/*
 * Copyright 2002-2017 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
 *
 *      http://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.web.reactive.function;

import java.util.List;
import java.util.Optional;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collectors;

import org.reactivestreams.Publisher;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

import org.springframework.core.ParameterizedTypeReference;
import org.springframework.core.ResolvableType;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.http.HttpMessage;
import org.springframework.http.MediaType;
import org.springframework.http.ReactiveHttpInputMessage;
import org.springframework.http.codec.HttpMessageReader;
import org.springframework.http.codec.multipart.Part;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.util.Assert;
import org.springframework.util.MultiValueMap;

/**
 * Implementations of {@link BodyExtractor} that read various bodies, such a reactive streams.
 *
 * @author Arjen Poutsma
 * @author Sebastien Deleuze
 * @since 5.0
 */
public abstract class BodyExtractors {

	private static final ResolvableType FORM_MAP_TYPE =
			ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class);

	private static final ResolvableType MULTIPART_MAP_TYPE = ResolvableType.forClassWithGenerics(
			MultiValueMap.class, String.class, Part.class);

	private static final ResolvableType PART_TYPE = ResolvableType.forClass(Part.class);

	private static final ResolvableType VOID_TYPE = ResolvableType.forClass(Void.class);

	/**
	 * Return a {@code BodyExtractor} that reads into a Reactor {@link Mono}.
	 * @param elementClass the class of element in the {@code Mono}
	 * @param <T> the element type
	 * @return a {@code BodyExtractor} that reads a mono
	 */
	public static <T> BodyExtractor<Mono<T>, ReactiveHttpInputMessage> toMono(Class<? extends T> elementClass) {
		Assert.notNull(elementClass, "'elementClass' must not be null");
		return toMono(ResolvableType.forClass(elementClass));
	}

	/**
	 * Return a {@code BodyExtractor} that reads into a Reactor {@link Mono}.
	 * The given {@link ParameterizedTypeReference} is used to pass generic type information, for
	 * instance when using the {@link org.springframework.web.reactive.function.client.WebClient WebClient}
	 * <pre class="code">
	 * Mono&lt;Map&lt;String, String&gt;&gt; body = this.webClient
	 *  .get()
	 *  .uri("http://example.com")
	 *  .exchange()
	 *  .flatMap(r -> r.body(toMono(new ParameterizedTypeReference&lt;Map&lt;String,String&gt;&gt;() {})));
	 * </pre>
	 * @param typeReference a reference to the type of element in the {@code Mono}
	 * @param <T> the element type
	 * @return a {@code BodyExtractor} that reads a mono
	 */
	public static <T> BodyExtractor<Mono<T>, ReactiveHttpInputMessage> toMono(
			ParameterizedTypeReference<T> typeReference) {

		Assert.notNull(typeReference, "'typeReference' must not be null");
		return toMono(ResolvableType.forType(typeReference.getType()));
	}

	static <T> BodyExtractor<Mono<T>, ReactiveHttpInputMessage> toMono(ResolvableType elementType) {
		Assert.notNull(elementType, "'elementType' must not be null");
		return (inputMessage, context) -> readWithMessageReaders(inputMessage, context,
				elementType,
				(HttpMessageReader<T> reader) -> {
					Optional<ServerHttpResponse> serverResponse = context.serverResponse();
					if (serverResponse.isPresent() && inputMessage instanceof ServerHttpRequest) {
						return reader.readMono(elementType, elementType, (ServerHttpRequest) inputMessage,
								serverResponse.get(), context.hints());
					}
					else {
						return reader.readMono(elementType, inputMessage, context.hints());
					}
				},
				ex -> (inputMessage.getHeaders().getContentType() == null) ?
						Mono.from(permitEmptyOrFail(inputMessage, ex)) : Mono.error(ex),
				Mono::empty);
	}

