingBuilder(other :: t :: Nil)
+  }
+
+  override def build: T = t
+}
+
+private class ListMergingBuilder[T <: Message](ts: List[T])(using cmp: Companion[T]) extends MergingBuilder[T] {
+  override def merge(other: T): MergingBuilder[T] = {
+    val empty = cmp.defaultInstance
+
+    if other == empty then this
+    else ListMergingBuilder(other :: ts)
+  }
+
+  override def build: T = {
+    val output = ByteString.newOutput(ts.foldLeft(0)(_ + _.serializedSize))
+    ts.reverse.foreach(_.writeTo(output))
+    output.close()
+    cmp.parseFrom(output.toByteString.newCodedInput())
+  }
+}

core/src/main/scala/org/ivovk/connect_rpc_scala/syntax/all.scala

@@ -1,9 +1,8 @@
 package org.ivovk.connect_rpc_scala.syntax
 
-import com.google.protobuf.ByteString
 import io.grpc.{StatusException, StatusRuntimeException}
 import org.ivovk.connect_rpc_scala.grpc.GrpcHeaders
-import scalapb.{GeneratedMessage, GeneratedMessageCompanion}
+import scalapb.GeneratedMessage
 
 object all extends ExceptionSyntax, ProtoMappingsSyntax
 
@@ -34,20 +33,6 @@ trait ExceptionSyntax {
 trait ProtoMappingsSyntax {
 
   extension [T <: GeneratedMessage](t: T) {
-    def concat(other: T, more: T*): T = {
-      val cmp   = t.companion.asInstanceOf[GeneratedMessageCompanion[T]]
-      val empty = cmp.defaultInstance
-
-      val els = (t :: other :: more.toList).filter(_ != empty)
-
-      els match
-        case Nil => empty
-        case el :: Nil => el
-        case _ =>
-          val is = els.foldLeft(ByteString.empty)(_ concat _.toByteString).newCodedInput()
-          cmp.parseFrom(is)
-    }
-
     def toProtoAny: com.google.protobuf.any.Any = {
       com.google.protobuf.any.Any(
         typeUrl = "type.googleapis.com/" + t.companion.scalaDescriptor.fullName,

core/src/test/scala/org/ivovk/connect_rpc_scala/grpc/MergingBuilderTest.scala

@@ -0,0 +1,15 @@
+package org.ivovk.connect_rpc_scala.grpc
+
+import org.scalatest.funsuite.AnyFunSuite
+import test.HttpCommunicationTest.AddRequest
+
+class MergingBuilderTest extends AnyFunSuite {
+  import MergingBuilder.*
+
+  test("merges three messages") {
+    val merge = AddRequest(a = 1).merge(AddRequest(b = 2)).merge(AddRequest(a = 3)).build
+
+    assert(merge.a == 3)
+    assert(merge.b == 2)
+  }
+}