Add BaseSurrogateSerializer (#120)

app/src/main/java/com/x8bit/bitwarden/data/auth/datasource/network/model/PreLoginResponseJson.kt

@@ -1,10 +1,7 @@
 package com.x8bit.bitwarden.data.auth.datasource.network.model
 
-import kotlinx.serialization.KSerializer
+import com.x8bit.bitwarden.data.platform.datasource.network.serializer.BaseSurrogateSerializer
 import kotlinx.serialization.Serializable
-import kotlinx.serialization.descriptors.SerialDescriptor
-import kotlinx.serialization.encoding.Decoder
-import kotlinx.serialization.encoding.Encoder
 
 private const val KDF_TYPE_ARGON2_ID = 1
 private const val KDF_TYPE_PBKDF2_SHA256 = 0
@@ -40,38 +37,37 @@ data class PreLoginResponseJson(
     }
 }
 
-private class PreLoginResponseSerializer : KSerializer<PreLoginResponseJson> {
+private class PreLoginResponseSerializer :
+    BaseSurrogateSerializer<PreLoginResponseJson, InternalPreLoginResponseJson>() {
 
-    private val surrogateSerializer = InternalPreLoginResponseJson.serializer()
+    override val surrogateSerializer = InternalPreLoginResponseJson.serializer()
 
-    override val descriptor: SerialDescriptor = surrogateSerializer.descriptor
+    override fun InternalPreLoginResponseJson.toExternalType(): PreLoginResponseJson =
+        PreLoginResponseJson(
+            kdfParams = when (this.kdfType) {
+                KDF_TYPE_PBKDF2_SHA256 -> {
+                    PreLoginResponseJson.KdfParams.Pbkdf2(
+                        iterations = this.kdfIterations,
+                    )
+                }
 
-    override fun deserialize(decoder: Decoder): PreLoginResponseJson {
-        val surrogate = decoder.decodeSerializableValue(surrogateSerializer)
-        val kdfParams = when (surrogate.kdfType) {
-            KDF_TYPE_PBKDF2_SHA256 -> {
-                PreLoginResponseJson.KdfParams.Pbkdf2(
-                    iterations = surrogate.kdfIterations,
+                KDF_TYPE_ARGON2_ID -> {
+                    @Suppress("UnsafeCallOnNullableType")
+                    PreLoginResponseJson.KdfParams.Argon2ID(
+                        iterations = this.kdfIterations,
+                        memory = this.kdfMemory!!,
+                        parallelism = this.kdfParallelism!!,
+                    )
+                }
+
+                else -> throw IllegalStateException(
+                    "Unable to parse KDF params for unknown kdfType: ${this.kdfType}",
                 )
-            }
+            },
+        )
 
-            KDF_TYPE_ARGON2_ID -> {
-                PreLoginResponseJson.KdfParams.Argon2ID(
-                    iterations = surrogate.kdfIterations,
-                    memory = surrogate.kdfMemory!!,
-                    parallelism = surrogate.kdfParallelism!!,
-                )
-            }
-
-            else -> throw IllegalStateException(
-                "Unable to parse KDF params for unknown kdfType: ${surrogate.kdfType}",
-            )
-        }
-        return PreLoginResponseJson(kdfParams = kdfParams)
-    }
-
-    override fun serialize(encoder: Encoder, value: PreLoginResponseJson) {
-        val surrogate = when (val params = value.kdfParams) {
+    override fun PreLoginResponseJson.toSurrogateType(): InternalPreLoginResponseJson =
+        when (val params = this.kdfParams) {
             is PreLoginResponseJson.KdfParams.Argon2ID -> {
                 InternalPreLoginResponseJson(
                     kdfType = KDF_TYPE_ARGON2_ID,
@@ -88,9 +84,4 @@ private class PreLoginResponseSerializer : KSerializer<PreLoginResponseJson> {
                 )
             }
         }
-        encoder.encodeSerializableValue(
-            surrogateSerializer,
-            surrogate,
-        )
-    }
 }

app/src/main/java/com/x8bit/bitwarden/data/platform/datasource/network/serializer/BaseSurrogateSerializer.kt

@@ -0,0 +1,52 @@
+package com.x8bit.bitwarden.data.platform.datasource.network.serializer
+
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+
+/**
+ * A helper class that simplifies the process of providing a "surrogate" [KSerializer]. These are
+ * used to provide mappings between an "internal" type [R] (the "surrogate") to an external type
+ * [T].
+ *
+ * See the [official surrogate documentation](https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#composite-serializer-via-surrogate)
+ * for details.
+ */
+abstract class BaseSurrogateSerializer<T, R> : KSerializer<T> {
+
+    /**
+     * The [KSerializer] naturally associated with the type [R], which is a typical class annotated
+     * with [Serializable].
+     */
+    abstract val surrogateSerializer: KSerializer<R>
+
+    /**
+     * A conversion from the internal/surrogate type [R] to external type [T].
+     */
+    abstract fun R.toExternalType(): T
+
+    /**
+     * A conversion from the external type [T] to the internal/surrogate type [R].
+     */
+    abstract fun T.toSurrogateType(): R
+
+    //region KSerializer overrides
+
+    override val descriptor: SerialDescriptor
+        get() = surrogateSerializer.descriptor
+
+    final override fun deserialize(decoder: Decoder): T =
+        decoder
+            .decodeSerializableValue(surrogateSerializer)
+            .toExternalType()
+
+    final override fun serialize(encoder: Encoder, value: T) {
+        encoder.encodeSerializableValue(
+            surrogateSerializer,
+            value.toSurrogateType(),
+        )
+    }
+
+    //endregion KSerializer overrides
+}

app/src/test/java/com/x8bit/bitwarden/data/platform/datasource/network/serializer/BaseSurrogateSerializerTest.kt

@@ -0,0 +1,71 @@
+package com.x8bit.bitwarden.data.platform.datasource.network.serializer
+
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.encodeToJsonElement
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+class BaseSurrogateSerializerTest {
+    private val json = Json
+
+    @Test
+    fun `properly deserializes raw JSON to the external model`() {
+        assertEquals(
+            ExternalData(
+                dataAsInt = 100,
+            ),
+            json.decodeFromString<ExternalData>(
+                """
+                {
+                 "dataAsString": "100"
+                }
+                """,
+            ),
+        )
+    }
+
+    @Test
+    fun `properly serializes external model back to raw JSON`() {
+        assertEquals(
+            json.parseToJsonElement(
+                """
+                {
+                  "dataAsString": "100"
+                }
+                """,
+            ),
+            json.encodeToJsonElement(
+                ExternalData(
+                    dataAsInt = 100,
+                ),
+            ),
+        )
+    }
+}
+
+@Serializable
+private data class InternalData(
+    val dataAsString: String,
+)
+
+@Serializable(TestSurrogateSerializer::class)
+private data class ExternalData(
+    val dataAsInt: Int,
+)
+
+private class TestSurrogateSerializer : BaseSurrogateSerializer<ExternalData, InternalData>() {
+    override val surrogateSerializer: KSerializer<InternalData>
+        get() = InternalData.serializer()
+
+    override fun InternalData.toExternalType(): ExternalData =
+        ExternalData(
+            dataAsInt = this.dataAsString.toInt(),
+        )
+
+    override fun ExternalData.toSurrogateType(): InternalData =
+        InternalData(
+            dataAsString = this.dataAsInt.toString(),
+        )
+}