idbloomfilter.java

pastry的java实现的2.0b版

/*************************************************************************

"FreePastry" Peer-to-Peer Application Development Substrate 

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package rice.p2p.util;

import java.io.*;
import java.math.*;
import java.util.*;

import rice.p2p.commonapi.*;
import rice.p2p.commonapi.rawserialization.*;

/**
 * @(#) IdBloomFilter.java Class which is an implementation of a bloom filter
 * which takes Ids as elements. This class simply wraps a normal BloomFilter,
 * but provides convienent methods for constructing and checking for the
 * existence of Ids.
 *
 * @version $Id: IdBloomFilter.java 3274 2006-05-15 16:17:47Z jeffh $
 * @author Alan Mislove
 */
public class IdBloomFilter implements Serializable {

  /**
   * An internal byte[] for managing ids in a memory-efficent manner
   */
  protected transient byte[] array;

  /**
   * The parameters to the hash functions for this bloom filter
   */
  protected BloomFilter filter;

  // serialver for backwards compatibility
  private final static long serialVersionUID = -9122948172786936161L;

  // note cannot configure these because Serializable
  /**
   * The number of bits per key in bloom filters
   */
  public static int NUM_BITS_PER_KEY = 4;

  /**
   * The number of different hash functions to use in bloom filters
   */
  public static int NUM_HASH_FUNCTIONS = 2;

  /**
   * Constructor which takes the number of hash functions to use and the length
   * of the set to use.
   *
   * @param set DESCRIBE THE PARAMETER
   */
  public IdBloomFilter(IdSet set) {
    this.filter = new BloomFilter(NUM_HASH_FUNCTIONS, NUM_BITS_PER_KEY * set.numElements());
    Iterator i = set.getIterator();

    while (i.hasNext()) {
      addId((Id) i.next());
    }
  }

  /**
   * Constructor for IdBloomFilter.
   *
   * @param buf DESCRIBE THE PARAMETER
   * @exception IOException DESCRIBE THE EXCEPTION
   */
  public IdBloomFilter(InputBuffer buf) throws IOException {
//    array = new byte[buf.readInt()];
//    buf.read(array);

    filter = new BloomFilter(buf);
  }

  /**
   * Internal method for checking to see if the array exists, and if not,
   * instanciating it. It also places the given Id into the array.
   *
   * @param id An id to build the array from
   */
  protected void checkArray(Id id) {
    if (array == null) {
      array = id.toByteArray();
    } else {
      id.toByteArray(array, 0);
    }
  }

  /**
   * Method which adds an Id to the underlying bloom filter
   *
   * @param id The id to add
   */
  protected void addId(Id id) {
    checkArray(id);
    filter.add(array);
  }

  /**
   * Method which returns whether or not an Id *may* be in the set.
   * Specifically, if this method returns false, the element is definately not
   * in the set. Otherwise, if true is returned, the element may be in the set,
   * but it is not guaranteed.
   *
   * @param id The id to check for
   * @return DESCRIBE THE RETURN VALUE
   */
  public boolean check(Id id) {
    checkArray(id);
    return filter.check(array);
  }

  /**
   * Method which checks an entire IdSet to see if they exist in this bloom
   * filter, and returns the response by adding elements to the other provided
   * id set.
   *
   * @param set THe set to check for
   * @param result The set to put the non-existing objects into
   * @param max The maximum number of keys to return
   */
  public void check(IdSet set, IdSet result, int max) {
    Iterator it = set.getIterator();
    int count = 0;

    while (it.hasNext() && (count < max)) {
      Id next = (Id) it.next();

      if (!check(next)) {
        result.addId(next);
        count++;
      }
    }
  }

  /**
   * DESCRIBE THE METHOD
   *
   * @param buf DESCRIBE THE PARAMETER
   * @exception IOException DESCRIBE THE EXCEPTION
   */
  public void serialize(OutputBuffer buf) throws IOException {
//    buf.writeInt(array.length);
//    buf.write(array, 0, array.length);

    filter.serialize(buf);
  }
}