/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.configuration2.event;

import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * <p>
 * A class for managing event listeners for an event source.
 * </p>
 * <p>
 * This class allows registering an arbitrary number of event listeners for specific event types. Event types are
 * specified using the {@link EventType} class. Due to the type parameters in method signatures, it is guaranteed that
 * registered listeners are compatible with the event types they are interested in.
 * </p>
 * <p>
 * There are also methods for firing events. Here all registered listeners are determined - based on the event type
 * specified at registration time - which should receive the event to be fired. So basically, the event type at listener
 * registration serves as a filter criterion. Because of the hierarchical nature of event types it can be determined in
 * a fine-grained way which events are propagated to which listeners. It is also possible to register a listener
 * multiple times for different event types.
 * </p>
 * <p>
 * Implementation note: This class is thread-safe.
 * </p>
 *
 * @since 2.0
 */
public class EventListenerList {
    /**
     * A special {@code Iterator} implementation used by the {@code getEventListenerIterator()} method. This iterator
     * returns only listeners compatible with a specified event type. It has a convenience method for invoking the current
     * listener in the iteration with an event.
     *
     * @param <T> the event type
     */
    public static final class EventListenerIterator<T extends Event> implements Iterator<EventListener<? super T>> {
        /** The underlying iterator. */
        private final Iterator<EventListenerRegistrationData<?>> underlyingIterator;

        /** The base event type. */
        private final EventType<T> baseEventType;

        /** The set with accepted event types. */
        private final Set<EventType<?>> acceptedTypes;

        /** The next element in the iteration. */
        private EventListener<? super T> nextElement;

        private EventListenerIterator(final Iterator<EventListenerRegistrationData<?>> it, final EventType<T> base) {
            underlyingIterator = it;
            baseEventType = base;
            acceptedTypes = EventType.fetchSuperEventTypes(base);
            initNextElement();
        }

        /**
         * Extracts the listener from the given data object and performs a cast to the target type. This is safe because it has
         * been checked before that the type is compatible.
         *
         * @param regData the data object
         * @return the extracted listener
         */
        @SuppressWarnings("unchecked")
        private EventListener<? super T> castListener(final EventListenerRegistrationData<?> regData) {
            @SuppressWarnings("rawtypes")
            final EventListener listener = regData.getListener();
            return listener;
        }

        @Override
        public boolean hasNext() {
            return nextElement != null;
        }

        /**
         * Determines the next element in the iteration.
         */
        private void initNextElement() {
            nextElement = null;
            while (underlyingIterator.hasNext() && nextElement == null) {
                final EventListenerRegistrationData<?> regData = underlyingIterator.next();
                if (acceptedTypes.contains(regData.getEventType())) {
                    nextElement = castListener(regData);
                }
            }
        }

        /**
         * Obtains the next event listener in this iteration and invokes it with the given event object.
         *
         * @param event the event object
         * @throws NoSuchElementException if iteration is at its end
         */
        public void invokeNext(final Event event) {
            validateEvent(event);
            invokeNextListenerUnchecked(event);
        }

        /**
         * Invokes the next event listener in the iteration without doing a validity check on the event. This method is called
         * internally to avoid duplicate event checks.
         *
         * @param event the event object
         */
        private void invokeNextListenerUnchecked(final Event event) {
            callListener(next(), event);
        }

        @Override
        public EventListener<? super T> next() {
            if (nextElement == null) {
                throw new NoSuchElementException("No more event listeners!");
            }

            final EventListener<? super T> result = nextElement;
            initNextElement();
            return result;
        }

        /**
         * {@inheritDoc} This implementation always throws an exception. Removing elements is not supported.
         */
        @Override
        public void remove() {
            throw new UnsupportedOperationException("Removing elements is not supported!");
        }

        /**
         * Checks whether the specified event can be passed to an event listener in this iteration. This check is done via the
         * hierarchy of event types.
         *
         * @param event the event object
         * @throws IllegalArgumentException if the event is invalid
         */
        private void validateEvent(final Event event) {
            if (event == null || !EventType.fetchSuperEventTypes(event.getEventType()).contains(baseEventType)) {
                throw new IllegalArgumentException("Event incompatible with listener iteration: " + event);
            }
        }
    }

    /**
     * Helper method for calling an event listener with an event. We have to operate on raw types to make this code compile.
     * However, this is safe because of the way the listeners have been registered and associated with event types - so it
     * is ensured that the event is compatible with the listener.
     *
     * @param listener the event listener to be called
     * @param event the event to be fired
     */
    @SuppressWarnings("unchecked")
    private static void callListener(final EventListener<?> listener, final Event event) {
        @SuppressWarnings("rawtypes")
        final EventListener rowListener = listener;
        rowListener.onEvent(event);
    }

    /** A list with the listeners added to this object. */
    private final List<EventListenerRegistrationData<?>> listeners;

    /**
     * Creates a new instance of {@code EventListenerList}.
     */
    public EventListenerList() {
        listeners = new CopyOnWriteArrayList<>();
    }

    /**
     * Adds all event listener registrations stored in the specified {@code EventListenerList} to this list.
     *
     * @param c the list to be copied (must not be <b>null</b>)
     * @throws IllegalArgumentException if the list to be copied is <b>null</b>
     */
    public void addAll(final EventListenerList c) {
        if (c == null) {
            throw new IllegalArgumentException("List to be copied must not be null!");
        }
        c.getRegistrations().forEach(this::addEventListener);
    }

