/*

 * Copyright (c) 2016, 2018, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code 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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 * questions.

 */

package jdk.jfr.internal;

import java.util.concurrent.ConcurrentHashMap;

import java.util.concurrent.atomic.AtomicLong;

import sun.misc.Unsafe;

public final class StringPool {

    private static final Unsafe unsafe = Unsafe.getUnsafe();

    static final int MIN_LIMIT = 16;

    static final int MAX_LIMIT = 128; /* 0 MAX means disabled */

    private static final long epochAddress;

    private static final SimpleStringIdPool sp = new SimpleStringIdPool();

    static {

        epochAddress = JVM.getJVM().getEpochAddress();

        sp.reset();

    }

    public static long addString(String s) {

        return sp.addString(s);

    }

    private static boolean getCurrentEpoch() {

        return unsafe.getByte(epochAddress) == 1;

    }

    private static class SimpleStringIdPool {

        /* string id index */

        private final AtomicLong sidIdx = new AtomicLong();

        /* epoch of cached strings */

        private boolean poolEpoch;

        /* the cache */

        private final ConcurrentHashMap<String, Long> cache;

        /* max size */

        private final int MAX_SIZE = 32*1024;

        /* max size bytes*/

        private final long MAX_SIZE_UTF16 = 16*1024*1024;

        /* max size bytes*/

        private long currentSizeUTF16;

        /* looking at a biased data set 4 is a good value */

        private final String[] preCache = new String[]{"", "" , "" ,""};

        /* index of oldest */

        private int preCacheOld = 0;

        /* loop mask */

        private static final int preCacheMask = 0x03;

        SimpleStringIdPool() {

            cache = new ConcurrentHashMap<>(MAX_SIZE, 0.75f);

        }

        void reset() {

            reset(getCurrentEpoch());

        }

        private void reset(boolean epoch) {

            this.cache.clear();

            this.poolEpoch = epoch;

            this.currentSizeUTF16 = 0;

        }

        private long addString(String s) {

            boolean currentEpoch = getCurrentEpoch();

            if (poolEpoch == currentEpoch) {

                /* pool is for current chunk */

                Long lsid = this.cache.get(s);

                if (lsid != null) {

                    return lsid.longValue();

                }

            } else {

                /* pool is for an old chunk */

                reset(currentEpoch);

            }

            if (!preCache(s)) {

                /* we should not pool this string */

                return -1;

            }

            if (cache.size() > MAX_SIZE || currentSizeUTF16 > MAX_SIZE_UTF16) {

                /* pool was full */

                reset(currentEpoch);

            }

            return storeString(s);

        }

        private long storeString(String s) {

            long sid = this.sidIdx.getAndIncrement();

            /* we can race but it is ok */

            this.cache.put(s, sid);

            boolean currentEpoch;

            synchronized(SimpleStringIdPool.class) {

                currentEpoch = JVM.addStringConstant(poolEpoch, sid, s);

                currentSizeUTF16 += s.length();

            }

            /* did we write in chunk that this pool represent */

            return currentEpoch == poolEpoch ? sid : -1;

        }

        private boolean preCache(String s) {

            if (preCache[0].equals(s)) {

                return true;

            }

            if (preCache[1].equals(s)) {

                return true;

            }

            if (preCache[2].equals(s)) {

                return true;

            }

            if (preCache[3].equals(s)) {

                return true;

            }

            preCacheOld = (preCacheOld - 1) & preCacheMask;

            preCache[preCacheOld] = s;

            return false;

        }

    }

}