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RocksDB fat jar that contains .so files for linux32 and linux64 (glibc and musl-libc), jnilib files for Mac OSX, and a .dll for Windows x64.
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// Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).

package org.rocksdb;

import java.nio.ByteBuffer;

/**
 * This class is intentionally private,
 * it holds methods which are called
 * from C++ to interact with a Comparator
 * written in Java.
 *
 * Placing these bridge methods in this
 * class keeps the API of the
 * {@link org.rocksdb.AbstractComparator} clean.
 */
class AbstractComparatorJniBridge {
  /**
   * Only called from JNI.
   * 
   * Simply a bridge to calling
   * {@link AbstractComparator#compare(ByteBuffer, ByteBuffer)},
   * which ensures that the byte buffer lengths are correct
   * before and after the call.
   *
   * @param comparator the comparator object on which to
   *     call {@link AbstractComparator#compare(ByteBuffer, ByteBuffer)}
   * @param a buffer access to first key
   * @param aLen the length of the a key,
   *     may be smaller than the buffer {@code a}
   * @param b buffer access to second key
   * @param bLen the length of the b key,
   *     may be smaller than the buffer {@code b}
   *
   * @return the result of the comparison
   */
  @SuppressWarnings("PMD.UnusedPrivateMethod")
  private static int compareInternal(final AbstractComparator comparator, final ByteBuffer a,
      final int aLen, final ByteBuffer b, final int bLen) {
    if (aLen != -1) {
      a.mark();
      a.limit(aLen);
    }
    if (bLen != -1) {
      b.mark();
      b.limit(bLen);
    }

    final int c = comparator.compare(a, b);

    if (aLen != -1) {
      a.reset();
    }
    if (bLen != -1) {
      b.reset();
    }

    return c;
  }

  /**
   * Only called from JNI.
   * 

   * Simply a bridge to calling
   * {@link AbstractComparator#findShortestSeparator(ByteBuffer, ByteBuffer)},
   * which ensures that the byte buffer lengths are correct
   * before the call.
   *
   * @param comparator the comparator object on which to
   *     call {@link AbstractComparator#findShortestSeparator(ByteBuffer, ByteBuffer)}
   * @param start buffer access to the start key
   * @param startLen the length of the start key,
   *     may be smaller than the buffer {@code start}
   * @param limit buffer access to the limit key
   * @param limitLen the length of the limit key,
   *     may be smaller than the buffer {@code limit}
   *
   * @return either {@code startLen} if the start key is unchanged, otherwise
   *     the new length of the start key
   */
  @SuppressWarnings("PMD.UnusedPrivateMethod")
  private static int findShortestSeparatorInternal(final AbstractComparator comparator,
      final ByteBuffer start, final int startLen, final ByteBuffer limit, final int limitLen) {
    if (startLen != -1) {
      start.limit(startLen);
    }
    if (limitLen != -1) {
      limit.limit(limitLen);
    }
    comparator.findShortestSeparator(start, limit);
    return start.remaining();
  }

  /**
   * Only called from JNI.
   * 
   * Simply a bridge to calling
   * {@link AbstractComparator#findShortestSeparator(ByteBuffer, ByteBuffer)},
   * which ensures that the byte buffer length is correct
   * before the call.
   *
   * @param comparator the comparator object on which to
   *     call {@link AbstractComparator#findShortSuccessor(ByteBuffer)}
   * @param key buffer access to the key
   * @param keyLen the length of the key,
   *     may be smaller than the buffer {@code key}
   *
   * @return either keyLen if the key is unchanged, otherwise the new length of the key
   */
  @SuppressWarnings("PMD.UnusedPrivateMethod")
  private static int findShortSuccessorInternal(
      final AbstractComparator comparator, final ByteBuffer key, final int keyLen) {
    if (keyLen != -1) {
      key.limit(keyLen);
    }
    comparator.findShortSuccessor(key);
    return key.remaining();
  }
}