/* 
    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.wiki.auth;

import java.security.Principal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.WeakHashMap;

import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

import org.apache.log4j.Logger;
import org.apache.wiki.WikiEngine;
import org.apache.wiki.WikiSession;
import org.apache.wiki.event.WikiEventListener;
import org.apache.wiki.event.WikiEventManager;
import org.apache.wiki.event.WikiSecurityEvent;

/**
 *  <p>Manages WikiSession's for different WikiEngine's.</p>
 *  <p>The WikiSession's are stored both in the remote user
 *  HttpSession and in the SessionMonitor for the WikeEngine.
 *  This class must be configured as a session listener in the
 *  web.xml for the wiki web application.
 *  </p>
 */
public class SessionMonitor implements HttpSessionListener
{
    private static Logger log = Logger.getLogger( SessionMonitor.class );

    /** Map with WikiEngines as keys, and SessionMonitors as values. */
    private static Map<WikiEngine, SessionMonitor>          c_monitors   = new HashMap<WikiEngine, SessionMonitor>();

    /** Weak hashmap with HttpSessions as keys, and WikiSessions as values. */
    private final Map<String, WikiSession>                 m_sessions   = new WeakHashMap<String, WikiSession>();

    private       WikiEngine          m_engine;

    private final PrincipalComparator m_comparator = new PrincipalComparator();

    /**
     * Returns the instance of the SessionMonitor for this wiki.
     * Only one SessionMonitor exists per WikiEngine.
     * @param engine the wiki engine
     * @return the session monitor
     */
    public static final SessionMonitor getInstance( WikiEngine engine )
    {
        if( engine == null )
        {
            throw new IllegalArgumentException( "Engine cannot be null." );
        }
        SessionMonitor monitor;

        synchronized( c_monitors )
        {
            monitor = c_monitors.get(engine);
            if( monitor == null )
            {
                monitor = new SessionMonitor(engine);

                c_monitors.put( engine, monitor );
            }
        }
        return monitor;
    }

    /**
     * Construct the SessionListener
     */
    public SessionMonitor()
    {
    }

    private SessionMonitor( WikiEngine engine )
    {
        m_engine = engine;
    }

    /**
     *  Just looks for a WikiSession; does not create a new one.
     * This method may return <code>null</code>, <em>and
     * callers should check for this value</em>.
     *
     *  @param session the user's HTTP session
     *  @return the WikiSession, if found
     */
    private WikiSession findSession( HttpSession session )
    {
        WikiSession wikiSession = null;
        String sid = ( session == null ) ? "(null)" : session.getId();
        WikiSession storedSession = m_sessions.get( sid );

        // If the weak reference returns a wiki session, return it
        if( storedSession != null )
        {
            if( log.isDebugEnabled() )
            {
                log.debug( "Looking up WikiSession for session ID=" + sid + "... found it" );
            }
            wikiSession = storedSession;
        }

        return wikiSession;
    }
    /**
     * <p>Looks up the wiki session associated with a user's Http session
     * and adds it to the session cache. This method will return the
     * "guest session" as constructed by {@link WikiSession#guestSession(WikiEngine)}
     * if the HttpSession is not currently associated with a WikiSession.
     * This method is guaranteed to return a non-<code>null</code> WikiSession.</p>
     * <p>Internally, the session is stored in a HashMap; keys are
     * the HttpSession objects, while the values are
     * {@link java.lang.ref.WeakReference}-wrapped WikiSessions.</p>
     * @param session the HTTP session
     * @return the wiki session
     */
    public final WikiSession find( HttpSession session )
    {
        WikiSession wikiSession = findSession(session);
        String sid = ( session == null ) ? "(null)" : session.getId();

        // Otherwise, create a new guest session and stash it.
        if( wikiSession == null )
        {
            if( log.isDebugEnabled() )
            {
                log.debug( "Looking up WikiSession for session ID=" + sid + "... not found. Creating guestSession()" );
            }
            wikiSession = WikiSession.guestSession( m_engine );
            synchronized( m_sessions )
            {
                m_sessions.put( sid, wikiSession );
            }
        }

        return wikiSession;
    }

    /**
     * Removes the wiki session associated with the user's HttpSession
     * from the session cache.
     * @param session the user's HTTP session
     */
    public final void remove( HttpSession session )
    {
        if ( session == null )
        {
            throw new IllegalArgumentException( "Session cannot be null." );
        }
        synchronized ( m_sessions )
        {
            m_sessions.remove( session.getId() );
        }
    }

    /**
     * Returns the current number of active wiki sessions.
     * @return the number of sessions
     */
    public final int sessions()
    {
        return userPrincipals().length;
    }

    /**
     * <p>Returns the current wiki users as a sorted array of
     * Principal objects. The principals are those returned by
     * each WikiSession's {@link WikiSession#getUserPrincipal()}'s
     * method.</p>
     * <p>To obtain the list of current WikiSessions, we iterate
     * through our session Map and obtain the list of values,
     * which are WikiSessions wrapped in {@link java.lang.ref.WeakReference}
     * objects. Those <code>WeakReference</code>s whose <code>get()</code>
     * method returns non-<code>null</code> values are valid
     * sessions.</p>
     * @return the array of user principals
     */
    public final Principal[] userPrincipals()
    {
        Collection<Principal> principals = new ArrayList<Principal>();
        for ( WikiSession session : m_sessions.values() )
        {
            principals.add( session.getUserPrincipal() );
        }
        Principal[] p = principals.toArray( new Principal[principals.size()] );
        Arrays.sort( p, m_comparator );
        return p;
    }

    /**
     * Registers a WikiEventListener with this instance.
     * @param listener the event listener
     * @since 2.4.75
     */
    public final synchronized void addWikiEventListener( WikiEventListener listener )
    {
        WikiEventManager.addWikiEventListener( this, listener );
    }

    /**
     * Un-registers a WikiEventListener with this instance.
     * @param listener the event listener
     * @since 2.4.75
     */
    public final synchronized void removeWikiEventListener( WikiEventListener listener )
    {
        WikiEventManager.removeWikiEventListener( this, listener );
    }

    /**
     * Fires a WikiSecurityEvent to all registered listeners.
     * @param type  the event type
     * @param principal the user principal associated with this session
     * @param session the wiki session
     * @since 2.4.75
     */
    protected final void fireEvent( int type, Principal principal, WikiSession session )
    {
        if( WikiEventManager.isListening(this) )
        {
            WikiEventManager.fireEvent(this,new WikiSecurityEvent(this,type,principal,session));
        }
    }

    /**
     * Fires when the web container creates a new HTTP session.
     * 
     * @param se the HTTP session event
     */
    public void sessionCreated( HttpSessionEvent se )
    {
        HttpSession session = se.getSession();
    }

    /**
     * Removes the user's WikiSession from the internal session cache when the web
     * container destoys an HTTP session.
     * @param se the HTTP session event
     */
    public void sessionDestroyed( HttpSessionEvent se )
    {
        HttpSession session = se.getSession();
        Iterator<SessionMonitor> it = c_monitors.values().iterator();
        while( it.hasNext() )
        {
            SessionMonitor monitor = it.next();

            WikiSession storedSession = monitor.findSession(session);

            monitor.remove(session);

            log.debug("Removed session "+session.getId()+".");

            if( storedSession != null )
            {
                fireEvent( WikiSecurityEvent.SESSION_EXPIRED,
                           storedSession.getLoginPrincipal(),
                           storedSession );
            }
        }
    }
}