com.facebook.stats.topk.TreeBasedIntegerTopK.java Source code

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Here is the source code for com.facebook.stats.topk.TreeBasedIntegerTopK.java

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/*
 * Copyright (C) 2012 Facebook, 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 com.facebook.stats.topk;

import com.facebook.collections.ComparablePair;
import com.google.common.base.Preconditions;

import java.util.LinkedList;
import java.util.List;
import java.util.TreeSet;

/*
 * Tree-based implementation of integer key streaming top-k.
 * This version is optimized for fast retrieval of the top k elements.
 * Time complexity of add() is O(log k) worst case, O(1) best case.
 * Time complexity of getTopK() is O(k).
 * Space usage is O(n + k).
 * (n = keySpaceSize)
 */
public class TreeBasedIntegerTopK implements TopK<Integer> {
    private final int k;
    private final long[] counts;
    private final boolean[] isInTop;
    private final TreeSet<ComparablePair<Long, Integer>> topPairs = new TreeSet<ComparablePair<Long, Integer>>();
    private long smallestTopCount = Long.MAX_VALUE;

    public TreeBasedIntegerTopK(int keySpaceSize, int k) {
        this.k = k;
        counts = new long[keySpaceSize];
        isInTop = new boolean[keySpaceSize];
    }

    /*
     * Cost of an add() operation is O(log k) if the candidate has to be inserted in
     * the current top-k elements, O(1) otherwise.
     */
    @Override
    public synchronized void add(Integer key, long count) {
        Preconditions.checkNotNull(key, "key can't be null");
        Preconditions.checkElementIndex(key, counts.length, "key");
        Preconditions.checkArgument(count >= 0, "count to add must be non-negative, got %s", count);

        if (count == 0) {
            return;
        }

        long currentCount = counts[key];

        counts[key] += count;

        if (isInTop[key]) {
            topPairs.remove(new ComparablePair<Long, Integer>(currentCount, key));
            topPairs.add(new ComparablePair<Long, Integer>(counts[key], key));
        } else if (topPairs.size() < k) {
            topPairs.add(new ComparablePair<Long, Integer>(counts[key], key));
            isInTop[key] = true;
            smallestTopCount = Math.min(smallestTopCount, counts[key]);
        } else if (counts[key] > smallestTopCount) {
            ComparablePair<Long, Integer> smallestTopPair = topPairs.pollFirst();

            isInTop[smallestTopPair.getSecond()] = false;
            topPairs.add(new ComparablePair<Long, Integer>(counts[key], key));
            isInTop[key] = true;
            smallestTopCount = topPairs.first().getFirst();
        }
    }

    @Override
    public synchronized List<Integer> getTopK() {
        LinkedList<Integer> topK = new LinkedList<Integer>();

        for (ComparablePair<Long, Integer> pair : topPairs) {
            topK.addFirst(pair.getSecond());
        }

        return topK;
    }
}