Add implementation for computing the required bytes to encode a message

formats/protobuf/api/kotlinx-serialization-protobuf.api

@@ -21,6 +21,10 @@ public final class kotlinx/serialization/protobuf/ProtoBufKt {
 	public static synthetic fun ProtoBuf$default (Lkotlinx/serialization/protobuf/ProtoBuf;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Lkotlinx/serialization/protobuf/ProtoBuf;
 }
 
+public final class kotlinx/serialization/protobuf/ProtoBufSerializedSizeCalculatorKt {
+	public static final fun getOrComputeSerializedSize (Lkotlinx/serialization/protobuf/ProtoBuf;Lkotlinx/serialization/SerializationStrategy;Ljava/lang/Object;)I
+}
+
 public final class kotlinx/serialization/protobuf/ProtoIntegerType : java/lang/Enum {
 	public static final field DEFAULT Lkotlinx/serialization/protobuf/ProtoIntegerType;
 	public static final field FIXED Lkotlinx/serialization/protobuf/ProtoIntegerType;

formats/protobuf/jsMain/src/kotlinx/serialization/protobuf/SerializedSizeCache.kt

@@ -0,0 +1,15 @@
+package kotlinx.serialization.protobuf
+
+import kotlinx.serialization.descriptors.SerialDescriptor
+
+internal actual fun createSerializedSizeCache(): SerializedSizeCache = JsHashMap()
+
+private class JsHashMap : SerializedSizeCache {
+    private val cache = mutableMapOf<SerialDescriptor, SerializedData>()
+
+    override fun get(descriptor: SerialDescriptor, key: SerializedSizeCacheKey): Int? = cache[descriptor]?.get(key)
+
+    override fun set(descriptor: SerialDescriptor, key: SerializedSizeCacheKey, serializedSize: Int) {
+        cache[descriptor] = mapOf(key to serializedSize)
+    }
+}

formats/protobuf/jvmMain/src/kotlinx/serialization/protobuf/SerializedSizeCache.kt

@@ -0,0 +1,18 @@
+package kotlinx.serialization.protobuf
+
+import kotlinx.serialization.descriptors.SerialDescriptor
+import java.util.concurrent.ConcurrentHashMap
+
+internal actual fun createSerializedSizeCache(): SerializedSizeCache {
+    return ConcurrentHashMapSerializedCache()
+}
+
+private class ConcurrentHashMapSerializedCache : SerializedSizeCache {
+    private val cache = ConcurrentHashMap<SerialDescriptor, SerializedData>()
+
+    override fun get(descriptor: SerialDescriptor, key: SerializedSizeCacheKey): Int? = cache[descriptor]?.get(key)
+
+    override fun set(descriptor: SerialDescriptor, key: SerializedSizeCacheKey, serializedSize: Int) {
+        cache[descriptor] = mapOf(key to serializedSize)
+    }
+}