	/**
	 * Return a {@code BodyExtractor} that reads into a Reactor {@link Flux}.
	 * @param elementClass the class of element in the {@code Flux}
	 * @param <T> the element type
	 * @return a {@code BodyExtractor} that reads a flux
	 */
	public static <T> BodyExtractor<Flux<T>, ReactiveHttpInputMessage> toFlux(Class<? extends T> elementClass) {
		Assert.notNull(elementClass, "'elementClass' must not be null");
		return toFlux(ResolvableType.forClass(elementClass));
	}

	/**
	 * Return a {@code BodyExtractor} that reads into a Reactor {@link Flux}.
	 * The given {@link ParameterizedTypeReference} is used to pass generic type information, for
	 * instance when using the {@link org.springframework.web.reactive.function.client.WebClient WebClient}
	 * <pre class="code">
	 * Flux&lt;ServerSentEvent&lt;String&gt;&gt; body = this.webClient
	 *  .get()
	 *  .uri("http://example.com")
	 *  .exchange()
	 *  .flatMap(r -> r.body(toFlux(new ParameterizedTypeReference&lt;ServerSentEvent&lt;String&gt;&gt;() {})));
	 * </pre>
	 * @param typeReference a reference to the type of element in the {@code Flux}
	 * @param <T> the element type
	 * @return a {@code BodyExtractor} that reads a flux
	 */
	public static <T> BodyExtractor<Flux<T>, ReactiveHttpInputMessage> toFlux(
			ParameterizedTypeReference<T> typeReference) {

		Assert.notNull(typeReference, "'typeReference' must not be null");
		return toFlux(ResolvableType.forType(typeReference.getType()));
	}

	@SuppressWarnings("unchecked")
	static <T> BodyExtractor<Flux<T>, ReactiveHttpInputMessage> toFlux(ResolvableType elementType) {
		Assert.notNull(elementType, "'elementType' must not be null");
		return (inputMessage, context) -> readWithMessageReaders(inputMessage, context,
				elementType,
				(HttpMessageReader<T> reader) -> {
					Optional<ServerHttpResponse> serverResponse = context.serverResponse();
					if (serverResponse.isPresent() && inputMessage instanceof ServerHttpRequest) {
						return reader.read(elementType, elementType, (ServerHttpRequest) inputMessage,
								serverResponse.get(), context.hints());
					}
					else {
						return reader.read(elementType, inputMessage, context.hints());
					}
				},
				ex -> (inputMessage.getHeaders().getContentType() == null) ?
						permitEmptyOrFail(inputMessage, ex) : Flux.error(ex),
				Flux::empty);
	}

	@SuppressWarnings("unchecked")
	private static <T> Flux<T> permitEmptyOrFail(ReactiveHttpInputMessage message, UnsupportedMediaTypeException ex) {
		return message.getBody().doOnNext(buffer -> {
			throw ex;
		}).map(o -> (T) o);
	}

	/**
	 * Return a {@code BodyExtractor} that reads form data into a {@link MultiValueMap}.
	 * @return a {@code BodyExtractor} that reads form data
	 */
	// Note that the returned BodyExtractor is parameterized to ServerHttpRequest, not
	// ReactiveHttpInputMessage like other methods, since reading form data only typically happens on
	// the server-side
	public static BodyExtractor<Mono<MultiValueMap<String, String>>, ServerHttpRequest> toFormData() {
		return (request, context) -> {
			ResolvableType type = FORM_MAP_TYPE;
			HttpMessageReader<MultiValueMap<String, String>> reader =
					messageReader(type, MediaType.APPLICATION_FORM_URLENCODED, context);
			return context.serverResponse()
					.map(response -> reader.readMono(type, type, request, response, context.hints()))
					.orElseGet(() -> reader.readMono(type, request, context.hints()));
		};
	}

