/*
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* The contents of this file are subject to the terms of the Common Development and
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* Distribution License (the License). You may not use this file except in compliance with the
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* License.
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*
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* You can obtain a copy of the License at legal/CDDLv1.0.txt. See the License for the
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* specific language governing permission and limitations under the License.
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*
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* When distributing Covered Software, include this CDDL Header Notice in each file and include
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* the License file at legal/CDDLv1.0.txt. If applicable, add the following below the CDDL
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* Header, with the fields enclosed by brackets [] replaced by your own identifying
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* information: "Portions Copyright [year] [name of copyright owner]".
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*
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* Copyright 2015 ForgeRock AS.
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*/
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package org.opends.server.backends.pluggable;
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import static org.opends.server.backends.pluggable.CursorTransformer.*;
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import java.util.NoSuchElementException;
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import org.forgerock.opendj.ldap.ByteSequence;
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import org.forgerock.opendj.ldap.ByteString;
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import org.forgerock.opendj.ldap.ByteStringBuilder;
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import org.forgerock.util.Function;
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import org.forgerock.util.promise.NeverThrowsException;
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import org.opends.server.backends.pluggable.CursorTransformer.ValueTransformer;
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import org.opends.server.backends.pluggable.OnDiskMergeImporter.SequentialCursorDecorator;
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import org.opends.server.backends.pluggable.spi.Cursor;
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import org.opends.server.backends.pluggable.spi.Importer;
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import org.opends.server.backends.pluggable.spi.ReadableTransaction;
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import org.opends.server.backends.pluggable.spi.SequentialCursor;
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import org.opends.server.backends.pluggable.spi.TreeName;
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import org.opends.server.backends.pluggable.spi.UpdateFunction;
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import org.opends.server.backends.pluggable.spi.WriteableTransaction;
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/**
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* Store counters associated to a key. Counter value is sharded amongst multiple keys to allow concurrent update without
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* contention (at the price of a slower read).
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*/
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final class ShardedCounter extends AbstractTree
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{
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/**
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* Must be a power of 2
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* @see <a href="http://en.wikipedia.org/wiki/Modulo_operation#Performance_issues">Performance issues</a>
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*/
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private static final long SHARD_COUNT = 256;
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private static final ValueTransformer<ByteString, ByteString, Long, NeverThrowsException> TO_LONG =
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new ValueTransformer<ByteString, ByteString, Long, NeverThrowsException>()
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{
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@Override
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public Long transform(ByteString key, ByteString value)
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{
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return decodeValue(value);
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}
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};
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private static final Function<ByteString, ByteString, NeverThrowsException> TO_KEY =
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new Function<ByteString, ByteString, NeverThrowsException>()
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{
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@Override
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public ByteString apply(ByteString shardedKey)
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{
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// -1 to remove the shard id.
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return shardedKey.subSequence(0, shardedKey.length() - 1);
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}
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};
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ShardedCounter(TreeName name)
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{
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super(name);
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}
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SequentialCursor<ByteString, Void> openCursor(ReadableTransaction txn)
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{
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return new UniqueKeysCursor<>(transformKeysAndValues(
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txn.openCursor(getName()), TO_KEY,
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CursorTransformer.<ByteString, ByteString, Void> constant(null)));
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}
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private Cursor<ByteString, Long> openCursor0(ReadableTransaction txn)
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{
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return transformKeysAndValues(txn.openCursor(getName()), TO_KEY, TO_LONG);
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}
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void addCount(final WriteableTransaction txn, ByteSequence key, final long delta)
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{
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txn.update(getName(), getShardedKey(key), new UpdateFunction()
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{
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@Override
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public ByteSequence computeNewValue(ByteSequence oldValue)
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{
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final long currentValue = oldValue == null ? 0 : decodeValue(oldValue.toByteString());
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return encodeValue(currentValue + delta);
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}
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});
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}
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void importPut(Importer importer, ByteSequence key, long delta)
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{
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if (delta != 0)
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{
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importer.put(getName(), getShardedKey(key), encodeValue(delta));
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}
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}
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long getCount(final ReadableTransaction txn, ByteSequence key)
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{
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long counterValue = 0;
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try (final SequentialCursor<ByteString, Long> cursor = new ShardCursor(openCursor0(txn), key))
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{
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while (cursor.next())
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{
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counterValue += cursor.getValue();
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}
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}
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return counterValue;
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}
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long removeCount(final WriteableTransaction txn, ByteSequence key)
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{
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long counterValue = 0;
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try (final SequentialCursor<ByteString, Long> cursor = new ShardCursor(openCursor0(txn), key))
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{
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// Iterate over and remove all the thread local shards
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while (cursor.next())
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{
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counterValue += cursor.getValue();
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cursor.delete();
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}
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}
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return counterValue;
