Properly cache serial names in order to improve performance of JSON p…

core/api/kotlinx-serialization-core.api

@@ -858,7 +858,7 @@ public final class kotlinx/serialization/internal/Platform_commonKt {
 	public static final fun cast (Lkotlinx/serialization/SerializationStrategy;)Lkotlinx/serialization/SerializationStrategy;
 }
 
-public class kotlinx/serialization/internal/PluginGeneratedSerialDescriptor : kotlinx/serialization/descriptors/SerialDescriptor {
+public class kotlinx/serialization/internal/PluginGeneratedSerialDescriptor : kotlinx/serialization/descriptors/SerialDescriptor, kotlinx/serialization/internal/CachedNames {
 	public fun <init> (Ljava/lang/String;Lkotlinx/serialization/internal/GeneratedSerializer;I)V
 	public synthetic fun <init> (Ljava/lang/String;Lkotlinx/serialization/internal/GeneratedSerializer;IILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun addElement (Ljava/lang/String;Z)V
@@ -872,6 +872,7 @@ public class kotlinx/serialization/internal/PluginGeneratedSerialDescriptor : ko
 	public final fun getElementsCount ()I
 	public fun getKind ()Lkotlinx/serialization/descriptors/SerialKind;
 	public fun getSerialName ()Ljava/lang/String;
+	public fun getSerialNames ()Ljava/util/Set;
 	public fun hashCode ()I
 	public fun isElementOptional (I)Z
 	public fun isNullable ()Z

core/commonMain/src/kotlinx/serialization/descriptors/SerialDescriptors.kt

@@ -270,9 +270,10 @@ internal class SerialDescriptorImpl(
     override val elementsCount: Int,
     typeParameters: List<SerialDescriptor>,
     builder: ClassSerialDescriptorBuilder
-) : SerialDescriptor {
+) : SerialDescriptor, CachedNames {
 
-    public override val annotations: List<Annotation> = builder.annotations
+    override val annotations: List<Annotation> = builder.annotations
+    override val serialNames: Set<String> = builder.elementNames.toHashSet()
 
     private val elementNames: Array<String> = builder.elementNames.toTypedArray()
     private val elementDescriptors: Array<SerialDescriptor> = builder.elementDescriptors.compactArray()

core/commonMain/src/kotlinx/serialization/internal/CachedNames.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+package kotlinx.serialization.internal
+
+import kotlinx.serialization.descriptors.*
+
+/**
+ * Internal interface used as a marker for [SerialDescriptor] in order
+ * to retrieve the set of all element names without allocations.
+ * Used by our implementations as a performance optimization.
+ * It's not an instance of [SerialDescriptor] to simplify implementation via delegation
+ */
+internal interface CachedNames {
+
+    /**
+     * A set of all names retrieved from [SerialDescriptor.getElementName]
+     */
+    public val serialNames: Set<String>
+}

core/commonMain/src/kotlinx/serialization/internal/NullableSerializer.kt

@@ -44,8 +44,12 @@ internal class NullableSerializer<T : Any>(private val serializer: KSerializer<T
 }
 
 @OptIn(ExperimentalSerializationApi::class)
-internal class SerialDescriptorForNullable(internal val original: SerialDescriptor) : SerialDescriptor by original {
+internal class SerialDescriptorForNullable(
+    internal val original: SerialDescriptor
+) : SerialDescriptor by original, CachedNames {
+
     override val serialName: String = original.serialName + "?"
+    override val serialNames: Set<String> = original.cachedSerialNames()
     override val isNullable: Boolean
         get() = true
 

core/commonMain/src/kotlinx/serialization/internal/Platform.common.kt

@@ -6,7 +6,6 @@ package kotlinx.serialization.internal
 
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
-import kotlinx.serialization.encoding.*
 import kotlin.native.concurrent.*
 import kotlin.reflect.*
 
@@ -56,8 +55,9 @@ internal object InternalHexConverter {
     }
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 internal fun SerialDescriptor.cachedSerialNames(): Set<String> {
-    if (this is PluginGeneratedSerialDescriptor) return namesSet
+    if (this is CachedNames) return serialNames
     val result = HashSet<String>(elementsCount)
     for (i in 0 until elementsCount) {
         result += getElementName(i)

core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt

@@ -18,7 +18,7 @@ internal open class PluginGeneratedSerialDescriptor(
     override val serialName: String,
     private val generatedSerializer: GeneratedSerializer<*>? = null,
     final override val elementsCount: Int
-) : SerialDescriptor {
+) : SerialDescriptor, CachedNames {
     override val kind: SerialKind get() = StructureKind.CLASS
     override val annotations: List<Annotation> get() = classAnnotations ?: emptyList()
 
@@ -29,7 +29,7 @@ internal open class PluginGeneratedSerialDescriptor(
     // Classes rarely have annotations, so we can save up a bit of allocations here
     private var classAnnotations: MutableList<Annotation>? = null
     private val elementsOptionality = BooleanArray(elementsCount)
-    internal val namesSet: Set<String> get() = indices.keys
+    public override val serialNames: Set<String> get() = indices.keys
 
     // don't change lazy mode: KT-32871, KT-32872
     private val indices: Map<String, Int> by lazy { buildIndices() }