	/**
	 * Return a {@code BodyExtractor} that reads multipart (i.e. file upload) form data into a
	 * {@link MultiValueMap}.
	 * @return a {@code BodyExtractor} that reads multipart data
	 */
	// Note that the returned BodyExtractor is parameterized to ServerHttpRequest, not
	// ReactiveHttpInputMessage like other methods, since reading form data only typically happens on
	// the server-side
	public static BodyExtractor<Mono<MultiValueMap<String, Part>>, ServerHttpRequest> toMultipartData() {
		return (serverRequest, context) -> {
			ResolvableType type = MULTIPART_MAP_TYPE;
			HttpMessageReader<MultiValueMap<String, Part>> reader =
					messageReader(type, MediaType.MULTIPART_FORM_DATA, context);
			return context.serverResponse()
					.map(response -> reader.readMono(type, type, serverRequest, response, context.hints()))
					.orElseGet(() -> reader.readMono(type, serverRequest, context.hints()));
		};
	}

	/**
	 * Return a {@code BodyExtractor} that reads multipart (i.e. file upload) form data into a
	 * {@link MultiValueMap}.
	 * @return a {@code BodyExtractor} that reads multipart data
	 */
	// Note that the returned BodyExtractor is parameterized to ServerHttpRequest, not
	// ReactiveHttpInputMessage like other methods, since reading form data only typically happens on
	// the server-side
	public static BodyExtractor<Flux<Part>, ServerHttpRequest> toParts() {
		return (serverRequest, context) -> {
			ResolvableType type = PART_TYPE;
			HttpMessageReader<Part> reader = messageReader(type, MediaType.MULTIPART_FORM_DATA, context);
			return context.serverResponse()
					.map(response -> reader.read(type, type, serverRequest, response, context.hints()))
					.orElseGet(() -> reader.read(type, serverRequest, context.hints()));
		};
	}

	/**
	 * Return a {@code BodyExtractor} that returns the body of the message as a {@link Flux} of
	 * {@link DataBuffer}s.
	 * <p><strong>Note</strong> that the returned buffers should be released after usage by calling
	 * {@link org.springframework.core.io.buffer.DataBufferUtils#release(DataBuffer)}
	 * @return a {@code BodyExtractor} that returns the body
	 * @see ReactiveHttpInputMessage#getBody()
	 */
	public static BodyExtractor<Flux<DataBuffer>, ReactiveHttpInputMessage> toDataBuffers() {
		return (inputMessage, context) -> inputMessage.getBody();
	}


	private static <T, S extends Publisher<T>> S readWithMessageReaders(
			ReactiveHttpInputMessage inputMessage, BodyExtractor.Context context, ResolvableType elementType,
			Function<HttpMessageReader<T>, S> readerFunction,
			Function<UnsupportedMediaTypeException, S> unsupportedError,
			Supplier<S> empty) {

		if (VOID_TYPE.equals(elementType)) {
			return empty.get();
		}
		MediaType contentType = contentType(inputMessage);
		List<HttpMessageReader<?>> messageReaders = context.messageReaders();
		return messageReaders.stream()
				.filter(r -> r.canRead(elementType, contentType))
				.findFirst()
				.map(BodyExtractors::<T>cast)
				.map(readerFunction)
				.orElseGet(() -> {
					List<MediaType> supportedMediaTypes = messageReaders.stream()
							.flatMap(reader -> reader.getReadableMediaTypes().stream())
							.collect(Collectors.toList());
					UnsupportedMediaTypeException error =
							new UnsupportedMediaTypeException(contentType, supportedMediaTypes);
					return unsupportedError.apply(error);
				});
	}

	private static <T> HttpMessageReader<T> messageReader(ResolvableType elementType,
			MediaType mediaType, BodyExtractor.Context context) {
		return context.messageReaders().stream()
				.filter(messageReader -> messageReader.canRead(elementType, mediaType))
				.findFirst()
				.map(BodyExtractors::<T>cast)
				.orElseThrow(() -> new IllegalStateException(
						"Could not find HttpMessageReader that supports \"" + mediaType +
								"\" and \"" + elementType + "\""));
	}

	private static MediaType contentType(HttpMessage message) {
		MediaType result = message.getHeaders().getContentType();
		return result != null ? result : MediaType.APPLICATION_OCTET_STREAM;
	}

	@SuppressWarnings("unchecked")
	private static <T> HttpMessageReader<T> cast(HttpMessageReader<?> messageReader) {
		return (HttpMessageReader<T>) messageReader;
	}

}
