Added EnumIndexSerializer.

core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/EnumIndexSerializer.kt

@@ -0,0 +1,32 @@
+package cn.rdlevel.rdkt.core.serialization
+
+import cn.rdlevel.rdkt.core.annotations.RDKTInternalAPI
+import kotlinx.serialization.builtins.serializer
+import kotlin.enums.EnumEntries
+import kotlin.enums.enumEntries
+
+/**
+ * A serializer that serializes an enum value as its ordinal index.
+ *
+ * @property entries The [EnumEntries] of the enum type being serialized.
+ */
+@RDKTInternalAPI
+public class EnumIndexSerializer<T : Enum<T>>(
+    private val entries: EnumEntries<T>
+) : TransformSerializer<T, Int>("EnumIndexSerializer", Int.serializer()) {
+    override fun toData(value: T): Int {
+        return value.ordinal
+    }
+
+    override fun fromData(data: Int): T {
+        return entries[data]
+    }
+}
+
+/**
+ * Creates an [EnumIndexSerializer] for the specified enum type [T].
+ */
+@RDKTInternalAPI
+public inline fun <reified T : Enum<T>> EnumIndexSerializer(): EnumIndexSerializer<T> {
+    return EnumIndexSerializer(enumEntries<T>())
+}

core/src/commonTest/kotlin/cn/rdlevel/rdkt/core/test/EnumIndexSerializerTest.kt

@@ -0,0 +1,33 @@
+@file:OptIn(RDKTInternalAPI::class)
+
+package cn.rdlevel.rdkt.core.test
+
+import cn.rdlevel.rdkt.core.annotations.RDKTInternalAPI
+import cn.rdlevel.rdkt.core.serialization.EnumIndexSerializer
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class EnumIndexSerializerTest {
+    @Serializable(TestEnum.Serializer::class)
+    enum class TestEnum {
+        FIRST, SECOND, THIRD;
+
+        object Serializer : KSerializer<TestEnum> by EnumIndexSerializer()
+    }
+
+    @Test
+    fun test() {
+        val map = mutableMapOf<String, TestEnum>(
+            "first" to TestEnum.FIRST,
+            "second" to TestEnum.SECOND,
+            "third" to TestEnum.THIRD,
+        )
+        val json = Json.encodeToString(map)
+        assertEquals("""{"first":0,"second":1,"third":2}""", json)
+        val map2 = Json.decodeFromString<Map<String, TestEnum>>(json)
+        assertEquals(map, map2)
+    }
+}