code: ///////////////////////////////////////////////////////////////////////////////
  // For information as to what this class does, see the Javadoc, below.       //
  // Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,       //
  // 2007, 2008, 2009, 2010, 2014, 2015 by Peter Spirtes, Richard Scheines, Joseph   //
  // Ramsey, and Clark Glymour.                                                //
  //                                                                           //
  // This program is free software; you can redistribute it and/or modify      //
  // it under the terms of the GNU General Public License as published by      //
  // the Free Software Foundation; either version 2 of the License, or         //
  // (at your option) any later version.                                       //
  //                                                                           //
  // This program 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 General Public License for more details.                              //
  //                                                                           //
  // You should have received a copy of the GNU General Public License         //
  // along with this program; if not, write to the Free Software               //
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA //
  ///////////////////////////////////////////////////////////////////////////////
  
  package edu.cmu.tetrad.util;
  
  /**
   * Generates (nonrecursively) all of the combinations of objects, where the
   * number of objects in each dimension is specified. The sequence of choices is
   * obtained by repeatedly calling the next() method.  When the sequence is
   * finished, null is returned.
   * <p>
   * A valid combination for the sequence of combinations for a choose b generated
   * by this class is an array x[] of b integers i, 0 <= i < a, such that x[j] <
   * x[j + 1] for each j from 0 to b - 1.
   *
   * @author Joseph Ramsey
   */
  public final class  CombinationGenerator {
  
      /**
       * The number of items for each dimension.
       */
      private final int[] dims;
  
      /**
       * The internally stored choice.
       */
      private final int[] local;
  
      /**
       * The choice that is returned. Used, since the returned array can be
       * modified by the user.
       */
      private final int[] returned;
  
      /**
       * Indicates whether the next() method has been called since the last
       * initialization.
       */
      private boolean begun;
  
      /**
       * Constructs a new combination of objects, choosing one object from
       * each dimension.
       *
       * @param dims the number of objects in each dimension. Each member must
       *             be >= 0.
       * @throws NullPointerException if dims is null.
       */
      public CombinationGenerator(int[] dims) {
          this.dims = dims;
          local = new int[dims.length];
          returned = new int[dims.length];
  
          // Initialize the combination array with successive integers [0 1 2 ...].
          // Set the value at the last index one less than it would be in such
          // a series, ([0 1 2 ... b - 2]) so that on the first call to next()
          // the first combination ([0 1 2 ... b - 1]) is returned correctly.
          for (int i = 0; i < dims.length - 1; i++) {
              local[i] = 0;
          }
  
          if (local.length > 0) {
              local[local.length - 1] = -1;
          }
  
          begun = false;
      }
  
      /**
       * @return the next combination in the series, or null if the series is
       * finished.
       */
      public int[] next() {
          int i = getNumObjects();
  
          // Scan from the right for the first index whose value is less than
          // its expected maximum (i + diff) and perform the fill() operation
          // at that index.
          while (--i > -1) {
              if (this.local[i] < dims[i] - 1) {
                  fill(i);
                  begun = true;
                  System.arraycopy(local, 0, returned, 0, getNumObjects());
                  return returned;
              }
          }
  
          if (this.begun) {
              return null;
          } else {
              begun = true;
              System.arraycopy(local, 0, returned, 0, getNumObjects());
              return returned;
          }
      }
  
      /**
       * This static method will print the series of combinations for a choose b
       * to System.out.
       *
       * @param dims An int array consisting of the number of dimensions for each
       *             variable, in order.
       */
      public static void testPrint(int[] dims) {
          CombinationGenerator cg = new CombinationGenerator(dims);
          int[] choice;
  
          System.out.println();
          System.out.print("Printing combinations for (");
  
          for (int i = 0; i < dims.length; i++) {
              System.out.print(dims[i]);
  
              if (i < dims.length - 1) {
                  System.out.print(", ");
              }
          }
  
          System.out.println(")\n");
  
          while ((choice = cg.next()) != null) {
              if (choice.length == 0) {
                  System.out.println("zero-length array");
              } else {
                  for (int aChoice : choice) {
                      System.out.print(aChoice + "\t");
                  }
  
                  System.out.println();
              }
          }
  
          System.out.println();
      }
  
      /**
       * @return Ibid.
       */
      private int getNumObjects() {
          return local.length;
      }
  
      /**
       * Fills the 'choice' array, from index 'index' to the end of the array,
       * with successive integers starting with choice[index] + 1.
       *
       * @param index the index to begin this incrementing operation.
       */
      private void fill(int index) {
          this.local[index]++;
  
          for (int i = index + 1; i < getNumObjects(); i++) {
              local[i] = 0;
          }
      }
  }
  
  
  
  