Implement Oneof field

protobuf_serialization/internal.nim

@@ -4,6 +4,7 @@
 
 import std/[options, sets, tables]
 import stew/shims/macros
+from std/strutils import parseEnum
 #Depending on the situation, one of these two are used.
 #Sometimes, one works where the other doesn't.
 #It all comes down to bugs in Nim and managing them.
@@ -56,6 +57,24 @@ proc fieldNumberOf*(T: type, fieldName: static string): int {.compileTime.} =
   else:
     fieldNum
 
+proc isOneof*(T: type, fieldName: static string): bool {.compileTime.} =
+  T.hasCustomPragmaFixed(fieldName, oneof)
+
+template setOneof*(obj: object, headerNum: int): untyped =
+  type T = typeof(obj)
+  for fieldName, fieldVar in fieldPairs(obj):
+    when fieldName == "kind":
+      enumInstanceSerializedFields(obj, fieldName2, fieldVar2):
+        if headerNum == T.fieldNumberOf(fieldName2):
+          try:
+            fieldVar = parseEnum[typeof(fieldVar)](fieldName2)
+          except ValueError:
+            raiseAssert "unexpected oneof field " & fieldName2
+    else:
+      # XXX skip dontSerialize
+      if headerNum != T.fieldNumberOf(fieldName):
+        fieldVar = default(typeof(fieldVar))
+
 template tableObject*(TableObject, K, V) =
   when K is SomePBInt and V is SomePBInt:
     type

protobuf_serialization/reader.nim

@@ -11,6 +11,8 @@ import
   serialization,
   "."/[codec, internal, types]
 
+from std/strutils import parseEnum
+
 export inputs, serialization, codec, types
 
 proc readValueInternal[T: object](stream: InputStream, value: var T, silent: bool = false) {.raises: [SerializationError, IOError].}
@@ -142,18 +144,46 @@ proc readValueInternal[T: object](stream: InputStream, value: var T, silent: boo
 
   while stream.readable():
     let header = stream.readHeader()
+    let headerNum = header.number()
     var i = -1
     var knownField = false
 
-    if not header.number().validFieldNumber(true):
-      raise newException(ProtobufReadError, "Invalid field number: " & $header.number())
+    if not headerNum.validFieldNumber(true):
+      raise newException(ProtobufReadError, "Invalid field number: " & $headerNum)
+
+    for fieldNameX, fieldVarX in fieldPairs(value):
+      when T.isOneof(fieldNameX):
+        type TT = typeof(fieldVarX)
+        enumInstanceSerializedFields(fieldVarX, fieldName, fieldVar):
+          const fieldNum = TT.fieldNumberOf(fieldName)
+          if headerNum == fieldNum:
+            protoType(ProtoType, TT, typeof(fieldVar), fieldName)
+            # TODO should we allow reading packed fields into non-repeated fields?
+            when ProtoType is SomePrimitive and fieldVar is seq and fieldVar isnot seq[byte]:
+              if header.kind() == WireKind.LengthDelim:
+                knownField = true
+                stream.readFieldPackedInto(fieldVar, header, ProtoType)
+              elif header.kind() == wireKind(ProtoType):
+                knownField = true
+                stream.readFieldInto(fieldVar, header, ProtoType)
+            elif typeof(fieldVar) is ref and defined(ConformanceTest):
+              if header.kind() == wireKind(ProtoType):
+                knownField = true
+                fieldVar = new typeof(fieldVar)
+                stream.readFieldInto(fieldVar[], header, ProtoType)
+            else:
+              if header.kind() == wireKind(ProtoType):
+                knownField = true
+                stream.readFieldInto(fieldVar, header, ProtoType)
+        if knownField:
+          setOneof(fieldVarX, headerNum)
 
     enumInstanceSerializedFields(value, fieldName, fieldVar):
       inc i
       const
         fieldNum = T.fieldNumberOf(fieldName)
 
-      if header.number() == fieldNum:
+      if headerNum == fieldNum:
         protoType(ProtoType, T, typeof(fieldVar), fieldName)
         # TODO should we allow reading packed fields into non-repeated fields?
         when ProtoType is SomePrimitive and fieldVar is seq and fieldVar isnot seq[byte]:

protobuf_serialization/types.nim

@@ -43,6 +43,7 @@ template packed*(v: bool) {.pragma.}
 template pint*() {.pragma.} # encode as `intXX`
 template sint*() {.pragma.} # encode as `sintXX`
 template fixed*() {.pragma.} # encode as `fixedXX`
+template oneof*() {.pragma.}
 
 func init*(
   T: type ProtobufWriter,

tests/test_oneof.nim

@@ -0,0 +1,47 @@
+# nim-protobuf-serialization
+# Copyright (c) 2026 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT))
+# at your option.
+# This file may not be copied, modified, or distributed except according to
+# those terms.
+
+import unittest2
+import stew/byteutils
+import ../protobuf_serialization
+
+type
+  OneOfKind {.pure.} = enum
+    unset
+    x
+    y
+  OneOf {.proto3.} = object
+    kind {.dontSerialize.}: OneOfKind
+    x {.fieldNumber: 1, pint.}: int64
+    y {.fieldNumber: 2, pint.}: int64
+  OneOfObj {.proto3.} = object
+    one {.oneof, dontSerialize.}: OneOf
+
+suite "Test oneof":
+  test "oneof unset":
+    let encoded = "".hexToSeqByte
+    check Protobuf.decode(encoded, OneOfObj) == OneOfObj(one: OneOf(kind: OneOfKind.unset))
+
+  test "oneof field 1 set":
+    # echo 'x: 1' | protoc --encode=OneOfObj test_oneof.proto | hexdump -ve '1/1 "%.2x"'
+    # 0801
+    let encoded = "0801".hexToSeqByte
+    check Protobuf.decode(encoded, OneOfObj) == OneOfObj(one: OneOf(kind: OneOfKind.x, x: 1))
+
+  test "oneof field 2 set":
+    let encoded = "1001".hexToSeqByte
+    check Protobuf.decode(encoded, OneOfObj) == OneOfObj(one: OneOf(kind: OneOfKind.y, y: 1))
+
+  test "oneof field 1 and 2 set":
+    let encoded = "08011001".hexToSeqByte
+    check Protobuf.decode(encoded, OneOfObj) == OneOfObj(one: OneOf(kind: OneOfKind.y, y: 1))
+
+  test "oneof field 1 and 2 set variant":
+    let encoded = "10010801".hexToSeqByte
+    check Protobuf.decode(encoded, OneOfObj) == OneOfObj(one: OneOf(kind: OneOfKind.x, x: 1))

tests/test_oneof.proto

@@ -0,0 +1,6 @@
+syntax = "proto3";
+
+message OneOfObj {
+  int64 x = 1;
+  int64 y = 2;
+}