/*
 * Copyright 2002-2020 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.result.method;

import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.function.Function;
import java.util.stream.Collectors;

import reactor.core.publisher.Mono;

import org.springframework.aop.support.AopUtils;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.core.MethodIntrospector;
import org.springframework.http.server.RequestPath;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.web.cors.CorsConfiguration;
import org.springframework.web.cors.reactive.CorsUtils;
import org.springframework.web.method.HandlerMethod;
import org.springframework.web.reactive.HandlerMapping;
import org.springframework.web.reactive.handler.AbstractHandlerMapping;
import org.springframework.web.server.ServerWebExchange;

/**
 * Abstract base class for {@link HandlerMapping} implementations that define
 * a mapping between a request and a {@link HandlerMethod}.
 *
 * <p>For each registered handler method, a unique mapping is maintained with
 * subclasses defining the details of the mapping type {@code <T>}.
 *
 * @author Rossen Stoyanchev
 * @author Brian Clozel
 * @author Sam Brannen
 * @since 5.0
 * @param <T> the mapping for a {@link HandlerMethod} containing the conditions
 * needed to match the handler method to an incoming request.
 */
public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {

        /**
         * Bean name prefix for target beans behind scoped proxies. Used to exclude those
         * targets from handler method detection, in favor of the corresponding proxies.
         * <p>We're not checking the autowire-candidate status here, which is how the
         * proxy target filtering problem is being handled at the autowiring level,
         * since autowire-candidate may have been turned to {@code false} for other
         * reasons, while still expecting the bean to be eligible for handler methods.
         * <p>Originally defined in {@link org.springframework.aop.scope.ScopedProxyUtils}
         * but duplicated here to avoid a hard dependency on the spring-aop module.
         */
        private static final String SCOPED_TARGET_NAME_PREFIX = "scopedTarget.";

        /**
         * HandlerMethod to return on a pre-flight request match when the request
         * mappings are more nuanced than the access control headers.
         */
        private static final HandlerMethod PREFLIGHT_AMBIGUOUS_MATCH =
                        new HandlerMethod(new PreFlightAmbiguousMatchHandler(),
                                        ClassUtils.getMethod(PreFlightAmbiguousMatchHandler.class, "handle"));

        private static final CorsConfiguration ALLOW_CORS_CONFIG = new CorsConfiguration();

        static {
                ALLOW_CORS_CONFIG.addAllowedOrigin("*");
                ALLOW_CORS_CONFIG.addAllowedMethod("*");
                ALLOW_CORS_CONFIG.addAllowedHeader("*");
                ALLOW_CORS_CONFIG.setAllowCredentials(true);
        }


        private final MappingRegistry mappingRegistry = new MappingRegistry();


        // TODO: handlerMethodMappingNamingStrategy

        /**
         * Return a (read-only) map with all mappings and HandlerMethod's.
         */
        public Map<T, HandlerMethod> getHandlerMethods() {
                this.mappingRegistry.acquireReadLock();
                try {
                        return Collections.unmodifiableMap(this.mappingRegistry.getMappings());
                }
                finally {
                        this.mappingRegistry.releaseReadLock();
                }
        }

        /**
         * Return the internal mapping registry. Provided for testing purposes.
         */
        MappingRegistry getMappingRegistry() {
                return this.mappingRegistry;
        }

        /**
         * Register the given mapping.
         * <p>This method may be invoked at runtime after initialization has completed.
         * @param mapping the mapping for the handler method
         * @param handler the handler
         * @param method the method
         */
        public void registerMapping(T mapping, Object handler, Method method) {
                if (logger.isTraceEnabled()) {
                        logger.trace("Register \"" + mapping + "\" to " + method.toGenericString());
                }
                this.mappingRegistry.register(mapping, handler, method);
        }

        /**
         * Un-register the given mapping.
         * <p>This method may be invoked at runtime after initialization has completed.
         * @param mapping the mapping to unregister
         */
        public void unregisterMapping(T mapping) {
                if (logger.isTraceEnabled()) {
                        logger.trace("Unregister mapping \"" + mapping);
                }
                this.mappingRegistry.unregister(mapping);
        }


        // Handler method detection

        /**
         * Detects handler methods at initialization.
         */
        @Override
        public void afterPropertiesSet() {

                initHandlerMethods();

                // Total includes detected mappings + explicit registrations via registerMapping..
                int total = this.getHandlerMethods().size();

                if ((logger.isTraceEnabled() && total == 0) || (logger.isDebugEnabled() && total > 0) ) {
                        logger.debug(total + " mappings in " + formatMappingName());
                }
        }

