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+/*
+ * 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;
+
+import org.apache.commons.math3.Field;
+import org.apache.commons.math3.RealFieldElement;
+import org.apache.commons.math3.ode.FieldEquationsMapper;
+import org.apache.commons.math3.ode.FieldODEStateAndDerivative;
+import org.apache.commons.math3.util.MathArrays;
+
+
+/**
+ * This class implements the Gill fourth order Runge-Kutta
+ * integrator for Ordinary Differential Equations .
+
+ * <p>This method is an explicit Runge-Kutta method, its Butcher-array
+ * is the following one :
+ * <pre>
+ * 0 | 0 0 0 0
+ * 1/2 | 1/2 0 0 0
+ * 1/2 | (q-1)/2 (2-q)/2 0 0
+ * 1 | 0 -q/2 (2+q)/2 0
+ * |-------------------------------
+ * | 1/6 (2-q)/6 (2+q)/6 1/6
+ * </pre>
+ * where q = sqrt(2)</p>
+ *
+ * @see EulerFieldIntegrator
+ * @see ClassicalRungeKuttaFieldIntegrator
+ * @see MidpointFieldIntegrator
+ * @see ThreeEighthesFieldIntegrator
+ * @see LutherFieldIntegrator
+ * @param <T> the type of the field elements
+ * @since 3.6
+ */
+
+public class GillFieldIntegrator<T extends RealFieldElement<T>>
+ extends RungeKuttaFieldIntegrator<T> {
+
+ /** Simple constructor.
+ * Build a fourth-order Gill integrator with the given step.
+ * @param field field to which the time and state vector elements belong
+ * @param step integration step
+ */
+ public GillFieldIntegrator(final Field<T> field, final T step) {
+ super(field, "Gill", step);
+ }
+
+ /** {@inheritDoc} */
+ public T[] getC() {
+ final T[] c = MathArrays.buildArray(getField(), 3);
+ c[0] = fraction(1, 2);
+ c[1] = c[0];
+ c[2] = getField().getOne();
+ return c;
+ }
+
+ /** {@inheritDoc} */
+ public T[][] getA() {
+
+ final T two = getField().getZero().add(2);
+ final T sqrtTwo = two.sqrt();
+
+ final T[][] a = MathArrays.buildArray(getField(), 3, -1);
+ for (int i = 0; i < a.length; ++i) {
+ a[i] = MathArrays.buildArray(getField(), i + 1);
+ }
+ a[0][0] = fraction(1, 2);
+ a[1][0] = sqrtTwo.subtract(1).multiply(0.5);
+ a[1][1] = sqrtTwo.subtract(2).multiply(-0.5);
+ a[2][0] = getField().getZero();
+ a[2][1] = sqrtTwo.multiply(-0.5);
+ a[2][2] = sqrtTwo.add(2).multiply(0.5);
+ return a;
+ }
+
+ /** {@inheritDoc} */
+ public T[] getB() {
+
+ final T two = getField().getZero().add(2);
+ final T sqrtTwo = two.sqrt();
+
+ final T[] b = MathArrays.buildArray(getField(), 4);
+ b[0] = fraction(1, 6);
+ b[1] = sqrtTwo.subtract(2).divide(-6);
+ b[2] = sqrtTwo.add(2).divide(6);
+ b[3] = b[0];
+
+ return b;
+
+ }
+
+ /** {@inheritDoc} */
+ @Override
+ protected GillFieldStepInterpolator<T>
+ createInterpolator(final boolean forward, T[][] yDotK,
+ final FieldODEStateAndDerivative<T> globalPreviousState,
+ final FieldODEStateAndDerivative<T> globalCurrentState,
+ final FieldEquationsMapper<T> mapper) {
+ return new GillFieldStepInterpolator<T>(getField(), forward, yDotK,
+ globalPreviousState, globalCurrentState,
+ globalPreviousState, globalCurrentState,
+ mapper);
+ }
+
+}