/*
    * 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.nonstiff;
  
  import org.apache.commons.math4.legacy.ode.sampling.StepInterpolator;
  
  /**
   * This class represents an interpolator over the last step during an
   * ODE integration for the 5(4) Higham and Hall integrator.
   *
   * @see HighamHall54Integrator
   *
   * @since 1.2
   */
  
  class HighamHall54StepInterpolator
    extends RungeKuttaStepInterpolator {
  
    /** Serializable version identifier. */
    private static final long serialVersionUID = 20111120L;
  
    /** Simple constructor.
     * This constructor builds an instance that is not usable yet, the
     * {@link
     * org.apache.commons.math4.legacy.ode.sampling.AbstractStepInterpolator#reinitialize}
     * method should be called before using the instance in order to
     * initialize the internal arrays. This constructor is used only
     * in order to delay the initialization in some cases. The {@link
     * EmbeddedRungeKuttaIntegrator} uses the prototyping design pattern
     * to create the step interpolators by cloning an uninitialized model
     * and later initializing the copy.
     */
    // CHECKSTYLE: stop RedundantModifier
    // the public modifier here is needed for serialization
    public HighamHall54StepInterpolator() {
      super();
    }
    // CHECKSTYLE: resume RedundantModifier
  
    /** Copy constructor.
     * @param interpolator interpolator to copy from. The copy is a deep
     * copy: its arrays are separated from the original arrays of the
     * instance
     */
    HighamHall54StepInterpolator(final HighamHall54StepInterpolator interpolator) {
      super(interpolator);
    }
  
    /** {@inheritDoc} */
    @Override
    protected StepInterpolator doCopy() {
      return new HighamHall54StepInterpolator(this);
    }
  
  
    /** {@inheritDoc} */
    @Override
    protected void computeInterpolatedStateAndDerivatives(final double theta,
                                            final double oneMinusThetaH) {
  
      final double bDot0 = 1 + theta * (-15.0/2.0 + theta * (16.0 - 10.0 * theta));
      final double bDot2 = theta * (459.0/16.0 + theta * (-729.0/8.0 + 135.0/2.0 * theta));
      final double bDot3 = theta * (-44.0 + theta * (152.0 - 120.0 * theta));
      final double bDot4 = theta * (375.0/16.0 + theta * (-625.0/8.0 + 125.0/2.0 * theta));
      final double bDot5 = theta * 5.0/8.0 * (2 * theta - 1);
  
      if (previousState != null && theta <= 0.5) {
          final double hTheta = h * theta;
          final double b0 = hTheta * (1.0 + theta * (-15.0/4.0  + theta * (16.0/3.0 - 5.0/2.0 * theta)));
          final double b2 = hTheta * (      theta * (459.0/32.0 + theta * (-243.0/8.0 + theta * 135.0/8.0)));
          final double b3 = hTheta * (      theta * (-22.0      + theta * (152.0/3.0  + theta * -30.0)));
          final double b4 = hTheta * (      theta * (375.0/32.0 + theta * (-625.0/24.0 + theta * 125.0/8.0)));
          final double b5 = hTheta * (      theta * (-5.0/16.0  + theta *  5.0/12.0));
          for (int i = 0; i < interpolatedState.length; ++i) {
              final double yDot0 = yDotK[0][i];
              final double yDot2 = yDotK[2][i];
              final double yDot3 = yDotK[3][i];
              final double yDot4 = yDotK[4][i];
              final double yDot5 = yDotK[5][i];
              interpolatedState[i] =
                      previousState[i] + b0 * yDot0 + b2 * yDot2 + b3 * yDot3 + b4 * yDot4 + b5 * yDot5;
              interpolatedDerivatives[i] =
                      bDot0 * yDot0 + bDot2 * yDot2 + bDot3 * yDot3 + bDot4 * yDot4 + bDot5 * yDot5;
          }
     } else {
         final double theta2 = theta * theta;
         final double b0 = h * (-1.0/12.0   + theta  * (1.0        + theta * (-15.0/4.0   + theta * (16.0/3.0 + theta * -5.0/2.0))));
         final double b2 = h * (-27.0/32.0  + theta2 * (459.0/32.0 + theta * (-243.0/8.0  + theta * 135.0/8.0)));
         final double b3 = h * (4.0/3.0     + theta2 * (-22.0      + theta * (152.0/3.0   + theta * -30.0)));
         final double b4 = h * (-125.0/96.0 + theta2 * (375.0/32.0 + theta * (-625.0/24.0 + theta * 125.0/8.0)));
         final double b5 = h * (-5.0/48.0   + theta2 * (-5.0/16.0  + theta *  5.0/12.0));
         for (int i = 0; i < interpolatedState.length; ++i) {
             final double yDot0 = yDotK[0][i];
             final double yDot2 = yDotK[2][i];
             final double yDot3 = yDotK[3][i];
             final double yDot4 = yDotK[4][i];
             final double yDot5 = yDotK[5][i];
             interpolatedState[i] =
                     currentState[i] + b0 * yDot0 + b2 * yDot2 + b3 * yDot3 + b4 * yDot4 + b5 * yDot5;
             interpolatedDerivatives[i] =
                     bDot0 * yDot0 + bDot2 * yDot2 + bDot3 * yDot3 + bDot4 * yDot4 + bDot5 * yDot5;
         }
     }
   }
 }