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/*
 Licensed to Diennea S.r.l. under one
 or more contributor license agreements. See the NOTICE file
 distributed with this work for additional information
 regarding copyright ownership. Diennea S.r.l. 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 herddb.core;

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.Random;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Simple randomic implementation of {@link PageReplacementPolicy}.
 * 
 * Discarding pegas will been choosen in a randomic way.
 * 
 *
 * @author diego.salvi
 */
public class RandomPageReplacementPolicy implements PageReplacementPolicy {

    private final PlainMetadata[] pages;
    private final Map positions;

    private final Random random;

    /**
     * Modification lock
     */
    private final Lock lock = new ReentrantLock();

    public RandomPageReplacementPolicy(int size) {
        this(size, new Random());
    }

    public RandomPageReplacementPolicy(int size, Random random) {
        this.random = random;
        pages = new PlainMetadata[size];

        positions = new HashMap<>(size);
    }

    @Override
    public PlainMetadata add(Page page) {
        final PlainMetadata metadata = new PlainMetadata(page);
        page.metadata = metadata;
        lock.lock();
        try {
            int count = positions.size();
            if (count < pages.length) {
                pages[count] = metadata;
                positions.put(metadata, count);
                return null;
            } else {
                int position = random.nextInt(count);

                PlainMetadata old = pages[position];
                positions.remove(old);

                pages[position] = metadata;
                positions.put(metadata, position);
                return old;
            }
        } finally {
            lock.unlock();
        }
    }

    public PlainMetadata pop() {
        lock.lock();
        try {
            int count = positions.size();
            int position = random.nextInt(count);

            PlainMetadata old = pages[position];
            positions.remove(old);

            if (count > 0) {
                PlainMetadata moving = pages[count - 1];
                pages[position] = moving;
                positions.put(moving, position);
            }

            return old;
        } finally {
            lock.unlock();
        }
    }

    @Override
    public int size() {
        return positions.size();
    }

    @Override
    public int capacity() {
        return pages.length;
    }

    @Override
    public > void remove(Collection pages) {
        for (Page page : pages) {
            remove(page);
        }
    }

    @Override
    public boolean remove(Page page) {
        final PlainMetadata metadata = (PlainMetadata) page.metadata;
        lock.lock();
        try {
            Integer position = positions.remove(metadata);

            if (position == null) {
                return false;
            }

            int count = positions.size();

            if (position == count) {
                pages[count] = null;
            } else {
                PlainMetadata moving = pages[count];
                pages[position] = moving;
                pages[count] = null;
                positions.put(moving, position);
            }

            return true;
        } finally {
            lock.unlock();
        }
    }

    @Override
    public void clear() {
        lock.lock();
        try {
            positions.clear();

            for (int i = 0; i < pages.length; ++i) {
                pages[i] = null;
            }
        } finally {
            lock.unlock();
        }
    }

    @Override
    public void pageHit(Page page) {
        /* No observation needed in a random strategy */
    }

    /**
     * Implementation of {@link Page.Metadata} with all data needed for {@link RandomPageReplacement}.
     *
     * @author diego.salvi
     */
    private static class PlainMetadata extends Page.Metadata {

        private final int hashcode;

        public PlainMetadata(Page datapage) {
            this(datapage.owner, datapage.pageId);
        }

        public PlainMetadata(Page.Owner owner, long pageId) {
            super(owner, pageId);

            hashcode = Objects.hash(owner, pageId);
        }

        @Override
        public int hashCode() {
            return hashcode;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) {
                return true;
            }
            if (obj == null) {
                return false;
            }
            if (!(obj instanceof PlainMetadata)) {
                return false;
            }
            PlainMetadata other = (PlainMetadata) obj;
            if (owner == null) {
                if (other.owner != null) {
                    return false;
                }
            } else if (!owner.equals(other.owner)) {
                return false;
            }
            if (pageId != other.pageId) {
                return false;
            }
            return true;
        }

        @Override
        public String toString() {
            return "PlainMetadata {pageId=" + pageId + ", owner=" + owner + '}';
        }
    }
}