code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
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  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               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 org.junit.jupiter.api.Test;
  import org.logicng.TestWithExampleFormulas;
  import org.logicng.io.parsers.ParserException;
  import org.logicng.io.parsers.PropositionalParser;
  
  /**
   * Unit Tests for NNF conversion.
   * @version 2.2.0
   * @since 1.0
   */
  public class NNFTest extends TestWithExampleFormulas {
  
      @Test
      public void testConstants() {
          assertThat(this.TRUE.nnf()).isEqualTo(this.TRUE);
          assertThat(this.FALSE.nnf()).isEqualTo(this.FALSE);
      }
  
      @Test
      public void testLiterals() {
          assertThat(this.A.nnf()).isEqualTo(this.A);
          assertThat(this.NA.nnf()).isEqualTo(this.NA);
      }
  
      @Test
      public void testBinaryOperators() throws ParserException {
          final PropositionalParser p = new PropositionalParser(this.f);
          assertThat(this.IMP1.nnf()).isEqualTo(p.parse("~a | b"));
          assertThat(this.IMP2.nnf()).isEqualTo(p.parse("a | ~b"));
          assertThat(this.IMP3.nnf()).isEqualTo(p.parse("~a | ~b | x | y"));
          assertThat(this.IMP4.nnf()).isEqualTo(p.parse("(~a | ~b) & (a | b) | (x | ~y) & (y | ~x)"));
          assertThat(this.EQ1.nnf()).isEqualTo(p.parse("(~a | b) & (~b | a)"));
          assertThat(this.EQ2.nnf()).isEqualTo(p.parse("(a | ~b) & (b | ~a)"));
          assertThat(this.EQ3.nnf()).isEqualTo(p.parse("(~a | ~b | x | y) & (~x & ~y | a & b)"));
      }
  
      @Test
      public void testNAryOperators() throws ParserException {
          final PropositionalParser p = new PropositionalParser(this.f);
          assertThat(this.AND1.nnf()).isEqualTo(this.AND1);
          assertThat(this.OR1.nnf()).isEqualTo(this.OR1);
          assertThat(p.parse("~(a | b) & c & ~(x & ~y) & (w => z)").nnf()).isEqualTo(p.parse("~a & ~b & c & (~x | y) & (~w | z)"));
          assertThat(p.parse("~(a & b) | c | ~(x | ~y) | (w => z)").nnf()).isEqualTo(p.parse("~a  | ~b | c | (~x & y) | (~w | z)"));
      }
  
      @Test
      public void testNot() throws ParserException {
          final PropositionalParser p = new PropositionalParser(this.f);
          assertThat(p.parse("~a").nnf()).isEqualTo(p.parse("~a"));
          assertThat(p.parse("~~a").nnf()).isEqualTo(p.parse("a"));
          assertThat(p.parse("~(a => b)").nnf()).isEqualTo(p.parse("a & ~b"));
          assertThat(p.parse("~(~(a | b) => ~(x | y))").nnf()).isEqualTo(p.parse("~a & ~b & (x | y)"));
          assertThat(p.parse("a <=> b").nnf()).isEqualTo(p.parse("(~a | b) & (~b | a)"));
          assertThat(p.parse("~(a <=> b)").nnf()).isEqualTo(p.parse("(~a | ~b) & (a | b)"));
          assertThat(p.parse("~(~(a | b) <=> ~(x | y))").nnf()).isEqualTo(p.parse("((a | b) | (x | y)) & ((~a & ~b) | (~x & ~y))"));
          assertThat(p.parse("~(a & b & ~x & ~y)").nnf()).isEqualTo(p.parse("~a | ~b | x | y"));
          assertThat(p.parse("~(a | b | ~x | ~y)").nnf()).isEqualTo(p.parse("~a & ~b & x & y"));
          assertThat(p.parse("~(a | b | ~x | ~y)").nnf()).isEqualTo(p.parse("~a & ~b & x & y"));
      }
  }
  