I have a Kotlin class representing some control data:

@Serializable(with = ControlSerializer::class)
class ControlData(
    val start: Double,
    val end: Double,
    val offset: Double?,
    val ramp: Double?,
)

Which should be deserialized from one of two JSON formats:

{ "control": {
    "start": 0.5,
    "end": 1.0,
    "offset": 3.0,
    "ramp": 2.0
    }
}

or

{ "control": 2.0 }

In the second format, the single float value should be expanded into start and end, with offset and ramp set to null.

I managed to get this to work using a JsonContentPolymorphicSerializer that selects a specific serializer based on the JSON type, but as the default serializer is not generated I needed to reimplement it:

object ControlSerializer: JsonContentPolymorphicSerializer(ControlData::class) {
    override fun selectDeserializer(content: JsonElement) {
        return when(content) {
            is JsonPrimitive -> FloatSerializer
            is JsonObject -> ObjectSerializer
            else -> throw Exception("Unrecognized data type")
        }
    }
}

object FloatSerializer: KSerializer<ControlData> {
    // Removed for brevity, but simple serialize/deserialize methods that use encodeDouble()
    // and decodeDouble()
}

object ObjectSerializer: KSerializer<ControlData> {
    // Also removed for brevity, but a more complicated class that uses
    // CompositeDecoder.decodeElementIndex to manually extract every element, which feels
    // unnecessarily complicated when the serialization package normally handles this by default
}

My question now is twofold:

1. Is there a way to access the default serializer for a class after setting up a custom serializer, so I don’t have to reimplement it manually?
2. In deference to the XY problem, is there a better way to achieve what I’m trying to do? I suspect there may be, but I haven’t been able to figure out what it might be.

Ideally I’d like the logic to be contained within the serializers so I don’t have to construct a custom Json object, but I realise this may not be possible. Currently I only need to deserialize data.

Thank you!