        /**
         * Scan beans in the ApplicationContext, detect and register handler methods.
         * @see #isHandler(Class)
         * @see #getMappingForMethod(Method, Class)
         * @see #handlerMethodsInitialized(Map)
         */
        protected void initHandlerMethods() {
                String[] beanNames = obtainApplicationContext().getBeanNamesForType(Object.class);

                for (String beanName : beanNames) {
                        if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                                Class<?> beanType = null;
                                try {
                                        beanType = obtainApplicationContext().getType(beanName);
                                }
                                catch (Throwable ex) {
                                        // An unresolvable bean type, probably from a lazy bean - let's ignore it.
                                        if (logger.isTraceEnabled()) {
                                                logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
                                        }
                                }
                                if (beanType != null && isHandler(beanType)) {
                                        detectHandlerMethods(beanName);
                                }
                        }
                }
                handlerMethodsInitialized(getHandlerMethods());
        }

        /**
         * Look for handler methods in a handler.
         * @param handler the bean name of a handler or a handler instance
         */
        protected void detectHandlerMethods(final Object handler) {
                Class<?> handlerType = (handler instanceof String ?
                                obtainApplicationContext().getType((String) handler) : handler.getClass());

                if (handlerType != null) {
                        final Class<?> userType = ClassUtils.getUserClass(handlerType);
                        Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                                        (MethodIntrospector.MetadataLookup<T>) method -> getMappingForMethod(method, userType));
                        if (logger.isTraceEnabled()) {
                                logger.trace(formatMappings(userType, methods));
                        }
                        methods.forEach((method, mapping) -> {
                                Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
                                registerHandlerMethod(handler, invocableMethod, mapping);
                        });
                }
        }

        private String formatMappings(Class<?> userType, Map<Method, T> methods) {
                String formattedType = Arrays.stream(ClassUtils.getPackageName(userType).split("\\."))
                                .map(p -> p.substring(0, 1))
                                .collect(Collectors.joining(".", "", "." + userType.getSimpleName()));
                Function<Method, String> methodFormatter = method -> Arrays.stream(method.getParameterTypes())
                                .map(Class::getSimpleName)
                                .collect(Collectors.joining(",", "(", ")"));
                return methods.entrySet().stream()
                                .map(e -> {
                                        Method method = e.getKey();
                                        return e.getValue() + ": " + method.getName() + methodFormatter.apply(method);
                                })
                                .collect(Collectors.joining("\n\t", "\n\t" + formattedType + ":" + "\n\t", ""));
        }

        /**
         * Register a handler method and its unique mapping. Invoked at startup for
         * each detected handler method.
         * @param handler the bean name of the handler or the handler instance
         * @param method the method to register
         * @param mapping the mapping conditions associated with the handler method
         * @throws IllegalStateException if another method was already registered
         * under the same mapping
         */
        protected void registerHandlerMethod(Object handler, Method method, T mapping) {
                this.mappingRegistry.register(mapping, handler, method);
        }

        /**
         * Create the HandlerMethod instance.
         * @param handler either a bean name or an actual handler instance
         * @param method the target method
         * @return the created HandlerMethod
         */
        protected HandlerMethod createHandlerMethod(Object handler, Method method) {
                if (handler instanceof String) {
                        return new HandlerMethod((String) handler,
                                        obtainApplicationContext().getAutowireCapableBeanFactory(), method);
                }
                return new HandlerMethod(handler, method);
        }

        /**
         * Extract and return the CORS configuration for the mapping.
         */
        @Nullable
        protected CorsConfiguration initCorsConfiguration(Object handler, Method method, T mapping) {
                return null;
        }

        /**
         * Invoked after all handler methods have been detected.
         * @param handlerMethods a read-only map with handler methods and mappings.
         */
        protected void handlerMethodsInitialized(Map<T, HandlerMethod> handlerMethods) {
        }


        // Handler method lookup

        /**
         * Look up a handler method for the given request.
         * @param exchange the current exchange
         */
        @Override
        public Mono<HandlerMethod> getHandlerInternal(ServerWebExchange exchange) {
                this.mappingRegistry.acquireReadLock();
                try {
                        HandlerMethod handlerMethod;
                        try {
                                handlerMethod = lookupHandlerMethod(exchange);
                        }
                        catch (Exception ex) {
                                return Mono.error(ex);
                        }
                        if (handlerMethod != null) {
                                handlerMethod = handlerMethod.createWithResolvedBean();
                        }
                        return Mono.justOrEmpty(handlerMethod);
                }
                finally {
                        this.mappingRegistry.releaseReadLock();
                }
        }

