com.hfg.math.Combinations Maven / Gradle / Ivy

Go to download
Show more of this group Show more artifacts with this name
Show all versions of com_hfg Show documentation
com.hfg xml, html, svg, and bioinformatics utility library
There is a newer version: 20240423
package com.hfg.math;


import com.hfg.util.collection.CollectionUtil;

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

//------------------------------------------------------------------------------
/**
 Generate combinations of the specified size using the specified list of objects.
 A combination is a set of objects in which position (or order) is NOT important.
 
  @author J. Alex Taylor, hairyfatguy.com
 
 */
//------------------------------------------------------------------------------
// com.hfg Library
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
//
// J. Alex Taylor, President, Founder, CEO, COO, CFO, OOPS hairyfatguy.com
// [email protected]
//------------------------------------------------------------------------------

public class Combinations
{
   //---------------------------------------------------------------------------
   public static> List> combinations(List inObjects, int inCombinationSize)
   {
      List> combinations = null;
      if (CollectionUtil.hasValues(inObjects))
      {
         if (inCombinationSize == 1)
         {
            combinations = new ArrayList<>(inObjects.size());
            for (T obj : inObjects)
            {
               combinations.add(Collections.singletonList(obj));
            }
         }
         else
         {
            combinations = new ArrayList<>();
            for (int i = 0; i <= inObjects.size() - inCombinationSize; i++)
            {
               T firstItem = inObjects.get(i);
               List> additionalItems = combinations(inObjects.subList(i + 1, inObjects.size()), inCombinationSize - 1);
               for (List additional : additionalItems)
               {
                  List combination = new ArrayList<>();
                  combination.add(firstItem);
                  combination.addAll(additional);
                  combinations.add(combination);
               }
            }
         }
      }

      return combinations;
   }
}