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

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Here is the source code for com.facebook.stats.topk.ArrayBasedIntegerTopK.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.google.common.base.Preconditions;
import com.google.common.primitives.Longs;

import java.util.Comparator;
import java.util.LinkedList;
import java.util.List;
import java.util.PriorityQueue;

/*
 * Array-based implementation of integer key streaming top-k.
 * This version is optimized for fast update of the counts.
 * Time complexity of add() is O(1).
 * Time complexity of getTopK() is O(n log k).
 * Space usage is O(n).
 * (n = keySpaceSize)
 */
public class ArrayBasedIntegerTopK implements TopK<Integer> {
    private final int k;
    // Java Language SE 7 spec, section 4.12.5 guarantees initial values of 0 for each entry
    private final long[] counts;

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

    @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);

        counts[key] += count;
    }

    @Override
    public List<Integer> getTopK() {
        Comparator<Integer> comparator = new Comparator<Integer>() {
            public int compare(Integer i, Integer j) {
                return Longs.compare(counts[i], counts[j]);
            }
        };
        PriorityQueue<Integer> topK = new PriorityQueue<Integer>(k, comparator);

        for (int key = 0; key < counts.length; ++key) {
            if (topK.size() < k) {
                if (counts[key] > 0) {
                    topK.offer(key);
                }
            } else if (counts[key] > counts[topK.peek()]) {
                topK.poll();
                topK.offer(key);
            }
        }

        LinkedList<Integer> sortedTopK = new LinkedList<Integer>();

        while (!topK.isEmpty()) {
            sortedTopK.addFirst(topK.poll());
        }
        return sortedTopK;

    }
}