        /**
         * Look up the best-matching handler method for the current request.
         * If multiple matches are found, the best match is selected.
         * @param exchange the current exchange
         * @return the best-matching handler method, or {@code null} if no match
         * @see #handleMatch
         * @see #handleNoMatch
         */
        @Nullable
        protected HandlerMethod lookupHandlerMethod(ServerWebExchange exchange) throws Exception {
                List<Match> matches = new ArrayList<>();
                addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, exchange);

                if (!matches.isEmpty()) {
                        Comparator<Match> comparator = new MatchComparator(getMappingComparator(exchange));
                        matches.sort(comparator);
                        Match bestMatch = matches.get(0);
                        if (matches.size() > 1) {
                                if (logger.isTraceEnabled()) {
                                        logger.trace(exchange.getLogPrefix() + matches.size() + " matching mappings: " + matches);
                                }
                                if (CorsUtils.isPreFlightRequest(exchange.getRequest())) {
                                        return PREFLIGHT_AMBIGUOUS_MATCH;
                                }
                                Match secondBestMatch = matches.get(1);
                                if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                                        Method m1 = bestMatch.handlerMethod.getMethod();
                                        Method m2 = secondBestMatch.handlerMethod.getMethod();
                                        RequestPath path = exchange.getRequest().getPath();
                                        throw new IllegalStateException(
                                                        "Ambiguous handler methods mapped for '" + path + "': {" + m1 + ", " + m2 + "}");
                                }
                        }
                        handleMatch(bestMatch.mapping, bestMatch.handlerMethod, exchange);
                        return bestMatch.handlerMethod;
                }
                else {
                        return handleNoMatch(this.mappingRegistry.getMappings().keySet(), exchange);
                }
        }

        private void addMatchingMappings(Collection<T> mappings, List<Match> matches, ServerWebExchange exchange) {
                for (T mapping : mappings) {
                        T match = getMatchingMapping(mapping, exchange);
                        if (match != null) {
                                matches.add(new Match(match, this.mappingRegistry.getMappings().get(mapping)));
                        }
                }
        }

        /**
         * Invoked when a matching mapping is found.
         * @param mapping the matching mapping
         * @param handlerMethod the matching method
         * @param exchange the current exchange
         */
        protected void handleMatch(T mapping, HandlerMethod handlerMethod, ServerWebExchange exchange) {
        }

        /**
         * Invoked when no matching mapping is not found.
         * @param mappings all registered mappings
         * @param exchange the current exchange
         * @return an alternative HandlerMethod or {@code null}
         * @throws Exception provides details that can be translated into an error status code
         */
        @Nullable
        protected HandlerMethod handleNoMatch(Set<T> mappings, ServerWebExchange exchange) throws Exception {
                return null;
        }

        @Override
        protected boolean hasCorsConfigurationSource(Object handler) {
                return super.hasCorsConfigurationSource(handler) ||
                                (handler instanceof HandlerMethod && this.mappingRegistry.getCorsConfiguration((HandlerMethod) handler) != null);
        }

        @Override
        protected CorsConfiguration getCorsConfiguration(Object handler, ServerWebExchange exchange) {
                CorsConfiguration corsConfig = super.getCorsConfiguration(handler, exchange);
                if (handler instanceof HandlerMethod) {
                        HandlerMethod handlerMethod = (HandlerMethod) handler;
                        if (handlerMethod.equals(PREFLIGHT_AMBIGUOUS_MATCH)) {
                                return ALLOW_CORS_CONFIG;
                        }
                        CorsConfiguration methodConfig = this.mappingRegistry.getCorsConfiguration(handlerMethod);
                        corsConfig = (corsConfig != null ? corsConfig.combine(methodConfig) : methodConfig);
                }
                return corsConfig;
        }


        // Abstract template methods

        /**
         * Whether the given type is a handler with handler methods.
         * @param beanType the type of the bean being checked
         * @return "true" if this a handler type, "false" otherwise.
         */
        protected abstract boolean isHandler(Class<?> beanType);

        /**
         * Provide the mapping for a handler method. A method for which no
         * mapping can be provided is not a handler method.
         * @param method the method to provide a mapping for
         * @param handlerType the handler type, possibly a sub-type of the method's
         * declaring class
         * @return the mapping, or {@code null} if the method is not mapped
         */
        @Nullable
        protected abstract T getMappingForMethod(Method method, Class<?> handlerType);

        /**
         * Check if a mapping matches the current request and return a (potentially
         * new) mapping with conditions relevant to the current request.
         * @param mapping the mapping to get a match for
         * @param exchange the current exchange
         * @return the match, or {@code null} if the mapping doesn't match
         */
        @Nullable
        protected abstract T getMatchingMapping(T mapping, ServerWebExchange exchange);

        /**
         * Return a comparator for sorting matching mappings.
         * The returned comparator should sort 'better' matches higher.
         * @param exchange the current exchange
         * @return the comparator (never {@code null})
         */
        protected abstract Comparator<T> getMappingComparator(ServerWebExchange exchange);


