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.functions;
  
  import static org.assertj.core.api.Assertions.assertThat;
  
  import org.junit.jupiter.api.Test;
  import org.logicng.TestWithExampleFormulas;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaFactory;
  import org.logicng.formulas.Variable;
  import org.logicng.formulas.cache.FunctionCacheEntry;
  import org.logicng.io.parsers.ParserException;
  
  /**
   * Unit Tests for the class {@link FormulaDepthFunction}.
   * @version 2.0.0
   * @since 2.0.0
   */
  public class FormulaDepthFunctionTest extends TestWithExampleFormulas {
  
      @Test
      public void testAtoms() {
          assertThat(this.TRUE.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.FALSE.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.A.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.NA.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.PBC1.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.PBC2.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.PBC3.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.PBC4.apply(FormulaDepthFunction.get())).isEqualTo(0);
          assertThat(this.PBC5.apply(FormulaDepthFunction.get())).isEqualTo(0);
      }
  
      @Test
      public void testDeepFormulas() {
          assertThat(this.AND1.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.AND2.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.AND3.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.OR1.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.OR2.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.OR3.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.NOT1.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.NOT2.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.IMP1.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.IMP2.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.IMP3.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.IMP4.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.EQ1.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.EQ2.apply(FormulaDepthFunction.get())).isEqualTo(1);
          assertThat(this.EQ3.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(this.EQ4.apply(FormulaDepthFunction.get())).isEqualTo(2);
      }
  
      @Test
      public void testDeeperFormulas() {
          Formula formula = this.PBC1;
          for (int i = 0; i < 10; i++) {
              final Variable var = this.f.variable("X" + i);
              formula = i % 2 == 0 ? this.f.or(formula, var) : this.f.and(formula, var);
          }
          assertThat(formula.apply(FormulaDepthFunction.get())).isEqualTo(10);
      }
  
      @Test
      public void testCache() throws ParserException {
          final FormulaFactory f = new FormulaFactory();
          final Formula formula = f.parse("A & B | C");
          assertThat(formula.functionCacheEntry(FunctionCacheEntry.DEPTH)).isNull();
          assertThat(formula.apply(FormulaDepthFunction.get())).isEqualTo(2);
          assertThat(formula.functionCacheEntry(FunctionCacheEntry.DEPTH)).isEqualTo(2);
          assertThat(f.variable("A").functionCacheEntry(FunctionCacheEntry.DEPTH)).isEqualTo(0);
  
          formula.setFunctionCacheEntry(FunctionCacheEntry.DEPTH, 3);
          assertThat(formula.apply(FormulaDepthFunction.get())).isEqualTo(3);
      }
  }
  