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.data;
  
  import edu.cmu.tetrad.util.TetradSerializable;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.util.*;
  
  /**
   * Represents a "Other Group" in Knowledge, which can be understood as: Group1 -> Group2 where there are
   * edges between all members of Group1 to Group2.
   * <p>
   * Immutable.
   *
   * @author Tyler Gibson
   */
  public final class KnowledgeGroup implements TetradSerializable {
      static final long serialVersionUID = 23L;
  
  
      /**
       * The left group of variables.
       *
       * @serial - not null
       */
      private Set<String> fromGroup;
  
  
      /**
       * The right group of variables.
       *
       * @serial - not null
       */
      private Set<String> toGroup;
  
  
      /**
       * The type of group that this is (currently either required or forbidden).
       */
      private int type;
  
  
      /**
       * The types of groups (Can an enum be used instead?)
       */
      public final static int REQUIRED = 1;
      public final static int FORBIDDEN = 2;
  
  
      /**
       * Constructs a group given the type.
       *
       * @param type - the type
       */
      public KnowledgeGroup(int type, Set<String> from, Set<String> to) {
          if (type != REQUIRED && type != FORBIDDEN) {
              throw new NullPointerException("The given type needs to be either REQUIRED or FORBIDDEN");
          }
          if (from == null) {
              throw new NullPointerException("The from set must not be null");
          }
          if (to == null) {
              throw new NullPointerException("The to set must not be null");
          }
          if (intersect(from, to)) {
              throw new IllegalArgumentException("The from and to sets must not intersect");
          }
          this.fromGroup = new HashSet<>(from);
          this.toGroup = new HashSet<>(to);
          this.type = type;
      }
  
  
      /**
       * Constructs an empty instance of a knowledge group.
       */
      public KnowledgeGroup(int type) {
          if (type != REQUIRED && type != FORBIDDEN) {
              throw new NullPointerException("The given type needs to be either REQUIRED or FORBIDDEN");
          }
          this.type = type;
          this.fromGroup = Collections.emptySet();
          this.toGroup = Collections.emptySet();
      }
  
  
      //====================== Public Methods =============================//
  
  
      /**
       * Generates a simple exemplar of this class to test serialization.
       */
      public static KnowledgeGroup serializableInstance() {
          return new KnowledgeGroup(REQUIRED, new HashSet<String>(0), new HashSet<String>(0));
      }
  
  
      public int getType() {
          return this.type;
      }
  
  
      /**
       * States whether this group is empty, that is there is no edges in it (Note there may be some
       * partial information though).
       */
      public boolean isEmpty() {
          return this.fromGroup.isEmpty() || this.toGroup.isEmpty();
      }
  
  
      public Set<String> getFromVariables() {
          return Collections.unmodifiableSet(this.fromGroup);
      }
  
  
      public Set<String> getToVariables() {
          return Collections.unmodifiableSet(this.toGroup);
      }
  
  
      /**
       * @return - edges.
       */
      public List<KnowledgeEdge> getEdges() {
          List<KnowledgeEdge> edges = new ArrayList<>(this.fromGroup.size() + this.toGroup.size());
          for (String from : this.fromGroup) {
              for (String to : this.toGroup) {
                  edges.add(new KnowledgeEdge(from, to));
              }
          }
          return edges;
      }
  
  
      public boolean containsEdge(KnowledgeEdge edge) {
          return this.fromGroup.contains(edge.getFrom()) && this.toGroup.contains(edge.getTo());
      }
  
  
      /**
       * Computes a hashcode.
       */
      public int hashCode() {
          int hash = 37;
          hash += 17 * this.fromGroup.hashCode() + 37;
          hash += 17 * this.toGroup.hashCode() + 37;
          hash += 17 * Integer.valueOf(this.type).hashCode() + 37;
          return hash;
      }
  
  
      /**
       * States whether this group has a conflict with the given one.
       *
       * @return true iff there is a conflict.
       */
      public boolean isConflict(KnowledgeGroup group) {
          // if they have different types, then if they share an edge there is a conflict.
          if (this.type != group.type) {
              return intersect(this.fromGroup, group.fromGroup) && intersect(this.toGroup, group.toGroup);
          }
          // otherwise they either don't share edges or are the same type and no conflicts
          // exist (as at worst they will have edges in common).
          return false;
      }
  
  
      /**
       * Equals when they are the same type and have the same edges.
       */
      public boolean equals(Object o) {
          if (o == this) {
              return true;
          }
  
          if (!(o instanceof KnowledgeGroup)) {
              return false;
          }
  
          KnowledgeGroup thatGroup = (KnowledgeGroup) o;
  
          return this.type == thatGroup.type && this.fromGroup.equals(thatGroup.fromGroup)
                  && this.toGroup.equals(thatGroup.toGroup);
  
      }
  
      //=========================== Private Methods ==========================//
  
  
      /**
       * States whether the two have a non-empty intersection or not.
       */
      private static boolean intersect(Set<String> set1, Set<String> set2) {
          for (String var : set1) {
              if (set2.contains(var)) {
                  return true;
              }
          }
          return false;
      }
  
  
      /**
       * Adds semantic checks to the default deserialization method. This method
       * must have the standard signature for a readObject method, and the body of
       * the method must begin with "s.defaultReadObject();". Other than that, any
       * semantic checks can be specified and do not need to stay the same from
       * version to version. A readObject method of this form may be added to any
       * class, even if Tetrad sessions were previously saved out using a version
       * of the class that didn't include it. (That's what the
       * "s.defaultReadObject();" is for. See J. Bloch, Effective Java, for help.
       *
       * @throws java.io.IOException
       * @throws ClassNotFoundException
       */
      private void readObject(ObjectInputStream s)
              throws IOException, ClassNotFoundException {
          s.defaultReadObject();
  
          if (this.type != REQUIRED && this.type != FORBIDDEN) {
              throw new IllegalStateException("Type must be REQUIRED or FORBIDDEN");
          }
  
          if (fromGroup == null) {
              throw new NullPointerException();
          }
  
          if (toGroup == null) {
              throw new NullPointerException();
          }
  
      }
  
  
  }
  
  
  
  