code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
  //                / /___/ /_/ / /_/ / / /__/ /|  / /_/ /                 //
  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  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 org.logicng.formulas;
  
  import org.logicng.datastructures.Assignment;
  import org.logicng.datastructures.Substitution;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.stream.Stream;
  
  /**
   * Boolean literals.
   * <p>
   * Literals are besides the constants true and false and pseudo Boolean constraints the
   * atomic formulas in LogicNG.  Each variable is a positive literal.
   * <p>
   * A literal consists of its name and its phase (also sign or polarity in the literature).
   * A new positive literal can be constructed with {@code f.literal("a", true)} or
   * - as a shortcut - {@code f.variable("a")}.  A new negative literal can be constructed
   * with {@code f.literal("a", false)} or if preferred with {@code f.not(f.variable("a"))}
   * or {@code f.variable("a").negate()}.
   * <p>
   * @version 2.2.0
   * @since 1.0
   */
  public class Literal extends Formula implements Comparable<Literal> {
  
      private static final Iterator<Formula> ITERATOR = new Iterator<Formula>() {
          @Override
          public boolean hasNext() {
              return false;
          }
  
          @Override
          public Formula next() {
              throw new NoSuchElementException();
          }
  
          @Override
          public void remove() {
              throw new UnsupportedOperationException();
          }
      };
  
      private final String name;
      private final boolean phase;
      private final Variable var;
      private volatile Literal negated;
      private volatile int hashCode;
  
      /**
       * Constructor.  A literal always has a name and a phase.  A positive literal can also
       * be constructed directly as a {@link Variable}.
       * @param name  the literal name
       * @param phase the phase of the literal (also found as sign or polarity in the literature)
       * @param f     the factory which created this literal
       */
      Literal(final String name, final boolean phase, final FormulaFactory f) {
          super(FType.LITERAL, f);
          this.name = name;
          this.phase = phase;
          this.var = phase ? (Variable) this : (Variable) this.negate();
      }
  
      @Override
      public FormulaFactory factory() {
          return this.f;
      }
  
      @Override
      public int numberOfOperands() {
          return 0;
      }
  
      @Override
      public boolean isConstantFormula() {
          return false;
      }
  
      @Override
      public boolean isAtomicFormula() {
          return true;
      }
  
      @Override
      public boolean containsVariable(final Variable variable) {
          return variable.name().equals(this.name);
      }
  
      @Override
      public boolean evaluate(final Assignment assignment) {
          return assignment.evaluateLit(this);
      }
  
      @Override
      public Formula restrict(final Assignment assignment) {
          return assignment.restrictLit(this);
      }
  
      @Override
      public boolean containsNode(final Formula formula) {
          return this.equals(formula);
      }
  
      @Override
      public Formula substitute(final Substitution substitution) {
          final Formula subst = substitution.getSubstitution(this.variable());
          return subst == null ? this : (this.phase ? subst : subst.negate());
      }
  
      @Override
      public Literal negate() {
          if (this.negated != null) {
              return this.negated;
          }
          this.negated = this.f.literal(this.name, !this.phase);
          return this.negated;
      }
  
      /**
       * Returns the name of the literal.
       * @return the name of the literal
       */
      public String name() {
          return this.name;
      }
  
      /**
       * Returns the phase of the literal.
       * @return the phase of the literal.
       */
      public boolean phase() {
          return this.phase;
      }
  
      /**
       * Returns a positive version of this literal (aka a variable).
       * @return a positive version of this literal
       */
      public Variable variable() {
          return this.var;
      }
  
      @Override
      public int hashCode() {
          final int result = this.hashCode;
          if (result == 0) {
              this.hashCode = this.name.hashCode() ^ (this.phase ? 1 : 0);
          }
          return this.hashCode;
      }
  
      @Override
      public boolean equals(final Object other) {
          if (other == this) {
              return true;
          }
          if (other instanceof Formula && this.f == ((Formula) other).f) {
              return false; // the same formula factory would have produced a == object
          }
          if (other instanceof Literal) {
              final Literal otherLit = (Literal) other;
              return this.phase == otherLit.phase && this.name.equals(otherLit.name);
          }
          return false;
      }
  
      @Override
      public int compareTo(final Literal lit) {
          if (this == lit) {
              return 0;
          }
          final int res = this.name.compareTo(lit.name);
          if (res == 0 && this.phase != lit.phase) {
              return this.phase ? -1 : 1;
          }
          return res;
      }
  
      @Override
      public Iterator<Formula> iterator() {
          return ITERATOR;
      }
  
      @Override
      public Stream<Formula> stream() {
          return Stream.empty();
      }
  }
  