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/**
*
*/
package de.fhswf.in.inf.java1.aufgabe08;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
/**
* Utility class for the Floyd and Bentley algorithm.
*
* @author $Author: $
* @version $Revision: $, $Date: $ UTC
*/
public final class FloydAndBentley
{
private static Random rnd = new Random();
/**
* Private constructor for utility class.
*
*/
private FloydAndBentley()
{
}
/**
* A function which chooses a sample from a List with the Floyd and Bentley
* algorithm. (Recursive algorithm)
*
* @param <T>
* Type of the Lists.
* @param s
* The set from which the sample is chosen.
* @param k
* The number of items to be chosen.
* @return Return the sample.
*/
public static <T> List<T> sample(List<T> s, int k)
{
if (s.isEmpty())
{
throw new IllegalArgumentException("Set is empty");
}
if (k > s.size())
{
throw new IllegalArgumentException("k is bigger than the Set");
}
List<T> ret = new ArrayList<>(k);
sampleRec(s, k, ret);
return ret;
}
/**
* The recursive implementation of the Floyd and Bentley algorithm.
*
* @param <T>
* Type of the Lists.
* @param s
* The set from which the sample is chosen.
* @param k
* The number of items to be chosen.
* @param ret
* Return the sample.
*/
private static <T> void sampleRec(List<T> s, int k, List<T> ret)
{
if (ret.size() >= k)
{
return;
}
T val = s.get(rnd.nextInt(s.size() - (k - ret.size() - 1)));
if (ret.contains(val))
{
val = s.get(s.size() - (k - ret.size() - 1));
}
ret.add(val);
sampleRec(s, k, ret);
}
/**
* A function which chooses a sample from a List with the Floyd and Bentley
* algorithm. (Non recursive algorithm)
*
* @param <T>
* Type of the Lists.
* @param s
* The set from which the sample is chosen.
* @param k
* The number of items to be chosen.
* @return Return the sample.
*/
public static <T> List<T> sample2(List<T> s, int k)
{
if (s.isEmpty())
{
throw new IllegalArgumentException("Set is empty");
}
if (k > s.size())
{
throw new IllegalArgumentException("k is bigger than the Set");
}
List<T> ret = new ArrayList<>(k);
for (int i = 0; i < k; i++)
{
T val = s.get(rnd.nextInt(s.size() - (k - ret.size() - 1)));
if (ret.contains(val))
{
val = s.get(s.size() - (k - ret.size() - 1));
}
ret.add(val);
}
return ret;
}
}
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