BloomFilter.java

/**
 * 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 com.facebook.infrastructure.utils;

import com.facebook.infrastructure.io.ICompactSerializer;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;

/**
 * Author : Avinash Lakshman ( alakshman@facebook.com) & Prashant Malik ( pmalik@facebook.com )
 */

public class BloomFilter extends Filter
{
    static ICompactSerializer<BloomFilter> serializer_ = new BloomFilterSerializer();  

    public static ICompactSerializer<BloomFilter> serializer()
    {
        return serializer_;
    }

    private BitSet filter_;
    
    public BloomFilter(int numElements, int bucketsPerElement)
    {
        this(BloomCalculations.computeBestK(bucketsPerElement), new BitSet(numElements * bucketsPerElement + 20));
    }

    public BloomFilter(int numElements, double maxFalsePosProbability)
    {
        BloomCalculations.BloomSpecification spec = BloomCalculations
                .computeBucketsAndK(maxFalsePosProbability);
        filter_ = new BitSet(numElements * spec.bucketsPerElement + 20);
        hashCount = spec.K;
    }

    /*
     * This version is only used by the deserializer. 
     */
    BloomFilter(int hashes, BitSet filter)
    {
        hashCount = hashes;
        filter_ = filter;
    }

    public void clear() {
        filter_.clear();
    }

    int buckets()
    {        
        return filter_.size();
    }

    BitSet filter()
    {
        return filter_;
    }

    public boolean isPresent(String key)
    {
        for (int bucketIndex : getHashBuckets(key)) {
            if (!filter_.get(bucketIndex))
            {
                return false;
            }
        }
        return true;
    }

    /*
     param@ key -- value whose hash is used to fill
     the filter_.
     This is a general purpose API.
     */
    public void add(String key)
    {
        for (int bucketIndex : getHashBuckets(key)) {
            filter_.set(bucketIndex);
        }
    }

    public String toString()
    {
        return filter_.toString();
    }

    ICompactSerializer tserializer() {
        return serializer_;
    }

    int emptyBuckets() {
        int n = 0;
        for (int i = 0; i < buckets(); i++) {
            if (!filter_.get(i)) {
                n++;
            }
        }
        return n;
    }
}

class BloomFilterSerializer implements ICompactSerializer<BloomFilter>
{
    /* 
     * The following methods are used for compact representation
     * of BloomFilter. This is essential, since we want to determine
     * the size of the serialized Bloom Filter blob before it is
     * populated armed with the knowledge of how many elements are
     * going to reside in it.
     */

    public void serialize(BloomFilter bf, DataOutputStream dos)
            throws IOException
    {
        /* write the number of hash functions used */
        dos.writeInt(bf.getHashCount());
        /* write the size of the BloomFilter, for backwards compatibility. */
        dos.writeInt(bf.buckets());
        BitSet.serializer().serialize(bf.filter(), dos);
    }

    public BloomFilter deserialize(DataInputStream dis) throws IOException
    {
        /* read the number of hash functions */
        int hashes = dis.readInt();
        /* read the size of the bloom filter.  only for backwards compatibility, since we don't actually need it. */
        dis.readInt();
        BitSet bs = BitSet.serializer().deserialize(dis);
        return new BloomFilter(hashes, bs);
    }
}