/*
 * 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);

	}

}
