channel.java

pastry的java实现的2.0b版

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

"FreePastry" Peer-to-Peer Application Development Substrate 

Copyright 2002, Rice University. All rights reserved.

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

package rice.p2p.splitstream;

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

import rice.p2p.commonapi.*;
import rice.p2p.scribe.*;

/**
 * The channel controls all the meta data associated with a group of stripes. It
 * contains the stripes themselves, and one of the stripe is the primary stripe
 * for this channel. A stripe whose topicId matches some prefix with local node
 * id, then it becomes its primary stripe. A channelId uniquely identifies a
 * channel. Number of stripes is obtained by pow(2, STRIPE_BASE), so for
 * STRIPE_BASE = 4, it is 16. Stripe identifiers are obtained by replacing first
 * digit by every possible value of a digit and second digit by 8. So, if
 * channelId is <0x1AB88..>, then stripe id's generated are <0x08B88..>,
 * <0x18B88..>, <0x28B88..> etc.
 *
 * @version $Id: Channel.java 2604 2005-06-24 21:51:46Z jeffh $
 * @author Ansley Post
 * @author Alan Mislove
 * @author Atul Singh
 */
public class Channel {

  /**
   * The default number of stripes to create
   */
  private int stripeBase;
  // = 4;

  /**
   * ChannelId for this channel
   */
  protected ChannelId channelId;

  /**
   * The list of stripeIds for this channel
   */
  protected Stripe[] stripes;

  /**
   * The Id of the local node
   */
  protected Id localId;

  /**
   * Constructor to create a new channel from scratch
   *
   * @param channelId The Id of the channel
   * @param scribe The underlying stripe object
   * @param factory The Id factory
   * @param localId The local Id
   * @param instance DESCRIBE THE PARAMETER
   * @param stripeBase DESCRIBE THE PARAMETER
   * @param maxFailedSubscriptions DESCRIBE THE PARAMETER
   */
  public Channel(ChannelId channelId, Scribe scribe, String instance, IdFactory factory, Id localId, int stripeBase, int maxFailedSubscriptions) {

    /*
     *  Initialize Member variables
     */
    this.stripeBase = stripeBase;
    this.channelId = channelId;
    this.localId = localId;

    /*
     *  Create the stripe id and stripe arrays
     */
    StripeId[] stripeIds = generateStripeIds(channelId, factory);
    stripes = new Stripe[stripeIds.length];

    /*
     *  Create the stripes
     */
    for (int i = 0; i < stripeIds.length; i++) {
      stripes[i] = new Stripe(stripeIds[i], scribe, instance, this, maxFailedSubscriptions);
    }
  }

  /**
   * Gets the local node id.
   *
   * @return The local node id
   */
  public Id getLocalId() {
    return localId;
  }

  /**
   * Gets the channelId for this channel
   *
   * @return ChannelId the channelId for this channel
   */
  public ChannelId getChannelId() {
    return channelId;
  }

  /**
   * At any moment a node is subscribed to at least 1 but possibly more stripes.
   * They will always be subscribed to their primary Stripe.
   *
   * @return Vector the Stripes this node is subscribed to.
   */
  public Stripe[] getStripes() {
    return stripes;
  }

  /**
   * The primary stripe is the stripe that the user must have.
   *
   * @return Stripe The Stripe object that is the primary stripe.
   */
  protected Stripe getPrimaryStripe() {
    for (int i = 0; i < stripes.length; i++) {
      if (SplitStreamScribePolicy.getPrefixMatch(this.localId, stripes[i].getStripeId().getId(), stripeBase) > 0) {
        return stripes[i];
      }
    }

    return null;
  }

  /**
   * Gets the StripeBase attribute of the Channel object
   *
   * @return The StripeBase value
   */
  public int getStripeBase() {
    return stripeBase;
  }

  /**
   * Creates and returns the Ids associated with the provided channelId
   *
   * @param id DESCRIBE THE PARAMETER
   * @param factory DESCRIBE THE PARAMETER
   * @return The array of stripeIds based on this channelId
   */
  protected StripeId[] generateStripeIds(ChannelId id, IdFactory factory) {
    int num = (int) Math.pow(2, stripeBase);
    StripeId[] stripeIds = new StripeId[num];

    for (int i = 0; i < num; i++) {
      byte[] array = id.getId().toByteArray();
      stripeIds[i] = new StripeId(factory.buildId(process(array, stripeBase, i)));
    }

    return stripeIds;
  }

  /**
   * Returns a new array, of the same length as array, with the first base bits
   * set to represent num.
   *
   * @param array The input array
   * @param base The number of bits to replace
   * @param num The number to replace the first base bits with
   * @return A new array
   */
  private static byte[] process(byte[] array, int base, int num) {
    int length = array.length * 8;
    BigInteger bNum = new BigInteger(num + "");
    bNum = bNum.shiftLeft(length - base);
    BigInteger bArray = new BigInteger(1, switchEndian(array));

    for (int i = length - 1; i > length - base - 1; i--) {
      if (bNum.testBit(i)) {
        bArray = bArray.setBit(i);
      } else {
        bArray = bArray.clearBit(i);
      }
    }

    byte[] newArray = bArray.toByteArray();
    byte[] result = new byte[array.length];

    if (newArray.length <= array.length) {
      System.arraycopy(newArray, 0, result, result.length - newArray.length, newArray.length);
    } else {
      System.arraycopy(newArray, newArray.length - array.length, result, 0, array.length);
    }

    return switchEndian(result);
  }

  /**
   * Switches the endianness of the array
   *
   * @param array The array to switch
   * @return THe switched array
   */
  private static byte[] switchEndian(byte[] array) {
    byte[] result = new byte[array.length];

    for (int i = 0; i < result.length; i++) {
      result[i] = array[result.length - 1 - i];
    }

    return result;
  }

}