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.search;
  
  import edu.cmu.tetrad.graph.Node;
  
  import java.text.DecimalFormat;
  import java.text.NumberFormat;
  import java.util.HashSet;
  import java.util.Set;
  
  /**
   * Represents a tetrad of variables.
   */
  public class Tetrad {
      private Node i;
      private Node j;
      private Node k;
      private Node l;
      private double pValue;
  
      public Tetrad(Node i, Node j, Node k, Node l) {
  //        testDistinctness(i, j, k, l);
          this.i = i;
          this.j = j;
          this.k = k;
          this.l = l;
          this.pValue = Double.NaN;
      }
  
      public Tetrad(Node i, Node j, Node k, Node l, double pValue) {
  //        testDistinctness(i, j, k, l);
          this.i = i;
          this.j = j;
          this.k = k;
          this.l = l;
          this.pValue = pValue;
      }
  
  //    private void testDistinctness(Node i, Node j, Node k, Node l) {
  //        if (i == j || i == k || i == l) {
  //            throw new IllegalArgumentException("Nodes not distinct.");
  //        }
  //
  //        if (j == k || j == l) {
  //            throw new IllegalArgumentException("Nodes not distinct.");
  //        }
  //
  //        if (k == l) {
  //            throw new IllegalArgumentException("Nodes not distinct.");
  //        }
  //    }
  
      public Node getI() {
          return i;
      }
  
      public Node getJ() {
          return j;
      }
  
      public Node getK() {
          return k;
      }
  
      public Node getL() {
          return l;
      }
  
      public int hashCode() {
  //        int hash = i.hashCode();
  //        hash += 17 * hash + j.hashCode();
  //        hash += 17 * hash + k.hashCode();
  //        hash += 17 * hash + l.hashCode();
  
          int hash = 17 * i.hashCode() * j.hashCode();
          hash += 29 * k.hashCode() * l.hashCode();
  
          return hash;
      }
  
      public boolean equals(Object o) {
          if (o == null) return false;
          Tetrad tetrad = (Tetrad) o;
          return (i == tetrad.i && j == tetrad.j && k == tetrad.k && l == tetrad.l)
                  || (i == tetrad.j && j == tetrad.i && k == tetrad.k && l == tetrad.l)
                  || (i == tetrad.i && j == tetrad.j && k == tetrad.l && l == tetrad.k)
                  || (i == tetrad.j && j == tetrad.i && k == tetrad.l && l == tetrad.k)
                  || (i == tetrad.k && j == tetrad.l && k == tetrad.i && l == tetrad.j)
                  || (i == tetrad.k && j == tetrad.l && k == tetrad.j && l == tetrad.i)
                  || (i == tetrad.l && j == tetrad.k && k == tetrad.i && l == tetrad.j)
                  || (i == tetrad.l && j == tetrad.k && k == tetrad.j && l == tetrad.i);
      }
  
      public String toString() {
          NumberFormat nf = new DecimalFormat("0.0000");
  
          if (Double.isNaN(pValue)) {
              return "s(" + i + "," + j + ")*s(" + k + "," + l + ")-s(" + i + "," + k + ")*s(" + j + "," + l + ")";
          } else {
              return "<" + i + ", " + j + ", " + k + ", " + l + ", " + nf.format(pValue) + ">";
          }
      }
  
      public double getPValue() {
          return pValue;
      }
  
      public Set<Node> getNodes() {
          Set<Node> nodes = new HashSet<>();
          nodes.add(i);
          nodes.add(j);
          nodes.add(k);
          nodes.add(l);
          return nodes;
      }
  }
  
  
  