ArrayBasedIntegerTopK.java

/*
 * 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;

  }
}