    /**
     * Adds the specified listener registration data object to the internal list of event listeners. This is an alternative
     * registration method; the event type and the listener are passed as a single data object.
     *
     * @param regData the registration data object (must not be <b>null</b>)
     * @param <T> the type of events processed by this listener
     * @throws IllegalArgumentException if the registration data object is <b>null</b>
     */
    public <T extends Event> void addEventListener(final EventListenerRegistrationData<T> regData) {
        if (regData == null) {
            throw new IllegalArgumentException("EventListenerRegistrationData must not be null!");
        }
        listeners.add(regData);
    }

    /**
     * Adds an event listener for the specified event type. This listener is notified about events of this type and all its
     * sub types.
     *
     * @param type the event type (must not be <b>null</b>)
     * @param listener the listener to be registered (must not be <b>null</b>)
     * @param <T> the type of events processed by this listener
     * @throws IllegalArgumentException if a required parameter is <b>null</b>
     */
    public <T extends Event> void addEventListener(final EventType<T> type, final EventListener<? super T> listener) {
        listeners.add(new EventListenerRegistrationData<>(type, listener));
    }

    /**
     * Removes all event listeners registered at this object.
     */
    public void clear() {
        listeners.clear();
    }

    /**
     * Fires an event to all registered listeners matching the event type.
     *
     * @param event the event to be fired (must not be <b>null</b>)
     * @throws IllegalArgumentException if the event is <b>null</b>
     */
    public void fire(final Event event) {
        if (event == null) {
            throw new IllegalArgumentException("Event to be fired must not be null!");
        }

        for (final EventListenerIterator<? extends Event> iterator = getEventListenerIterator(event.getEventType()); iterator.hasNext();) {
            iterator.invokeNextListenerUnchecked(event);
        }
    }

    /**
     * Gets a specialized iterator for obtaining all event listeners stored in this list which are compatible with the
     * specified event type.
     *
     * @param eventType the event type object
     * @param <T> the event type
     * @return an {@code Iterator} with the selected event listeners
     */
    public <T extends Event> EventListenerIterator<T> getEventListenerIterator(final EventType<T> eventType) {
        return new EventListenerIterator<>(listeners.iterator(), eventType);
    }

    /**
     * Gets an {@code Iterable} allowing access to all event listeners stored in this list which are compatible with the
     * specified event type.
     *
     * @param eventType the event type object
     * @param <T> the event type
     * @return an {@code Iterable} with the selected event listeners
     */
    public <T extends Event> Iterable<EventListener<? super T>> getEventListeners(final EventType<T> eventType) {
        return () -> getEventListenerIterator(eventType);
    }

    /**
     * Gets an (unmodifiable) list with registration information about all event listeners registered at this object.
     *
     * @return a list with event listener registration information
     */
    public List<EventListenerRegistrationData<?>> getRegistrations() {
        return Collections.unmodifiableList(listeners);
    }

    /**
     * Gets a list with {@code EventListenerRegistrationData} objects for all event listener registrations of the
     * specified event type or an event type having this type as super type (directly or indirectly). Note that this is the
     * opposite direction than querying event types for firing events: in this case event listener registrations are
     * searched which are super event types from a given type. This method in contrast returns event listener registrations
     * for listeners that extend a given super type.
     *
     * @param eventType the event type object
     * @param <T> the event type
     * @return a list with the matching event listener registration objects
     */
    public <T extends Event> List<EventListenerRegistrationData<? extends T>> getRegistrationsForSuperType(final EventType<T> eventType) {
        final Map<EventType<?>, Set<EventType<?>>> superTypes = new HashMap<>();
        final List<EventListenerRegistrationData<? extends T>> results = new LinkedList<>();

        listeners.forEach(reg -> {
            final Set<EventType<?>> base = superTypes.computeIfAbsent(reg.getEventType(), EventType::fetchSuperEventTypes);
            if (base.contains(eventType)) {
                @SuppressWarnings("unchecked")
                final
                // This is safe because we just did a check
                EventListenerRegistrationData<? extends T> result = (EventListenerRegistrationData<? extends T>) reg;
                results.add(result);
            }
        });

        return results;
    }

    /**
     * Removes the event listener registration defined by the passed in data object. This is an alternative method for
     * removing a listener which expects the event type and the listener in a single data object.
     *
     * @param regData the registration data object
     * @param <T> the type of events processed by this listener
     * @return a flag whether a listener registration was removed
     * @see #removeEventListener(EventType, EventListener)
     */
    public <T extends Event> boolean removeEventListener(final EventListenerRegistrationData<T> regData) {
        return listeners.remove(regData);
    }

    /**
     * Removes the event listener registration for the given event type and listener. An event listener instance may be
     * registered multiple times for different event types. Therefore, when removing a listener the event type of the
     * registration in question has to be specified. The return value indicates whether a registration was removed. A value
     * of <b>false</b> means that no such combination of event type and listener was found.
     *
     * @param eventType the event type
     * @param listener the event listener to be removed
     * @param <T> the type of events processed by this listener
     * @return a flag whether a listener registration was removed
     */
    public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
        return !(listener == null || eventType == null) && removeEventListener(new EventListenerRegistrationData<>(eventType, listener));
    }
}
