org.apache.druid.query.groupby.orderby.TopNSequence.java Source code

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Here is the source code for org.apache.druid.query.groupby.orderby.TopNSequence.java

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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.druid.query.groupby.orderby;

import com.google.common.collect.MinMaxPriorityQueue;
import com.google.common.collect.Ordering;
import org.apache.druid.java.util.common.guava.Accumulator;
import org.apache.druid.java.util.common.guava.BaseSequence;
import org.apache.druid.java.util.common.guava.Sequence;

import java.util.Collections;
import java.util.Iterator;

public class TopNSequence<T> extends BaseSequence<T, Iterator<T>> {
    public TopNSequence(final Sequence<T> input, final Ordering<T> ordering, final int limit) {
        super(new IteratorMaker<T, Iterator<T>>() {
            @Override
            public Iterator<T> make() {
                if (limit <= 0) {
                    return Collections.emptyIterator();
                }

                // Materialize the topN values
                final MinMaxPriorityQueue<T> queue = MinMaxPriorityQueue.orderedBy(ordering).maximumSize(limit)
                        .create();

                input.accumulate(queue, new Accumulator<MinMaxPriorityQueue<T>, T>() {
                    @Override
                    public MinMaxPriorityQueue<T> accumulate(MinMaxPriorityQueue<T> theQueue, T row) {
                        theQueue.offer(row);
                        return theQueue;
                    }
                });

                // Now return them when asked
                return new Iterator<T>() {
                    @Override
                    public boolean hasNext() {
                        return !queue.isEmpty();
                    }

                    @Override
                    public T next() {
                        return queue.poll();
                    }

                    @Override
                    public void remove() {
                        throw new UnsupportedOperationException();
                    }
                };
            }

            @Override
            public void cleanup(Iterator<T> rowIterator) {
                // Nothing to do
            }
        });
    }
}