io.druid.query.groupby.orderby.TopNSorter.java Source code

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Here is the source code for io.druid.query.groupby.orderby.TopNSorter.java

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/*
 * Druid - a distributed column store.
 * Copyright 2012 - 2015 Metamarkets Group Inc.
 *
 * Licensed 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 io.druid.query.groupby.orderby;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.MinMaxPriorityQueue;
import com.google.common.collect.Ordering;

/**
 * A utility class that sorts a list of comparable items in the given order, and keeps only the
 * top N sorted items.
 */
public class TopNSorter<T> {
    private Ordering<T> ordering;

    /**
     * Constructs a sorter that will sort items with given ordering.
     * @param ordering the order that this sorter instance will use for sorting
     */
    public TopNSorter(Ordering<T> ordering) {
        this.ordering = ordering;
    }

    /**
     * Sorts a list of rows and retain the top n items
     * @param items the collections of items to be sorted
     * @param n the number of items to be retained
     * @return Top n items that are sorted in the order specified when this instance is constructed.
     */
    public Iterable<T> toTopN(Iterable<T> items, int n) {
        if (n <= 0) {
            return ImmutableList.of();
        }

        MinMaxPriorityQueue<T> queue = MinMaxPriorityQueue.orderedBy(ordering).maximumSize(n).create(items);

        return new OrderedPriorityQueueItems<T>(queue);
    }
}