/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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
    *
    *      https://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.
   */
  
  /*
   * This is not the original file distributed by the Apache Software Foundation
   * It has been modified by the Hipparchus project
   */
  
  package org.hipparchus.ode;
  
  import org.hipparchus.exception.MathIllegalArgumentException;
  import org.hipparchus.exception.MathIllegalStateException;
  import org.hipparchus.ode.nonstiff.ClassicalRungeKuttaIntegrator;
  import org.hipparchus.util.FastMath;
  import org.junit.jupiter.api.Test;
  
  import static org.junit.jupiter.api.Assertions.assertEquals;
  import static org.junit.jupiter.api.Assertions.assertTrue;
  
  
  class FirstOrderConverterTest {
  
      @Test
      void testDoubleDimension() {
          for (int i = 1; i < 10; ++i) {
              SecondOrderODE eqn2 = new Equations(i, 0.2);
              FirstOrderConverter eqn1 = new FirstOrderConverter(eqn2);
              assertEquals(eqn1.getDimension(), (2 * eqn2.getDimension()));
          }
      }
  
      @Test
      void testDecreasingSteps()
          throws MathIllegalArgumentException, MathIllegalStateException {
  
          double previousError = Double.NaN;
          for (int i = 0; i < 10; ++i) {
  
              double step  = FastMath.pow(2.0, -(i + 1));
              double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, step)
                              - FastMath.sin(4.0);
              if (i > 0) {
                  assertTrue(FastMath.abs(error) < FastMath.abs(previousError));
              }
              previousError = error;
  
          }
      }
  
      @Test
      void testSmallStep()
          throws MathIllegalArgumentException, MathIllegalStateException {
          double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, 1.0e-4)
                          - FastMath.sin(4.0);
          assertTrue(FastMath.abs(error) < 1.0e-10);
      }
  
      @Test
      void testBigStep()
          throws MathIllegalArgumentException, MathIllegalStateException {
          double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, 0.5)
                          - FastMath.sin(4.0);
          assertTrue(FastMath.abs(error) > 0.1);
      }
  
      private static class Equations
      implements SecondOrderODE {
  
          private int n;
  
          private double omega2;
  
          public Equations(int n, double omega) {
              this.n = n;
              omega2 = omega * omega;
          }
  
          public int getDimension() {
              return n;
          }
  
          public double[] computeSecondDerivatives(double t, double[] y, double[] yDot) {
              final double[] yDDot = new double[n];
              for (int i = 0; i < n; ++i) {
                  yDDot[i] = -omega2 * y[i];
             }
             return yDDot;
         }
 
     }
 
     private double integrateWithSpecifiedStep(double omega,
                                               double t0, double t,
                                               double step) throws MathIllegalArgumentException, MathIllegalStateException {
         double[] y0 = new double[2];
         y0[0] = FastMath.sin(omega * t0);
         y0[1] = omega * FastMath.cos(omega * t0);
         ClassicalRungeKuttaIntegrator i = new ClassicalRungeKuttaIntegrator(step);
         final ODEStateAndDerivative finalstate =
                         i.integrate(new FirstOrderConverter(new Equations(1, omega)), new ODEState(t0, y0), t);
         return finalstate.getPrimaryState()[0];
     }
 
 }