fix: Fix MoveOption serialization

lib/src/commonMain/kotlin/xyz/mcxross/kaptos/model/MoveStruct.kt

@@ -18,6 +18,7 @@ package xyz.mcxross.kaptos.model
 import kotlinx.serialization.Serializable
 import xyz.mcxross.bcs.Bcs
 import xyz.mcxross.kaptos.core.Hex
+import xyz.mcxross.kaptos.serialize.MoveOptionSerializer
 import xyz.mcxross.kaptos.serialize.MoveStringSerializer
 import xyz.mcxross.kaptos.serialize.MoveVectorSerializer
 
@@ -141,7 +142,7 @@ data class MoveString(val value: String) : TransactionArgument() {
   }
 }
 
-@Serializable
+@Serializable(with = MoveOptionSerializer::class)
 data class MoveOption<T : EntryFunctionArgument>(val value: T?) : TransactionArgument() {
   fun unwrap(): T {
     return value ?: throw IllegalArgumentException("Option is empty")

lib/src/commonMain/kotlin/xyz/mcxross/kaptos/serialize/MoveOptionSerializer.kt

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2024 McXross
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package xyz.mcxross.kaptos.serialize
+
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.descriptors.listSerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.encoding.decodeStructure
+import kotlinx.serialization.encoding.encodeCollection
+import xyz.mcxross.kaptos.model.EntryFunctionArgument
+import xyz.mcxross.kaptos.model.MoveOption
+
+class MoveOptionSerializer<T : EntryFunctionArgument>(
+  private val elementSerializer: KSerializer<T>
+) : KSerializer<MoveOption<T>> {
+  @OptIn(ExperimentalSerializationApi::class)
+  override val descriptor: SerialDescriptor = listSerialDescriptor(elementSerializer.descriptor)
+
+  override fun serialize(encoder: Encoder, value: MoveOption<T>) {
+     value.value?.let {
+         encoder.encodeCollection(descriptor, 1) { elementSerializer.serialize(encoder, it) }
+     } ?: run {
+         encoder.encodeByte(0)
+     }
+  }
+
+  override fun deserialize(decoder: Decoder): MoveOption<T> {
+      return MoveOption(
+          decoder.decodeStructure(descriptor) {
+              val isSome = decodeCollectionSize(descriptor)
+              require(isSome < 2) { "Expected length of 0 or 1 for MoveOption, found $isSome" }
+
+              if (isSome == 1) {
+                  decodeSerializableElement(descriptor, 0, elementSerializer)
+              } else {
+                  null
+              }
+          }
+      )
+  }
+}

lib/src/commonTest/kotlin/xyz/mcxross/kaptos/unit/serialize/MoveOptionSerializerTest.kt

@@ -0,0 +1,70 @@
+package xyz.mcxross.kaptos.unit.serialize
+
+import xyz.mcxross.bcs.Bcs
+import kotlin.test.Test
+import xyz.mcxross.kaptos.model.MoveOption
+import xyz.mcxross.kaptos.model.U8
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertNull
+
+class MoveOptionSerializerTest {
+    @Test
+    fun `can deserialize a Some option (vector of length 1)`() {
+        val encoded = listOf(1.toByte(), 42.toByte()).toByteArray()
+
+        val decoded: MoveOption<U8> = Bcs.decodeFromByteArray(encoded)
+        val expected = MoveOption(U8(42.toByte()))
+
+        assertEquals(expected.value?.value, decoded.value?.value)
+    }
+
+    @Test
+    fun `can deserialize a None option (vector of length 0)`() {
+        val encoded = listOf(0.toByte()).toByteArray()
+
+        val decoded: MoveOption<U8> = Bcs.decodeFromByteArray(encoded)
+        val expected = MoveOption<U8>(null)
+
+        assertNull(decoded.value)
+        assertEquals(expected.value, decoded.value)
+    }
+
+    @Test
+    fun `unwrap returns value when Some`() {
+        val encoded = listOf(1.toByte(), 100.toByte()).toByteArray()
+
+        val decoded: MoveOption<U8> = Bcs.decodeFromByteArray(encoded)
+
+        assertEquals(100.toByte(), decoded.unwrap().value)
+    }
+
+    @Test
+    fun `unwrap throws when None`() {
+        val encoded = listOf(0.toByte()).toByteArray()
+
+        val decoded: MoveOption<U8> = Bcs.decodeFromByteArray(encoded)
+
+        assertFailsWith<IllegalArgumentException> {
+            decoded.unwrap()
+        }
+    }
+
+    @Test
+    fun `should throw error when deserializing empty byte array`() {
+        val emptyInput = byteArrayOf()
+
+        assertFailsWith<Exception> {
+            Bcs.decodeFromByteArray<MoveOption<U8>>(emptyInput)
+        }
+    }
+
+    @Test
+    fun `should throw error when deserializing vector with length greater than 1`() {
+        val encoded = listOf(2.toByte(), 1.toByte(), 2.toByte()).toByteArray()
+
+        assertFailsWith<IllegalArgumentException> {
+            Bcs.decodeFromByteArray<MoveOption<U8>>(encoded)
+        }
+    }
+}