/*
 * 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.sshd.common;

import java.util.Objects;

import org.apache.sshd.common.channel.Channel;
import org.apache.sshd.common.session.Session;

/**
 * Provides the capability to attach in-memory attributes to the entity
 *
 * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
 */
public interface AttributeStore {
    /**
     * <P>
     * Type safe key for storage of user attributes. Typically it is used as a static
     * variable that is shared between the producer and the consumer. To further
     * restrict access the setting or getting it from the store one can add static
     * {@code get/set methods} e.g:
     * </P>
     *
     * <pre>
     * public static final AttributeKey<MyValue> MY_KEY = new AttributeKey<MyValue>();
     *
     * public static MyValue getMyValue(Session s) {
     *   return s.getAttribute(MY_KEY);
     * }
     *
     * public static void setMyValue(Session s, MyValue value) {
     *   s.setAttribute(MY_KEY, value);
     * }
     * </pre>
     *
     * @param <T> type of value stored in the attribute.
     * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
     */
    // CHECKSTYLE:OFF
    class AttributeKey<T> {
        public AttributeKey() {
            super();
        }
    }
    // CHECKSTYLE:ON

    /**
     * Returns the value of the user-defined attribute.
     *
     * @param <T> The generic attribute type
     * @param key The key of the attribute; must not be {@code null}.
     * @return <tt>null</tt> if there is no value associated with the specified key
     */
    <T> T getAttribute(AttributeKey<T> key);

    /**
     * Sets a user-defined attribute.
     *
     * @param <T>   The generic attribute type
     * @param key   The key of the attribute; must not be {@code null}.
     * @param value The value of the attribute; must not be {@code null}.
     * @return The old value of the attribute; {@code null} if it is new.
     */
    <T> T setAttribute(AttributeKey<T> key, T value);

    /**
     * Removes the user-defined attribute
     *
     * @param <T> The generic attribute type
     * @param key The key of the attribute; must not be {@code null}.
     * @return The removed value; {@code null} if no previous value
     */
    <T> T removeAttribute(AttributeKey<T> key);

    /**
     * Attempts to resolve the associated value by going up the store's
     * hierarchy (if any)
     *
     * @param <T> The generic attribute type
     * @param key The key of the attribute; must not be {@code null}.
     * @return <tt>null</tt> if there is no value associated with the specified key
     */
    <T> T resolveAttribute(AttributeKey<T> key);

    /**
     * @param <T> The generic attribute type
     * @param manager The {@link FactoryManager} - ignored if {@code null}
     * @param key The attribute key - never {@code null}
     * @return Associated value - {@code null} if not found
     */
    static <T> T resolveAttribute(FactoryManager manager, AttributeKey<T> key) {
        Objects.requireNonNull(key, "No key");
        return (manager == null) ? null : manager.getAttribute(key);
    }

    /**
     * Attempts to use the session's attribute, if not found then tries the factory manager
     *
     * @param <T> The generic attribute type
     * @param session The {@link Session} - ignored if {@code null}
     * @param key The attribute key - never {@code null}
     * @return Associated value - {@code null} if not found
     * @see Session#getFactoryManager()
     * @see #resolveAttribute(FactoryManager, AttributeKey)
     */
    static <T> T resolveAttribute(Session session, AttributeKey<T> key) {
        Objects.requireNonNull(key, "No key");
        if (session == null) {
            return null;
        }

        T value = session.getAttribute(key);
        return (value != null) ? value : resolveAttribute(session.getFactoryManager(), key);
    }

    /**
     * Attempts to use the channel attribute, if not found then tries the session
     *
     * @param <T> The generic attribute type
     * @param channel The {@link Channel} - ignored if {@code null}
     * @param key The attribute key - never {@code null}
     * @return Associated value - {@code null} if not found
     * @see Session#getFactoryManager()
     * @see #resolveAttribute(Session, AttributeKey)
     */
    static <T> T resolveAttribute(Channel channel, AttributeKey<T> key) {
        Objects.requireNonNull(key, "No key");
        if (channel == null) {
            return null;
        }

        T value = channel.getAttribute(key);
        return (value != null) ? value : resolveAttribute(channel.getSession(), key);
    }
}