CombinationGenerator.java

/* 
 * Copyright 2015 Themistoklis Mavridis <themis.mavridis@issel.ee.auth.gr>.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.thesmartweb.swebrank;


import java.math.BigInteger;

/**
 * Method to Systematically generate combinations.
 * @author Michael Gilleland - http://www.merriampark.com/comb.htm
 * <a href ="http://www.ccs.neu.edu/home/lieber/courses/csu670/f08/sdg-players/winning-player/src/logic/CombinationGenerator.java">http://www.ccs.neu.edu/home/lieber/courses/csu670/f08/sdg-players/winning-player/src/logic/CombinationGenerator.java</a>
 * 
 * 
 */
public class CombinationGenerator {

  private int[] a;
  private int n;
  private int r;
  private BigInteger numLeft;
  private BigInteger total;

  //------------
  // Constructor
  //------------

    /**
     * Generator of the combinations
     * @param n number 1
     * @param r number 2
     */
    
  public CombinationGenerator (int n, int r) {
    if (r > n) {
      throw new IllegalArgumentException ();
    }
    if (n < 1) {
      throw new IllegalArgumentException ();
    }
    this.n = n;
    this.r = r;
    a = new int[r];
    BigInteger nFact = getFactorial (n);
    BigInteger rFact = getFactorial (r);
    BigInteger nminusrFact = getFactorial (n - r);
    total = nFact.divide (rFact.multiply (nminusrFact));
    reset ();


  }

    /**
     *
     */
    public void reset () {
    for (int i = 0; i < a.length; i++) {
      a[i] = i;
    }
    numLeft = new BigInteger (total.toString ());
  }

  //------------------------------------------------
  // Return number of combinations not yet generated
  //------------------------------------------------

    /**
     * Getter of the number left
     * @return number left
     */
    
  public BigInteger getNumLeft () {
    return numLeft;
  }

  //-----------------------------
  // Are there more combinations?
  //-----------------------------

    /**
     * Getter of the flag if more is left
     * @return get if more numbers are left
     */
    
  public boolean hasMore () {
    return numLeft.compareTo (BigInteger.ZERO) == 1;
  }

  //------------------------------------
  // Return total number of combinations
  //------------------------------------

    /**
     * Getter of the total number of combinations
     * @return the total number of combinations
     */
    
  public BigInteger getTotal () {
    return total;
  }

  //------------------
  // Compute factorial
  //------------------

  private static BigInteger getFactorial (int n) {
    BigInteger fact = BigInteger.ONE;
    for (int i = n; i > 1; i--) {
      fact = fact.multiply (new BigInteger (Integer.toString (i)));
    }
    return fact;
  }

  //--------------------------------------------------------
  // Generate next combination (algorithm from Rosen p. 286)
  //--------------------------------------------------------

    /**
     * Get the next combination (algorithm from Rosen p. 286)
     * @return next combination (algorithm from Rosen p. 286)
     */
    
  public int[] getNext () {

    if (numLeft.equals (total)) {
      numLeft = numLeft.subtract (BigInteger.ONE);
      return a;
    }

    int i = r - 1;
    while (a[i] == n - r + i) {
      i--;
    }
    a[i] = a[i] + 1;
    for (int j = i + 1; j < r; j++) {
      a[j] = a[i] + j - i;
    }

    numLeft = numLeft.subtract (BigInteger.ONE);
    return a;

  }
}