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Collections and collection utilities for Neo4j.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.collection.pool;

import static java.time.Duration.ofMinutes;

import java.time.Duration;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.LongSupplier;
import org.neo4j.function.Factory;

public class LinkedQueuePool implements Pool {
    public interface Monitor {
        void updatedCurrentPeakSize(int currentPeakSize);

        void updatedTargetSize(int targetSize);

        void created(R resource);

        void acquired(R resource);

        void disposed(R resource);

        class Adapter implements Monitor {
            @Override
            public void updatedCurrentPeakSize(int currentPeakSize) {}

            @Override
            public void updatedTargetSize(int targetSize) {}

            @Override
            public void created(R resource) {}

            @Override
            public void acquired(R resource) {}

            @Override
            public void disposed(R resource) {}
        }
    }

    public interface CheckStrategy {
        boolean shouldCheck();

        class TimeoutCheckStrategy implements CheckStrategy {
            private final long intervalNanos;
            private long lastCheckTimeNanos;
            private final LongSupplier clock;

            TimeoutCheckStrategy(Duration duration) {
                this(duration, System::nanoTime);
            }

            TimeoutCheckStrategy(Duration duration, LongSupplier nanoClock) {
                this.intervalNanos = duration.toNanos();
                this.lastCheckTimeNanos = nanoClock.getAsLong();
                this.clock = nanoClock;
            }

            @Override
            public boolean shouldCheck() {
                long currentTimeNanos = clock.getAsLong();
                if (currentTimeNanos > lastCheckTimeNanos + intervalNanos) {
                    lastCheckTimeNanos = currentTimeNanos;
                    return true;
                }
                return false;
            }
        }
    }

    private final Queue unused = new ConcurrentLinkedQueue<>();
    private final Monitor monitor;
    private final int minSize;
    private final Factory factory;
    private final CheckStrategy checkStrategy;
    // Guarded by nothing. Those are estimates, losing some values doesn't matter much
    private final AtomicInteger allocated = new AtomicInteger(0);
    private final AtomicInteger queueSize = new AtomicInteger(0);
    private int currentPeakSize;
    private int targetSize;

    public LinkedQueuePool(int minSize, Factory factory) {
        this(minSize, factory, new CheckStrategy.TimeoutCheckStrategy(ofMinutes(1)), new Monitor.Adapter<>());
    }

    public LinkedQueuePool(int minSize, Factory factory, CheckStrategy strategy, Monitor monitor) {
        this.minSize = minSize;
        this.factory = factory;
        this.currentPeakSize = 0;
        this.targetSize = minSize;
        this.checkStrategy = strategy;
        this.monitor = monitor;
    }

    protected R create() {
        return factory.newInstance();
    }

    @Override
    public void dispose(R resource) {
        monitor.disposed(resource);
        allocated.decrementAndGet();
    }

    @Override
    public final R acquire() {
        R resource = unused.poll();
        if (resource == null) {
            resource = create();
            allocated.incrementAndGet();
            monitor.created(resource);
        } else {
            queueSize.decrementAndGet();
        }
        currentPeakSize = Math.max(currentPeakSize, allocated.get() - queueSize.get());
        if (checkStrategy.shouldCheck()) {
            targetSize = Math.max(minSize, currentPeakSize);
            monitor.updatedCurrentPeakSize(currentPeakSize);
            currentPeakSize = 0;
            monitor.updatedTargetSize(targetSize);
        }

        monitor.acquired(resource);
        return resource;
    }

    @Override
    public void release(R toRelease) {
        if (queueSize.get() < targetSize) {
            unused.offer(toRelease);
            queueSize.incrementAndGet();
        } else {
            dispose(toRelease);
        }
    }

    /**
     * Dispose of all pooled objects.
     */
    @Override
    public void close() {
        for (R resource = unused.poll(); resource != null; resource = unused.poll()) {
            dispose(resource);
        }
    }
}