/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.classic.turbo;

import java.io.File;
import java.net.URL;
import java.util.List;

import ch.qos.logback.core.CoreConstants;
import ch.qos.logback.core.joran.event.SaxEvent;
import ch.qos.logback.core.joran.spi.ConfigurationWatchList;
import ch.qos.logback.core.joran.util.ConfigurationWatchListUtil;
import ch.qos.logback.core.status.StatusUtil;
import org.slf4j.Marker;

import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.joran.JoranConfigurator;
import ch.qos.logback.core.joran.spi.JoranException;
import ch.qos.logback.core.spi.FilterReply;

import static ch.qos.logback.core.CoreConstants.MILLIS_IN_ONE_SECOND;

/**
 * Reconfigure a LoggerContext when the configuration file changes.
 *
 * @author Ceki Gulcu
 */
public class ReconfigureOnChangeFilter extends TurboFilter {

    /**
     * Scan for changes in configuration file once every minute.
     */
    // 1 minute - value mentioned in documentation
    public final static long DEFAULT_REFRESH_PERIOD = 60 * MILLIS_IN_ONE_SECOND;

    long refreshPeriod = DEFAULT_REFRESH_PERIOD;
    URL mainConfigurationURL;
    protected volatile long nextCheck;

    ConfigurationWatchList configurationWatchList;

    @Override
    public void start() {
        configurationWatchList = ConfigurationWatchListUtil.getConfigurationWatchList(context);
        if (configurationWatchList != null) {
            mainConfigurationURL = configurationWatchList.getMainURL();
            if (mainConfigurationURL == null) {
                addWarn("Due to missing top level configuration file, automatic reconfiguration is impossible.");
                return;
            }
            List<File> watchList = configurationWatchList.getCopyOfFileWatchList();
            long inSeconds = refreshPeriod / 1000;
            addInfo("Will scan for changes in [" + watchList + "] every " + inSeconds + " seconds. ");
            synchronized (configurationWatchList) {
                updateNextCheck(System.currentTimeMillis());
            }
            super.start();
        } else {
            addWarn("Empty ConfigurationWatchList in context");
        }
    }

    @Override
    public String toString() {
        return "ReconfigureOnChangeFilter{" + "invocationCounter=" + invocationCounter + '}';
    }

    // The next fields counts the number of time the decide method is called
    //
    // IMPORTANT: This field can be updated by multiple threads. It follows that
    // its values may *not* be incremented sequentially. However, we don't care
    // about the actual value of the field except that from time to time the
    // expression (invocationCounter++ & mask) == mask) should be true.
    private long invocationCounter = 0;

    private volatile long mask = 0xF;
    private volatile long lastMaskCheck = System.currentTimeMillis();

    @Override
    public FilterReply decide(Marker marker, Logger logger, Level level, String format, Object[] params, Throwable t) {
        if (!isStarted()) {
            return FilterReply.NEUTRAL;
        }

        // for performance reasons, skip change detection (MASK-1) times out of MASK.
        // Only once every MASK calls is change detection code executed
        // Note that MASK is a variable itself.
        if (((invocationCounter++) & mask) != mask) {
            return FilterReply.NEUTRAL;
        }

        long now = System.currentTimeMillis();

        synchronized (configurationWatchList) {
            updateMaskIfNecessary(now);
            if (changeDetected(now)) {
                // Even though reconfiguration involves resetting the loggerContext,
                // which clears the list of turbo filters including this instance, it is
                // still possible for this instance to be subsequently invoked by another
                // thread if it was already executing when the context was reset.
                disableSubsequentReconfiguration();
                detachReconfigurationToNewThread();
            }
        }

        return FilterReply.NEUTRAL;
    }

    // experiments indicate that even for CPU intensive applications with 200 or more threads MASK
    // values in the order of 0xFFFF is appropriate
    private static final int MAX_MASK = 0xFFFF;

