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package org.aksw.commons.collections.random;

import com.google.common.collect.Iterables;

import java.util.*;

/**
 * Created by Claus Stadler
 * Date: Oct 9, 2010
 * Time: 5:43:50 PM
 * Updated by Konrad Höffner 12.8.2011
 */
public class RandomUtils
{
	/**
	 * Returns a random sample of the given universe. If n <= universe.length, then Arrays.copyOf of the universe is returned.
	 * The implementation runs in O(n).
	 * Principally the same as the method shuffleRandomSample() but works on arrays, does not shuffle the source, is not a void method and has a better complexity.
	 * For a discussion and benchmark of various methods of creating random samples see  http://www.java-forum.org/mathematik/116289-random-sample.html
	 * @author Konrad Höffner*/
	public static  T[] randomSample(T[] universe, int n)
	{
		int m = universe.length;
		if(m<=n) return universe.clone();
		// new T[n] does not work because you cannot create a generic array
		T[] sample =  Arrays.copyOf(universe, n);
		int rndIndex;
		Random rnd = new Random();	  
		for (int i = 0; i < n; i++)
		{
			rndIndex = rnd.nextInt(m - i);
			sample[i] = universe[rndIndex];
			universe[rndIndex] = universe[m - i -1];
		}	  	     		
		return sample;
	}

	/**
	 * Extracts a random sample of a specified maximum size from the given collection.
	 * Warning: Shuffles elements of the source list.
	 *
	 * @param source
	 * @param sampleSize
	 * @param 
	 * @return
	 */
	public static  void shuffleRandomSample(List source, Collection out, int sampleSize, Random random)
	{
		//if(source.size() < sampleSize)
		//throw new IllegalArgumentException("Cannot create a random sample of size " + sampleSize + " from a collection containing only " + source.size() + " items");

		Collections.shuffle(source, random);
		Iterator it = source.iterator();
		while(out.size() < sampleSize && it.hasNext()) {
			out.add(it.next());
		}
	}

	public static  void shuffleRemoveRandomSample(List source, Collection out, int sampleSize, Random random)
	{
		Collections.shuffle(source, random);

		List tmp = source.subList(0, sampleSize);
		out.addAll(tmp);

		tmp.clear();
	}

    public static  void retainRandomSample(List inout, int sampleSize, Random random)
    {
        Collections.shuffle(inout, random);

        inout.subList(Math.min(sampleSize, inout.size()), inout.size());
    }

	public static  Set randomSampleSet(Collection source, int sampleSize, Random random)
	{
		List src = new ArrayList(source);
		Set result = new HashSet();

		shuffleRandomSample(src, result, sampleSize, random);

		return result;
	}

	public static  Set randomSampleSet(Collection source, int sampleSize)
	{
		return randomSampleSet(source, sampleSize, new Random());
	}


	public static  Set shuffleRemoveRandomSampleSet(List source, int sampleSize, Random random)
	{
		Set result = new HashSet();

		shuffleRemoveRandomSample(source, result, sampleSize, random);

		return result;
	}

	public static  T randomItem(Collection collection, Random random)
	{
		if(collection.isEmpty())
			throw new IllegalArgumentException("Collection must not be empty");

		int index = random.nextInt(collection.size());

		return Iterables.get(collection, index);   
	}

	public static  T randomItem(Iterable iterable, Random random)
	{
		if(Iterables.isEmpty(iterable))
			throw new IllegalArgumentException("Iterable must not be empty");

		int index = random.nextInt(Iterables.size(iterable));

		return Iterables.get(iterable, index);
	}
}