From 96f475ea2a58863aa3d3d728f1040d41add9c963 Mon Sep 17 00:00:00 2001
From: Sebastian Schuberth <sschuberth@gmail.com>
Date: Wed, 25 Aug 2021 22:27:09 +0200
Subject: [PATCH] Support serialization of collections that are not lists

So far there was an implicit assumption hard-coded that collections are
always lists. However, sets are also collections, and can be serialized
to JSON arrays just like lists.

This change allows to serialize generic collections independently of the
concrete implementation.

Fixes #1421.
---
 .../internal/CollectionSerializers.kt         | 23 ++++++-----
 .../features/CollectionSerializerTest.kt      | 41 +++++++++++++++++++
 2 files changed, 53 insertions(+), 11 deletions(-)
 create mode 100644 formats/json/commonTest/src/kotlinx/serialization/features/CollectionSerializerTest.kt

diff --git a/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt b/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt
index 91b4e713f..d66e1c545 100644
--- a/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt
+++ b/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt
@@ -202,12 +202,17 @@ internal class ReferenceArraySerializer<ElementKlass : Any, Element : ElementKla
     }
 }
 
+@PublishedApi
+internal abstract class CollectionSerializer<E, C: Collection<E>, B>(element: KSerializer<E>) : ListLikeSerializer<E, C, B>(element) {
+    override fun C.collectionSize(): Int = size
+    override fun C.collectionIterator(): Iterator<E> = iterator()
+}
+
 @InternalSerializationApi
 @PublishedApi
-internal class ArrayListSerializer<E>(element: KSerializer<E>) : ListLikeSerializer<E, List<E>, ArrayList<E>>(element) {
+internal class ArrayListSerializer<E>(element: KSerializer<E>) : CollectionSerializer<E, List<E>, ArrayList<E>>(element) {
     override val descriptor: SerialDescriptor = ArrayListClassDesc(element.descriptor)
-    override fun List<E>.collectionSize(): Int = size
-    override fun List<E>.collectionIterator(): Iterator<E> = iterator()
+
     override fun builder(): ArrayList<E> = arrayListOf()
     override fun ArrayList<E>.builderSize(): Int = size
     override fun ArrayList<E>.toResult(): List<E> = this
@@ -219,11 +224,9 @@ internal class ArrayListSerializer<E>(element: KSerializer<E>) : ListLikeSeriali
 @PublishedApi
 internal class LinkedHashSetSerializer<E>(
     eSerializer: KSerializer<E>
-) : ListLikeSerializer<E, Set<E>, LinkedHashSet<E>>(eSerializer) {
-
+) : CollectionSerializer<E, Set<E>, LinkedHashSet<E>>(eSerializer) {
     override val descriptor: SerialDescriptor = LinkedHashSetClassDesc(eSerializer.descriptor)
-    override fun Set<E>.collectionSize(): Int = size
-    override fun Set<E>.collectionIterator(): Iterator<E> = iterator()
+
     override fun builder(): LinkedHashSet<E> = linkedSetOf()
     override fun LinkedHashSet<E>.builderSize(): Int = size
     override fun LinkedHashSet<E>.toResult(): Set<E> = this
@@ -235,11 +238,9 @@ internal class LinkedHashSetSerializer<E>(
 @PublishedApi
 internal class HashSetSerializer<E>(
     eSerializer: KSerializer<E>
-) : ListLikeSerializer<E, Set<E>, HashSet<E>>(eSerializer) {
-
+) : CollectionSerializer<E, Set<E>, HashSet<E>>(eSerializer) {
     override val descriptor: SerialDescriptor = HashSetClassDesc(eSerializer.descriptor)
-    override fun Set<E>.collectionSize(): Int = size
-    override fun Set<E>.collectionIterator(): Iterator<E> = iterator()
+
     override fun builder(): HashSet<E> = HashSet()
     override fun HashSet<E>.builderSize(): Int = size
     override fun HashSet<E>.toResult(): Set<E> = this
diff --git a/formats/json/commonTest/src/kotlinx/serialization/features/CollectionSerializerTest.kt b/formats/json/commonTest/src/kotlinx/serialization/features/CollectionSerializerTest.kt
new file mode 100644
index 000000000..ca8116a0b
--- /dev/null
+++ b/formats/json/commonTest/src/kotlinx/serialization/features/CollectionSerializerTest.kt
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2017-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.features
+
+import kotlinx.serialization.*
+import kotlinx.serialization.builtins.*
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class CollectionSerializerTest {
+
+    @Serializable
+    data class CollectionWrapper(
+        val collection: Collection<String>
+    )
+
+    @Test
+    fun testListJson() {
+        val list = listOf("foo", "bar", "foo", "bar")
+
+        val string = Json.encodeToString(CollectionWrapper(list))
+        assertEquals("""{"collection":["foo","bar","foo","bar"]}""", string)
+
+        val wrapper = Json.decodeFromString<CollectionWrapper>(string)
+        assertEquals(list, wrapper.collection)
+    }
+
+    @Test
+    fun testSetJson() {
+        val set = setOf("foo", "bar", "foo", "bar")
+
+        val string = Json.encodeToString(CollectionWrapper(set))
+        assertEquals("""{"collection":["foo","bar"]}""", string)
+
+        val wrapper = Json.decodeFromString<CollectionWrapper>(string)
+        assertEquals(set.toList(), wrapper.collection)
+    }
+}