btalgorithmpeerselectionatpeer.java

p2p仿真

/*
 * @(#)BTAlgorithmPeerSelectionAtPeer.java	ver 1.2  6/20/2005
 *
 * Copyright 2005 Weishuai Yang ([e-mail]). 
 * All rights reserved.
 * 
 */

package gps.protocol.BT.algorithm;
import gps.Simulator;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Collections;

/**
 * selects peers to send request
 * 
 * @author  Weishuai Yang
 * @version 1.2,  6/20/2005
 */

public class BTAlgorithmPeerSelectionAtPeer {
    /**
     * default peer number to send request
     */
	public static int DEFAULT_PEER_NUM_LIMIT=10;
	/**
	 * peer number to send request
	 */
	public int peerNumLimit=DEFAULT_PEER_NUM_LIMIT;
	/**
	 * random number generator
	 */
	protected Random rand=null;
	
	/**
	 * constructs peer selection algorithm object
	  * @param n seed for random object
	 */
	public BTAlgorithmPeerSelectionAtPeer(int n){
		rand=new Random(n+Integer.parseInt(Simulator.getInstance().getProperties().getProperty("RandomSeed")));

	}
	
	
	/**
	 * selects peers having documents from peers local record
	 * @param list local record of peers having this document
	 * @return peer list to send request to
	 * 
	 * one difference of the slectPeers algorithm between peer side 
	 * and tracker side is that selected nodes are removed from the 
	 * passed-in list for the peer side selection, but not for tracker
	 * side selection. 
	 */
	
	public ArrayList selectPeers(List list){
		//this algorithm simply randomly select peerNumLimit of nodes.
		ArrayList listClone=null;

		if(list.size()<=peerNumLimit){
			listClone=new ArrayList(list);
			list.clear();
			listClone.trimToSize();
			Collections.shuffle(listClone, rand);
			return listClone;
		}
		
	 	int index=0;

		
		//for performance, different treatment for list size between 
		//peerNumLimit and 2*peerNumLimit
		
		if(list.size()<2*peerNumLimit){
			//remove list.size-peerNumLimit;
		    listClone=new ArrayList(list);
			while(listClone.size()>peerNumLimit){
				index=rand.nextInt()%listClone.size();
				if(index<0) index=-index;
				listClone.remove(index);
			}
		}
		
		else {
			//adding and increase to peerNumLimit
			listClone=new ArrayList(peerNumLimit);
			while(listClone.size()<peerNumLimit){
				index=rand.nextInt()%list.size();
				if(index<0) index=-index;
				listClone.add(list.get(index));
			}
	 	}
		//currently, the difference between the selection algorithm 
		//at tracker and peer is that, the selected elements are 
		//removed at peer, but not at tracker
	 	list.removeAll(listClone);
	 	Collections.shuffle(listClone, rand);
		return listClone;
	 } 

}