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With inspiration from other libraries

There is a newer version: 2.12.15

/*
 * 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.math3.ode.nonstiff;


/**
 * This class implements a second order Runge-Kutta integrator for
 * Ordinary Differential Equations.
 *
 * This method is an explicit Runge-Kutta method, its Butcher-array
 * is the following one :
 * 
 *    0  |  0    0
 *   1/2 | 1/2   0
 *       |----------
 *       |  0    1
 * 
 * 
 *
 * @see EulerIntegrator
 * @see ClassicalRungeKuttaIntegrator
 * @see GillIntegrator
 * @see ThreeEighthesIntegrator
 * @see LutherIntegrator
 *
 * @since 1.2
 */

public class MidpointIntegrator extends RungeKuttaIntegrator {

  /** Time steps Butcher array. */
  private static final double[] STATIC_C = {
    1.0 / 2.0
  };

  /** Internal weights Butcher array. */
  private static final double[][] STATIC_A = {
    { 1.0 / 2.0 }
  };

  /** Propagation weights Butcher array. */
  private static final double[] STATIC_B = {
    0.0, 1.0
  };

  /** Simple constructor.
   * Build a midpoint integrator with the given step.
   * @param step integration step
   */
  public MidpointIntegrator(final double step) {
    super("midpoint", STATIC_C, STATIC_A, STATIC_B, new MidpointStepInterpolator(), step);
  }

}