/*
 * Copyright 2002-2021 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.converter.json;

import java.io.IOException;
import java.lang.reflect.Type;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Map;

import kotlinx.serialization.KSerializer;
import kotlinx.serialization.SerializationException;
import kotlinx.serialization.SerializersKt;
import kotlinx.serialization.descriptors.PolymorphicKind;
import kotlinx.serialization.json.Json;

import org.springframework.core.GenericTypeResolver;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.AbstractGenericHttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.http.converter.HttpMessageNotWritableException;
import org.springframework.lang.Nullable;
import org.springframework.util.ConcurrentReferenceHashMap;
import org.springframework.util.StreamUtils;

Implementation of HttpMessageConverter that can read and write JSON using kotlinx.serialization.
This converter can be used to bind @Serializable Kotlin classes, open polymorphic serialization is not supported. It supports application/json and application/*+json with various character sets, UTF-8 being the default. 
Author:
Andreas Ahlenstorf, Sebastien Deleuze, Juergen Hoeller
Since:
5.3/**
 * Implementation of {@link org.springframework.http.converter.HttpMessageConverter}
 * that can read and write JSON using
 * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
 *
 * <p>This converter can be used to bind {@code @Serializable} Kotlin classes,
 * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
 * is not supported.
 * It supports {@code application/json} and {@code application/*+json} with
 * various character sets, {@code UTF-8} being the default.
 *
 * @author Andreas Ahlenstorf
 * @author Sebastien Deleuze
 * @author Juergen Hoeller
 * @since 5.3
 */
public class KotlinSerializationJsonHttpMessageConverter extends AbstractGenericHttpMessageConverter<Object> {

	private static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

	private static final Map<Type, KSerializer<Object>> serializerCache = new ConcurrentReferenceHashMap<>();

	private final Json json;


	
Construct a new KotlinSerializationJsonHttpMessageConverter with the default configuration. /**
	 * Construct a new {@code KotlinSerializationJsonHttpMessageConverter} with the default configuration.
	 */
	public KotlinSerializationJsonHttpMessageConverter() {
		this(Json.Default);
	}

	
Construct a new KotlinSerializationJsonHttpMessageConverter with a custom configuration. /**
	 * Construct a new {@code KotlinSerializationJsonHttpMessageConverter} with a custom configuration.
	 */
	public KotlinSerializationJsonHttpMessageConverter(Json json) {
		super(MediaType.APPLICATION_JSON, new MediaType("application", "*+json"));
		this.json = json;
	}


	@Override
	protected boolean supports(Class<?> clazz) {
		try {
			serializer(clazz);
			return true;
		}
		catch (Exception ex) {
			return false;
		}
	}

	@Override
	public boolean canRead(Type type, @Nullable Class<?> contextClass, @Nullable MediaType mediaType) {
		try {
			serializer(GenericTypeResolver.resolveType(type, contextClass));
			return canRead(mediaType);
		}
		catch (Exception ex) {
			return false;
		}
	}

	@Override
	public boolean canWrite(@Nullable Type type, Class<?> clazz, @Nullable MediaType mediaType) {
		try {
			serializer(type != null ? GenericTypeResolver.resolveType(type, clazz) : clazz);
			return canWrite(mediaType);
		}
		catch (Exception ex) {
			return false;
		}
	}

	@Override
	public final Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException {

		return decode(serializer(GenericTypeResolver.resolveType(type, contextClass)), inputMessage);
	}

	@Override
	protected final Object readInternal(Class<?> clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException {

		return decode(serializer(clazz), inputMessage);
	}

	private Object decode(KSerializer<Object> serializer, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException {

		MediaType contentType = inputMessage.getHeaders().getContentType();
		String jsonText = StreamUtils.copyToString(inputMessage.getBody(), getCharsetToUse(contentType));
		try {
			// TODO Use stream based API when available
			return this.json.decodeFromString(serializer, jsonText);
		}
		catch (SerializationException ex) {
			throw new HttpMessageNotReadableException("Could not read JSON: " + ex.getMessage(), ex, inputMessage);
		}
	}

	@Override
	protected final void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException {

		encode(object, serializer(type != null ? type : object.getClass()), outputMessage);
	}

	private void encode(Object object, KSerializer<Object> serializer, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException {

		try {
			String json = this.json.encodeToString(serializer, object);
			MediaType contentType = outputMessage.getHeaders().getContentType();
			outputMessage.getBody().write(json.getBytes(getCharsetToUse(contentType)));
			outputMessage.getBody().flush();
		}
		catch (IOException ex) {
			throw ex;
		}
		catch (Exception ex) {
			throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getMessage(), ex);
		}
	}

	private Charset getCharsetToUse(@Nullable MediaType contentType) {
		if (contentType != null && contentType.getCharset() != null) {
			return contentType.getCharset();
		}
		return DEFAULT_CHARSET;
	}

	
Tries to find a serializer that can marshall or unmarshall instances of the given type
using kotlinx.serialization. If no serializer can be found, an exception is thrown.
Resolved serializers are cached and cached results are returned on successive calls.
TODO Avoid relying on throwing exception when https://github.com/Kotlin/kotlinx.serialization/pull/1164 is fixed
Params:
type – the type to find a serializer for
Throws:
RuntimeException – if no serializer supporting the given type can be found
Returns:
a resolved serializer for the given type/**
	 * Tries to find a serializer that can marshall or unmarshall instances of the given type
	 * using kotlinx.serialization. If no serializer can be found, an exception is thrown.
	 * <p>Resolved serializers are cached and cached results are returned on successive calls.
	 * TODO Avoid relying on throwing exception when https://github.com/Kotlin/kotlinx.serialization/pull/1164 is fixed
	 * @param type the type to find a serializer for
	 * @return a resolved serializer for the given type
	 * @throws RuntimeException if no serializer supporting the given type can be found
	 */
	private KSerializer<Object> serializer(Type type) {
		KSerializer<Object> serializer = serializerCache.get(type);
		if (serializer == null) {
			serializer = SerializersKt.serializer(type);
			if (serializer.getDescriptor().getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
			}
			serializerCache.put(type, serializer);
		}
		return serializer;
	}

}