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 static org.assertj.core.api.Assertions.assertThat;
  import static org.assertj.core.api.Assertions.assertThatThrownBy;
  
  import org.junit.jupiter.api.Test;
  import org.logicng.io.parsers.ParserException;
  import org.logicng.io.parsers.PropositionalParser;
  import org.logicng.io.parsers.PseudoBooleanParser;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.stream.Collectors;
  
  /**
   * Tests for the formula iterators and streams.
   * @version 2.0.0
   * @since 1.0
   */
  public class FormulaIteratorTest {
  
      private final FormulaFactory f = new FormulaFactory();
      private final PropositionalParser p = new PropositionalParser(this.f);
  
      @Test
      public void testTrue() {
          final Formula formula = this.f.verum();
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.hasNext()).isFalse();
          assertThat(formula.stream().count()).isEqualTo(0);
          assertThatThrownBy(() -> formula.iterator().next()).isInstanceOf(NoSuchElementException.class);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testFalse() {
          final Formula formula = this.f.falsum();
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.hasNext()).isFalse();
          assertThat(formula.stream().count()).isEqualTo(0);
          assertThatThrownBy(() -> formula.iterator().next()).isInstanceOf(NoSuchElementException.class);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testLiteral() {
          final Formula formula = this.f.variable("a");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.hasNext()).isFalse();
          assertThat(formula.stream().count()).isEqualTo(0);
          assertThatThrownBy(() -> formula.iterator().next()).isInstanceOf(NoSuchElementException.class);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testNot() throws ParserException {
          final Formula formula = this.p.parse("~(a & (b | c))");
          final Formula operand = this.p.parse("a & (b | c)");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.next()).isEqualTo(operand);
          assertThatThrownBy(it::next).isInstanceOf(NoSuchElementException.class);
          assertThat(formula.stream().count()).isEqualTo(1);
          assertThat(formula.stream().collect(Collectors.toList())).containsExactly(operand);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testImplication() throws ParserException {
          final Formula formula = this.p.parse("a => c | d");
          final Formula left = this.f.variable("a");
          final Formula right = this.p.parse("c | d");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.next()).isEqualTo(left);
          assertThat(it.next()).isEqualTo(right);
          assertThatThrownBy(it::next).isInstanceOf(NoSuchElementException.class);
          assertThat(formula.stream().count()).isEqualTo(2);
          assertThat(formula.stream().collect(Collectors.toList())).containsExactly(left, right);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testEquivalence() throws ParserException {
          final Formula formula = this.p.parse("a <=> c | d");
          final Formula left = this.f.variable("a");
          final Formula right = this.p.parse("c | d");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.next()).isEqualTo(left);
          assertThat(it.next()).isEqualTo(right);
          assertThatThrownBy(it::next).isInstanceOf(NoSuchElementException.class);
          assertThat(formula.stream().count()).isEqualTo(2);
          assertThat(formula.stream().collect(Collectors.toList())).containsExactly(left, right);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testAnd() throws ParserException {
          final Formula formula = this.p.parse("a & (c | d) & ~e");
          final Formula op1 = this.p.parse("a");
          final Formula op2 = this.p.parse("c | d");
          final Formula op3 = this.p.parse("~e");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.next()).isEqualTo(op1);
          assertThat(it.next()).isEqualTo(op2);
          assertThat(it.next()).isEqualTo(op3);
          assertThatThrownBy(it::next).isInstanceOf(NoSuchElementException.class);
          assertThat(formula.stream().count()).isEqualTo(3);
          assertThat(formula.stream().collect(Collectors.toList())).containsExactly(op1, op2, op3);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testOr() throws ParserException {
          final Formula formula = this.p.parse("a | (c & d) | ~e");
          final Formula op1 = this.p.parse("a");
          final Formula op2 = this.p.parse("c & d");
          final Formula op3 = this.p.parse("~e");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.next()).isEqualTo(op1);
          assertThat(it.next()).isEqualTo(op2);
          assertThat(it.next()).isEqualTo(op3);
          assertThatThrownBy(it::next).isInstanceOf(NoSuchElementException.class);
          assertThat(formula.stream().count()).isEqualTo(3);
          assertThat(formula.stream().collect(Collectors.toList())).containsExactly(op1, op2, op3);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  
      @Test
      public void testPBC() throws ParserException {
          final Formula formula = new PseudoBooleanParser(this.f).parse("3*a + 4*b + 5*c <= 8");
          final Iterator<Formula> it = formula.iterator();
          assertThat(it.hasNext()).isFalse();
          assertThat(formula.stream().count()).isEqualTo(0);
          assertThatThrownBy(() -> formula.iterator().next()).isInstanceOf(NoSuchElementException.class);
          assertThatThrownBy(() -> formula.iterator().remove()).isInstanceOf(UnsupportedOperationException.class);
      }
  }
  