/*
 * 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.math.ode.nonstiff;

import org.apache.commons.math.ode.DerivativeException;
import org.apache.commons.math.ode.sampling.AbstractStepInterpolator;
import org.apache.commons.math.ode.sampling.StepInterpolator;

/**
 * This class implements a linear interpolator for step.
 *
 * <p>This interpolator computes dense output inside the last
 * step computed. The interpolation equation is consistent with the
 * integration scheme :
 *
 * <pre>
 *   y(t_n + theta h) = y (t_n + h) - (1-theta) h y'
 * </pre>
 *
 * where theta belongs to [0 ; 1] and where y' is the evaluation of
 * the derivatives already computed during the step.</p>
 *
 * @see EulerIntegrator
 * @version $Revision$ $Date$
 * @since 1.2
 */

class EulerStepInterpolator
  extends RungeKuttaStepInterpolator {

  /** Serializable version identifier */
  private static final long serialVersionUID = -7179861704951334960L;

  /** Simple constructor.
   * This constructor builds an instance that is not usable yet, the
   * {@link 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 RungeKuttaIntegrator} class uses the
   * prototyping design pattern to create the step interpolators by
   * cloning an uninitialized model and latter initializing the copy.
   */
  public EulerStepInterpolator() {
  }

  /** 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
   */
  public EulerStepInterpolator(final EulerStepInterpolator interpolator) {
    super(interpolator);
  }

  /** {@inheritDoc} */
  @Override
  protected StepInterpolator doCopy() {
    return new EulerStepInterpolator(this);
  }


  /** {@inheritDoc} */
  @Override
  protected void computeInterpolatedStateAndDerivatives(final double theta,
                                          final double oneMinusThetaH)
    throws DerivativeException {

    for (int i = 0; i < interpolatedState.length; ++i) {
      interpolatedState[i] = currentState[i] - oneMinusThetaH * yDotK[0][i];
    }
    System.arraycopy(yDotK[0], 0, interpolatedDerivatives, 0, interpolatedDerivatives.length);

  }

}