        /**
         * A registry that maintains all mappings to handler methods, exposing methods
         * to perform lookups and providing concurrent access.
         *
         * <p>Package-private for testing purposes.
         */
        class MappingRegistry {

                private final Map<T, MappingRegistration<T>> registry = new HashMap<>();

                private final Map<T, HandlerMethod> mappingLookup = new LinkedHashMap<>();

                private final Map<HandlerMethod, CorsConfiguration> corsLookup = new ConcurrentHashMap<>();

                private final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();

                /**
                 * Return all mappings and handler methods. Not thread-safe.
                 * @see #acquireReadLock()
                 */
                public Map<T, HandlerMethod> getMappings() {
                        return this.mappingLookup;
                }

                /**
                 * Return CORS configuration. Thread-safe for concurrent use.
                 */
                @Nullable
                public CorsConfiguration getCorsConfiguration(HandlerMethod handlerMethod) {
                        HandlerMethod original = handlerMethod.getResolvedFromHandlerMethod();
                        return this.corsLookup.get(original != null ? original : handlerMethod);
                }

                /**
                 * Acquire the read lock when using getMappings and getMappingsByUrl.
                 */
                public void acquireReadLock() {
                        this.readWriteLock.readLock().lock();
                }

                /**
                 * Release the read lock after using getMappings and getMappingsByUrl.
                 */
                public void releaseReadLock() {
                        this.readWriteLock.readLock().unlock();
                }

                public void register(T mapping, Object handler, Method method) {
                        this.readWriteLock.writeLock().lock();
                        try {
                                HandlerMethod handlerMethod = createHandlerMethod(handler, method);
                                validateMethodMapping(handlerMethod, mapping);
                                this.mappingLookup.put(mapping, handlerMethod);

                                CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping);
                                if (corsConfig != null) {
                                        this.corsLookup.put(handlerMethod, corsConfig);
                                }

                                this.registry.put(mapping, new MappingRegistration<>(mapping, handlerMethod));
                        }
                        finally {
                                this.readWriteLock.writeLock().unlock();
                        }
                }

                private void validateMethodMapping(HandlerMethod handlerMethod, T mapping) {
                        // Assert that the supplied mapping is unique.
                        HandlerMethod existingHandlerMethod = this.mappingLookup.get(mapping);
                        if (existingHandlerMethod != null && !existingHandlerMethod.equals(handlerMethod)) {
                                throw new IllegalStateException(
                                                "Ambiguous mapping. Cannot map '" + handlerMethod.getBean() + "' method \n" +
                                                handlerMethod + "\nto " + mapping + ": There is already '" +
                                                existingHandlerMethod.getBean() + "' bean method\n" + existingHandlerMethod + " mapped.");
                        }
                }

                public void unregister(T mapping) {
                        this.readWriteLock.writeLock().lock();
                        try {
                                MappingRegistration<T> definition = this.registry.remove(mapping);
                                if (definition == null) {
                                        return;
                                }

                                this.mappingLookup.remove(definition.getMapping());
                                this.corsLookup.remove(definition.getHandlerMethod());
                        }
                        finally {
                                this.readWriteLock.writeLock().unlock();
                        }
                }
        }


        private static class MappingRegistration<T> {

                private final T mapping;

                private final HandlerMethod handlerMethod;

                public MappingRegistration(T mapping, HandlerMethod handlerMethod) {
                        Assert.notNull(mapping, "Mapping must not be null");
                        Assert.notNull(handlerMethod, "HandlerMethod must not be null");
                        this.mapping = mapping;
                        this.handlerMethod = handlerMethod;
                }

                public T getMapping() {
                        return this.mapping;
                }

                public HandlerMethod getHandlerMethod() {
                        return this.handlerMethod;
                }

        }


        /**
         * A thin wrapper around a matched HandlerMethod and its mapping, for the purpose of
         * comparing the best match with a comparator in the context of the current request.
         */
        private class Match {

                private final T mapping;

                private final HandlerMethod handlerMethod;

                public Match(T mapping, HandlerMethod handlerMethod) {
                        this.mapping = mapping;
                        this.handlerMethod = handlerMethod;
                }

                @Override
                public String toString() {
                        return this.mapping.toString();
                }
        }


        private class MatchComparator implements Comparator<Match> {

                private final Comparator<T> comparator;

                public MatchComparator(Comparator<T> comparator) {
                        this.comparator = comparator;
                }

                @Override
                public int compare(Match match1, Match match2) {
                        return this.comparator.compare(match1.mapping, match2.mapping);
                }
        }


        private static class PreFlightAmbiguousMatchHandler {

                @SuppressWarnings("unused")
                public void handle() {
                        throw new UnsupportedOperationException("Not implemented");
                }
        }

}