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With inspiration from other libraries

There is a newer version: 2.12.15

/**
 * 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.util.LinkedHashMap;
import java.util.Map;

/**
 * Clients of this class should only use the  {@link #getMessageCountAndThenIncrement} method. Other methods inherited
 * via LinkedHashMap are not thread safe.
 */
class LRUMessageCache extends LinkedHashMap {

    private static final long serialVersionUID = 1L;
    final int cacheSize;

    LRUMessageCache(int cacheSize) {
        super((int) (cacheSize * (4.0f / 3)), 0.75f, true);
        if (cacheSize < 1) {
            throw new IllegalArgumentException("Cache size cannot be smaller than 1");
        }
        this.cacheSize = cacheSize;
    }

    int getMessageCountAndThenIncrement(String msg) {
        // don't insert null elements
        if (msg == null) {
            return 0;
        }

        Integer i;
        // LinkedHashMap is not LinkedHashMap. See also LBCLASSIC-255
        synchronized (this) {
            i = super.get(msg);
            if (i == null) {
                i = 0;
            } else {
                i = i + 1;
            }
            super.put(msg, i);
        }
        return i;
    }

    // called indirectly by get() or put() which are already supposed to be
    // called from within a synchronized block
    protected boolean removeEldestEntry(Map.Entry eldest) {
        return (size() > cacheSize);
    }

    @Override
    synchronized public void clear() {
        super.clear();
    }
}