/**
 * ============LICENSE_START====================================================
 * org.onap.aaf
 * ===========================================================================
 * Copyright (c) 2018 AT&T Intellectual Property. All rights reserved.
 * ===========================================================================
 * 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
 * 
 *      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.
 * ============LICENSE_END====================================================
 *
 */

/*
 * Pool
 * 
 * Author: Jonathan
 * 5/27/2011
 */
package org.onap.aaf.cadi.util;

import java.util.Iterator;
import java.util.LinkedList;

import org.onap.aaf.cadi.CadiException;

/**
 * This Class pools on an As-Needed-Basis any particular kind of class, which is
 * quite suitable for expensive operations.
 * 
 * The user calls "get" on a Pool, and if a waiting resource (T) is available,
 * it will be returned. Otherwise, one will be created with the "Creator" class
 * (must be defined for (T)).
 * 
 * You can Prime the instances to avoid huge startup costs
 * 
 * The returned "Pooled" object simply has to call "done()" and the object is
 * returned to the pool. If the developer does not return the object, a memory
 * leak does not occur. There are no references to the object once "get" is
 * called. However, the developer who does not return the object when done
 * obviates the point of the pool, as new Objects are created in place of the
 * Object not returned when another call to "get" is made.
 * 
 * There is a cushion of extra objects, currently defaulted to MAX_RANGE. If the
 * items returned become higher than the MAX_RANGE, the object is allowed to go
 * out of scope, and be cleaned up. the default can be changed on a per-pool
 * basis.
 * 
 * Class revamped for CadiExceptions and Access logging 10/4/2017
 * 
 * @author Jonathan
 * 
 * @param <T>
 */
public class Pool<T> {
    /**
     * This is a constant which specified the default maximum number of unused
     * objects to be held at any given time.
     */
    private static final int MAX_RANGE = 6; // safety

    /**
     * only Simple List needed.
     * 
     * NOTE TO MAINTAINERS: THIS OBJECT DOES IT'S OWN SYNCHRONIZATION. All
     * changes that touch list must account for correctly synchronizing list.
     */
    private LinkedList<Pooled<T>> list;

    /**
     * keep track of how many elements exist, to avoid asking list.
     */
    private int count;

    /**
     * Spares are those Object that are primed and ready to go.
     */
    private int spares;

    /**
     * Actual MAX number of spares allowed to hang around. Can be set to
     * something besides the default MAX_RANGE.
     */
    private int max_range = MAX_RANGE;

    /**
     * The Creator for this particular pool. It must work for type T.
     */
    private Creator<T> creator;

    private Log logger;

    /**
     * Create a new Pool, given the implementation of Creator<T>, which must be
     * able to create/destroy T objects at will.
     * 
     * @param creator
     */
    public Pool(Creator<T> creator) {
        count = spares = 0;
        this.creator = creator;
        list = new LinkedList<>();
        logger = Log.NULL;
    }
    
    /**
     * Attach Pool Logging activities to any other Logging Mechanism.
     * @param logger
     */
    public void setLogger(Log logger) {
        this.logger = logger;
    }
    
    public void log(Object ...objects) {
        logger.log(objects);
    }

    /**
     * Preallocate a certain number of T Objects. Useful for services so that
     * the first transactions don't get hit with all the Object creation costs
     * 
     * @param lt
     * @param prime
     * @throws CadiException 
     */
    public void prime(int prime) throws CadiException  {
        for (int i = 0; i < prime; ++i) {
            Pooled<T> pt = new Pooled<T>(creator.create(), this);
            synchronized (list) {
                list.addFirst(pt);
                ++count;
            }
        }

    }

    /**
     * Destroy and remove all remaining objects. This is valuable for closing
     * down all Allocated objects cleanly for exiting. It is also a good method
     * for removing objects when, for instance, all Objects are invalid because
     * of broken connections, etc.
     */
    public void drain() {
        synchronized (list) {
            for (int i = 0; i < list.size(); ++i) {
                Pooled<T> pt = list.remove();
                creator.destroy(pt.content);
                logger.log("Pool drained ", creator.toString());
            }
            count = spares = 0;
        }

    }

    /**
     * This is the essential function for Pool. Get an Object "T" inside a
     * "Pooled<T>" object. If there is a spare Object, then use it. If not, then
     * create and pass back.
     * 
     * This one uses a Null LogTarget
     * 
     * IMPORTANT: When the use of this object is done (and the object is still
     * in a valid state), then "done()" should be called immediately to allow
     * the object to be reused. That is the point of the Pool...
     * 
     * If the Object is in an invalid state, then "toss()" should be used so the
     * Pool doesn't pass on invalid objects to others.
     * 
     * @param lt
     * @return
     * @throws CadiException
     */
    public Pooled<T> get() throws CadiException {
        Pooled<T> pt;
        synchronized (list) {
            if (list.isEmpty()) {
                pt = null;
            } else {
                pt = list.removeLast();
                --count;
                creator.reuse(pt.content);
            }
        }
        if (pt == null) {
            if (spares < max_range)
                ++spares;
            pt = new Pooled<T>(creator.create(), this);
        } else {
            if (spares > 1)
                --spares;
        }
        return pt;
    }

