/*
    * 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
    *
    *      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.apache.commons.math4.legacy.ode;
  
  import org.apache.commons.math4.legacy.exception.DimensionMismatchException;
  import org.apache.commons.math4.legacy.exception.MaxCountExceededException;
  import org.apache.commons.math4.legacy.exception.NoBracketingException;
  import org.apache.commons.math4.legacy.exception.NumberIsTooSmallException;
  import org.apache.commons.math4.legacy.ode.nonstiff.ClassicalRungeKuttaIntegrator;
  import org.apache.commons.math4.core.jdkmath.JdkMath;
  import org.junit.Assert;
  import org.junit.Test;
  
  
  public class FirstOrderConverterTest {
  
    @Test
    public void testDoubleDimension() {
      for (int i = 1; i < 10; ++i) {
        SecondOrderDifferentialEquations eqn2 = new Equations(i, 0.2);
        FirstOrderConverter eqn1 = new FirstOrderConverter(eqn2);
        Assert.assertEquals(eqn1.getDimension(), 2 * eqn2.getDimension());
      }
    }
  
    @Test
    public void testDecreasingSteps()
        throws DimensionMismatchException, NumberIsTooSmallException, MaxCountExceededException, NoBracketingException {
  
      double previousError = Double.NaN;
      for (int i = 0; i < 10; ++i) {
  
        double step  = JdkMath.pow(2.0, -(i + 1));
        double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, step)
                     - JdkMath.sin(4.0);
        if (i > 0) {
          Assert.assertTrue(JdkMath.abs(error) < JdkMath.abs(previousError));
        }
        previousError = error;
      }
    }
  
    @Test
    public void testSmallStep()
        throws DimensionMismatchException, NumberIsTooSmallException, MaxCountExceededException, NoBracketingException {
      double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, 1.0e-4)
                     - JdkMath.sin(4.0);
      Assert.assertTrue(JdkMath.abs(error) < 1.0e-10);
    }
  
    @Test
    public void testBigStep()
        throws DimensionMismatchException, NumberIsTooSmallException, MaxCountExceededException, NoBracketingException {
      double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, 0.5)
                     - JdkMath.sin(4.0);
      Assert.assertTrue(JdkMath.abs(error) > 0.1);
    }
  
    private static final class Equations
      implements SecondOrderDifferentialEquations {
  
       private int n;
  
        private double omega2;
  
        Equations(int n, double omega) {
          this.n = n;
          omega2 = omega * omega;
        }
  
        @Override
      public int getDimension() {
          return n;
        }
  
        @Override
      public void computeSecondDerivatives(double t, double[] y, double[] yDot,
                                             double[] yDDot) {
          for (int i = 0; i < n; ++i) {
            yDDot[i] = -omega2 * y[i];
          }
      }
    }
  
   private double integrateWithSpecifiedStep(double omega,
                                             double t0, double t,
                                             double step) throws DimensionMismatchException, NumberIsTooSmallException, MaxCountExceededException, NoBracketingException {
     double[] y0 = new double[2];
     y0[0] = JdkMath.sin(omega * t0);
     y0[1] = omega * JdkMath.cos(omega * t0);
     ClassicalRungeKuttaIntegrator i = new ClassicalRungeKuttaIntegrator(step);
     double[] y = new double[2];
     i.integrate(new FirstOrderConverter(new Equations(1, omega)), t0, y0, t, y);
     return y[0];
   }
 }