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}
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static long decodeValue(ByteString value)
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{
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switch (value.length())
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{
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case 1:
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return value.byteAt(0);
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case (Integer.SIZE / Byte.SIZE):
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return value.toInt();
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case (Long.SIZE / Byte.SIZE):
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return value.toLong();
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default:
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throw new IllegalArgumentException("Unsupported sharded-counter value format.");
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}
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}
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static ByteString encodeValue(long value)
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{
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final byte valueAsByte = (byte) value;
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if (valueAsByte == value)
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{
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return ByteString.wrap(new byte[] { valueAsByte });
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}
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final int valueAsInt = (int) value;
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if (valueAsInt == value)
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{
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return ByteString.valueOfInt(valueAsInt);
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}
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return ByteString.valueOfLong(value);
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}
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@Override
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public String valueToString(ByteString value)
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{
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return String.valueOf(decodeValue(value));
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}
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private static ByteSequence getShardedKey(ByteSequence key)
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{
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final byte bucket = (byte) (Thread.currentThread().getId() & (SHARD_COUNT - 1));
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return new ByteStringBuilder(key.length() + ByteStringBuilder.MAX_COMPACT_SIZE).appendBytes(key).appendByte(bucket);
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}
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/** Restricts a cursor to the shards of a specific key. */
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private final class ShardCursor extends SequentialCursorDecorator<Cursor<ByteString, Long>, ByteString, Long>
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{
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private final ByteSequence targetKey;
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private boolean initialized;
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ShardCursor(Cursor<ByteString, Long> delegate, ByteSequence targetKey)
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{
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super(delegate);
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this.targetKey = targetKey;
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}
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@Override
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public boolean next()
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{
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if (!initialized)
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{
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initialized = true;
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return delegate.positionToKeyOrNext(targetKey) && isOnTargetKey();
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}
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return delegate.next() && isOnTargetKey();
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}
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private boolean isOnTargetKey()
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{
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return targetKey.equals(delegate.getKey());
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}
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}
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/**
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* Cursor that returns unique keys and null values. Ensure that {@link #getKey()} will return a different key after
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* each {@link #next()}.
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*/
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private static final class UniqueKeysCursor<K> implements SequentialCursor<K, Void>
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{
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private final Cursor<K, ?> delegate;
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private boolean isDefined;
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private K key;
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private UniqueKeysCursor(Cursor<K, ?> cursor)
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{
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this.delegate = cursor;
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if (!delegate.isDefined())
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{
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delegate.next();
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}
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}
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@Override
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public boolean next()
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{
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isDefined = delegate.isDefined();
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if (isDefined)
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{
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key = delegate.getKey();
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skipEntriesWithSameKey();
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}
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return isDefined;
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}
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private void skipEntriesWithSameKey()
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{
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throwIfUndefined(this);
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while (delegate.next() && key.equals(delegate.getKey()))
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{
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// Skip all entries having the same key.
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}
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// Delegate is one step beyond. When delegate.isDefined() return false, we have to return true once more.
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isDefined = true;
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}
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@Override
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public boolean isDefined()
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{
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return isDefined;
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}
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@Override
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public K getKey() throws NoSuchElementException
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{
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throwIfUndefined(this);
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return key;
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}
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@Override
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public Void getValue() throws NoSuchElementException
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{
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throwIfUndefined(this);
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return null;
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}
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@Override
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public void delete() throws NoSuchElementException, UnsupportedOperationException
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{
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throw new UnsupportedOperationException();
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}
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@Override
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public void close()
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{
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key = null;
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delegate.close();
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}
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private static void throwIfUndefined(SequentialCursor<?, ?> cursor)
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{
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if (!cursor.isDefined())
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{
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throw new NoSuchElementException();
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}
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}
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}
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}
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