    /**
     * This function will validate whether the Objects are still in a usable
     * state. If not, they are tossed from the Pool. This is valuable to have
     * when Remote Connections go down, and there is a question on whether the
     * Pooled Objects are still functional.
     * 
     * @return
     */
    public boolean validate() {
        boolean rv = true;
        synchronized (list) {
            for (Iterator<Pooled<T>> iter = list.iterator(); iter.hasNext();) {
                Pooled<T> t = iter.next();
                if (!creator.isValid(t.content)) {
                    rv = false;
                    t.toss();
                    iter.remove();
                }
            }
        }
        return rv;
    }

    /**
     * This is an internal method, used only by the Internal Pooled<T> class.
     * 
     * The Pooled<T> class "offers" it's Object back after use. It is an
     * "offer", because Pool will simply destroy and remove the object if it has
     * more than enough spares.
     * 
     * @param lt
     * @param used
     * @return
     */
    // Used only by Pooled<T>
    private boolean offer(Pooled<T> used) {
        if (count < spares) {
            synchronized (list) {
                list.addFirst(used);
                ++count;
            }
            logger.log("Pool recovered ", creator);
        } else {
            logger.log("Pool destroyed ", creator);
            creator.destroy(used.content);
        }
        return false;
    }

    /**
     * The Creator Interface give the Pool the ability to Create, Destroy and
     * Validate the Objects it is maintaining. Thus, it is a specially written
     * Implementation for each type.
     * 
     * @author Jonathan
     * 
     * @param <T>
     */
    public interface Creator<T> {
        public T create() throws CadiException;

        public void destroy(T t);

        public boolean isValid(T t);

        public void reuse(T t);
    }

    public interface Log {
        public void log(Object ... o);
        
        public final static Log NULL = new Log() {
            @Override
            public void log(Object ... o) {
            }
        };
    }
    /**
     * The "Pooled<T>" class is the transient class that wraps the actual Object
     * T for API use/ It gives the ability to return ("done()", or "toss()") the
     * Object to the Pool when processing is finished.
     * 
     * For Safety, i.e. to avoid memory leaks and invalid Object States, there
     * is a "finalize" method. It is strictly for when coder forgets to return
     * the object, or perhaps hasn't covered the case during Exceptions or
     * Runtime Exceptions with finally (preferred). This should not be
     * considered normal procedure, as finalize() is called at an undetermined
     * time during garbage collection, and is thus rather useless for a Pool.
     * However, we don't want Coding Mistakes to put the whole program in an
     * invalid state, so if something happened such that "done()" or "toss()"
     * were not called, the resource is still cleaned up as well as possible.
     * 
     * @author Jonathan
     * 
     * @param <T>
     */
    public static class Pooled<T> {
        public final T content;
        private Pool<T> pool;

        /**
         * Create the Wrapping Object Pooled<T>.
         * 
         * @param t
         * @param pool
         * @param logTarget
         */
        public Pooled(T t, Pool<T> pool) {
            content = t;
            this.pool = pool;

        }

        /**
         * This is the key API for the Pool, as calling "done()" offers this
         * object back to the Pool for reuse.
         * 
         * Do not use the Pooled<T> object again after calling "done()".
         */
        public void done() {
            if (pool != null) {
                pool.offer(this);
            }
        }

        /**
         * The user of the Object may discover that the Object t is no longer in
         * a valid state. Don't put Garbage back in the Refrigerator... Toss it,
         * if it's no longer valid.
         * 
         * toss() is also used for draining the Pool, etc.
         * 
         * toss() will attempt to destroy the Object by using the Creator
         * Interface.
         * 
         */
        public void toss() {
            if (pool != null) {
                pool.creator.destroy(content);
            }
            // Don't allow finalize to put it back in.
            pool = null;
        }

        /**
         * Just in case someone neglected to offer back object... Do not rely on
         * this, as there is no specific time when finalize is called, which
         * rather defeats the purpose of a Pool.
         */
        @Override
        protected void finalize() throws Throwable {
            if (pool != null) {
                done();
                pool = null;
            }
        }
    }

    /**
     * Get the maximum number of spare objects allowed at any moment
     * 
     * @return
     */
    public int getMaxRange() {
        return max_range;
    }

    /**
     * Set a Max Range for numbers of spare objects waiting to be used.
     * 
     * No negative numbers are allowed
     * 
     * @return
     */
    public void setMaxRange(int max_range) {
        // Do not allow negative numbers
        this.max_range = Math.max(0, max_range);
    }

}