Efficient "nullable" on Enum codec

src/main/kotlin/ru/dbotthepony/mc/otm/network/MatteryStreamCodec.kt

@@ -189,6 +189,35 @@ interface MatteryStreamCodec<in S : ByteBuf, V> : StreamCodec<@UnsafeVariance S,
 		val values: List<V> = listOf(*this.clazz.enumConstants!!)
 		val valuesMap = values.associateBy { it.name }
 
+		private val nullable = object : MatteryStreamCodec<S, V?> {
+			override fun decode(stream: S): V? {
+				val value = stream.readVarInt()
+
+				if (value == 0) {
+					return null
+				} else {
+					val id = value ushr 1
+					return values.getOrNull(id) ?: throw NoSuchElementException("No such enum with index $id")
+				}
+			}
+
+			override fun encode(stream: S, value: V?) {
+				if (value == null) {
+					stream.writeVarInt(0)
+				} else {
+					stream.writeVarInt(1 or (value.ordinal shl 1))
+				}
+			}
+
+			override fun copy(value: V?): V? {
+				return value
+			}
+
+			override fun compare(a: V?, b: V?): Boolean {
+				return a === b
+			}
+		}
+
 		override fun decode(stream: S): V {
 			val id = stream.readVarInt()
 			return values.getOrNull(id) ?: throw NoSuchElementException("No such enum with index $id")
@@ -206,6 +235,10 @@ interface MatteryStreamCodec<in S : ByteBuf, V> : StreamCodec<@UnsafeVariance S,
 			return a === b
 		}
 
+		fun nullable(): MatteryStreamCodec<S, V?> {
+			return nullable
+		}
+
 		companion object {
 			/**
 			 * FIXME: enums with abstract methods which get compiled to subclasses, whose DO NOT expose "parent's" enum constants array