/*
 * Copyright 2002-2018 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.web.reactive.function.server;

import java.util.function.Function;

import reactor.core.publisher.Mono;

import org.springframework.util.Assert;
import org.springframework.web.reactive.function.server.support.ServerRequestWrapper;

/**
 * Represents a function that filters a {@linkplain HandlerFunction handler function}.
 *
 * @author Arjen Poutsma
 * @since 5.0
 * @param <T> the type of the {@linkplain HandlerFunction handler function} to filter
 * @param <R> the type of the response of the function
 * @see RouterFunction#filter(HandlerFilterFunction)
 */
@FunctionalInterface
public interface HandlerFilterFunction<T extends ServerResponse, R extends ServerResponse> {

	/**
	 * Apply this filter to the given handler function. The given
	 * {@linkplain HandlerFunction handler function} represents the next entity in the chain,
	 * and can be {@linkplain HandlerFunction#handle(ServerRequest) invoked} in order to
	 * proceed to this entity, or not invoked to block the chain.
	 * @param request the request
	 * @param next the next handler or filter function in the chain
	 * @return the filtered response
	 * @see ServerRequestWrapper
	 */
	Mono<R> filter(ServerRequest request, HandlerFunction<T> next);

	/**
	 * Return a composed filter function that first applies this filter, and then applies the
	 * {@code after} filter.
	 * @param after the filter to apply after this filter is applied
	 * @return a composed filter that first applies this function and then applies the
	 * {@code after} function
	 */
	default HandlerFilterFunction<T, R> andThen(HandlerFilterFunction<T, T> after) {
		Assert.notNull(after, "HandlerFilterFunction must not be null");
		return (request, next) -> {
			HandlerFunction<T> nextHandler = handlerRequest -> after.filter(handlerRequest, next);
			return filter(request, nextHandler);
		};
	}

	/**
	 * Apply this filter to the given handler function, resulting in a filtered handler function.
	 * @param handler the handler function to filter
	 * @return the filtered handler function
	 */
	default HandlerFunction<R> apply(HandlerFunction<T> handler) {
		Assert.notNull(handler, "HandlerFunction must not be null");
		return request -> this.filter(request, handler);
	}

	/**
	 * Adapt the given request processor function to a filter function that only operates
	 * on the {@code ClientRequest}.
	 * @param requestProcessor the request processor
	 * @return the filter adaptation of the request processor
	 */
	static HandlerFilterFunction<?, ?> ofRequestProcessor(
			Function<ServerRequest, Mono<ServerRequest>> requestProcessor) {

		Assert.notNull(requestProcessor, "Function must not be null");
		return (request, next) -> requestProcessor.apply(request).flatMap(next::handle);
	}

	/**
	 * Adapt the given response processor function to a filter function that only operates
	 * on the {@code ClientResponse}.
	 * @param responseProcessor the response processor
	 * @return the filter adaptation of the request processor
	 */
	static <T extends ServerResponse, R extends ServerResponse> HandlerFilterFunction<T, R> ofResponseProcessor(
			Function<T, Mono<R>> responseProcessor) {

		Assert.notNull(responseProcessor, "Function must not be null");
		return (request, next) -> next.handle(request).flatMap(responseProcessor);
	}

}
