1 files changed, 54 insertions, 0 deletions
diff --git a/runtime/commonMain/src/kotlinx/serialization/internal/NullableSerializer.kt b/runtime/commonMain/src/kotlinx/serialization/internal/NullableSerializer.kt
new file mode 100644
index 00000000..23309beb
--- /dev/null
+++ b/runtime/commonMain/src/kotlinx/serialization/internal/NullableSerializer.kt
@@ -0,0 +1,54 @@
+/*
+ * Copyright 2017-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.internal
+
+import kotlinx.serialization.*
+import kotlin.reflect.KClass
+
+fun <T : Any> makeNullable(actualSerializer: KSerializer<T>): KSerializer<T?> = NullableSerializer(actualSerializer)
+
+class NullableSerializer<T : Any>(public val actualSerializer: KSerializer<T>) : KSerializer<T?> {
+    private class SerialDescriptorForNullable(val original: SerialDescriptor): SerialDescriptor by original {
+        override val isNullable: Boolean
+            get() = true
+
+        override fun equals(other: Any?): Boolean {
+            if (this === other) return true
+            if (other !is SerialDescriptorForNullable) return false
+
+            if (original != other.original) return false
+
+            return true
+        }
+
+        override fun hashCode(): Int {
+            return original.hashCode() * 31
+        }
+    }
+
+    override val descriptor: SerialDescriptor = SerialDescriptorForNullable(actualSerializer.descriptor)
+
+    override fun serialize(encoder: Encoder, obj: T?) {
+        if (obj != null) {
+            encoder.encodeNotNullMark()
+            encoder.encodeSerializableValue(actualSerializer, obj)
+        }
+        else {
+            encoder.encodeNull()
+        }
+    }
+
+    override fun deserialize(decoder: Decoder): T? {
+        return if (decoder.decodeNotNullMark()) decoder.decodeSerializableValue(actualSerializer) else decoder.decodeNull()
+    }
+
+    override fun patch(decoder: Decoder, old: T?): T? {
+        return when {
+            old == null -> deserialize(decoder)
+            decoder.decodeNotNullMark() -> decoder.updateSerializableValue(actualSerializer, old)
+            else -> decoder.decodeNull().let { old }
+        }
+    }
+}