    // if less than MASK_INCREASE_THRESHOLD milliseconds elapse between invocations of updateMaskIfNecessary() method,
    // then the mask should be increased
    private static final long MASK_INCREASE_THRESHOLD = 100;

    // if more than MASK_DECREASE_THRESHOLD milliseconds elapse between invocations of updateMaskIfNecessary() method,
    // then the mask should be decreased
    private static final long MASK_DECREASE_THRESHOLD = MASK_INCREASE_THRESHOLD * 8;

    // update the mask so as to execute change detection code about once every 100 to 8000 milliseconds.
    private void updateMaskIfNecessary(long now) {
        final long timeElapsedSinceLastMaskUpdateCheck = now - lastMaskCheck;
        lastMaskCheck = now;
        if (timeElapsedSinceLastMaskUpdateCheck < MASK_INCREASE_THRESHOLD && (mask < MAX_MASK)) {
            mask = (mask << 1) | 1;
        } else if (timeElapsedSinceLastMaskUpdateCheck > MASK_DECREASE_THRESHOLD) {
            mask = mask >>> 2;
        }
    }

    // by detaching reconfiguration to a new thread, we release the various
    // locks held by the current thread, in particular, the AppenderAttachable
    // reader lock.
    void detachReconfigurationToNewThread() {
        addInfo("Detected change in [" + configurationWatchList.getCopyOfFileWatchList() + "]");
        context.getExecutorService().submit(new ReconfiguringThread());
    }

    void updateNextCheck(long now) {
        nextCheck = now + refreshPeriod;
    }

    protected boolean changeDetected(long now) {
        if (now >= nextCheck) {
            updateNextCheck(now);
            return configurationWatchList.changeDetected();
        }
        return false;
    }

    void disableSubsequentReconfiguration() {
        nextCheck = Long.MAX_VALUE;
    }

    public long getRefreshPeriod() {
        return refreshPeriod;
    }

    public void setRefreshPeriod(long refreshPeriod) {
        this.refreshPeriod = refreshPeriod;
    }

    class ReconfiguringThread implements Runnable {
        public void run() {
            if (mainConfigurationURL == null) {
                addInfo("Due to missing top level configuration file, skipping reconfiguration");
                return;
            }
            LoggerContext lc = (LoggerContext) context;
            addInfo(CoreConstants.RESET_MSG_PREFIX + "named [" + context.getName() + "]");
            if (mainConfigurationURL.toString().endsWith("xml")) {
                performXMLConfiguration(lc);
            } 
        }

        private void performXMLConfiguration(LoggerContext lc) {
            JoranConfigurator jc = new JoranConfigurator();
            jc.setContext(context);
            StatusUtil statusUtil = new StatusUtil(context);
            List<SaxEvent> eventList = jc.recallSafeConfiguration();
            URL mainURL = ConfigurationWatchListUtil.getMainWatchURL(context);
            lc.reset();
            long threshold = System.currentTimeMillis();
            try {
                jc.doConfigure(mainConfigurationURL);
                if (statusUtil.hasXMLParsingErrors(threshold)) {
                    fallbackConfiguration(lc, eventList, mainURL);
                }
            } catch (JoranException e) {
                fallbackConfiguration(lc, eventList, mainURL);
            }
        }

        private void fallbackConfiguration(LoggerContext lc, List<SaxEvent> eventList, URL mainURL) {
            JoranConfigurator joranConfigurator = new JoranConfigurator();
            joranConfigurator.setContext(context);
            if (eventList != null) {
                addWarn("Falling back to previously registered safe configuration.");
                try {
                    lc.reset();
                    JoranConfigurator.informContextOfURLUsedForConfiguration(context, mainURL);
                    joranConfigurator.doConfigure(eventList);
                    addInfo("Re-registering previous fallback configuration once more as a fallback configuration point");
                    joranConfigurator.registerSafeConfiguration(eventList);
                } catch (JoranException e) {
                    addError("Unexpected exception thrown by a configuration considered safe.", e);
                }
            } else {
                addWarn("No previous configuration to fall back on.");
            }
        }
    }
}
