/*

  * 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.nabla.differences;

 import org.apache.commons.nabla.core.DifferentialPair;

 import org.apache.commons.nabla.core.UnivariateDerivative;

 import org.apache.commons.nabla.core.UnivariateDifferentiable;

 /** Two-points finite differences scheme.

  * The error model for the two-points scheme is

  * <code>h<sup>2</sup>/6 f<sup>(3)</sup>(x) + O(h<sup>4</sup>)</code>.

  */

 public class TwoPointsScheme extends FiniteDifferencesDifferentiator {

     /** Serializable UID. */

     private static final long serialVersionUID = -5824475098109485044L;

     /** Scheme denominator. */

     private final double denominator;

     /** Build a 2-points finite differences scheme.

      * @param h differences step size

      */

     public TwoPointsScheme(final double h) {

         super(h, h * h / 6, 3);

         denominator = 2 * h;

     }

     /** {@inheritDoc} */

     public UnivariateDerivative differentiate(final UnivariateDifferentiable d) {

         return new UnivariateDerivative() {

             public UnivariateDifferentiable getPrimitive() {

                 return d;

             }

             public DifferentialPair f(final DifferentialPair t) {

                 final double h = getStepSize();

                 final double u0 = t.getValue();

                 final double ft = d.f(u0);

                 final double d1 = d.f(u0 + h) - d.f(u0 - h);

                 return new DifferentialPair(ft, t.getFirstDerivative() * d1 / denominator);

             }

         };

     }

 }