Check-in commons-math 3.6.1 am: 1354beaf45 am: 0018f64b87 am: b3715644fb

Original change: https://android-review.googlesource.com/c/platform/external/apache-commons-math/+/2702413

Change-Id: I5ad9b2a0822d668b5b6a62933c6d4c1f0b802001
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

1 files changed, 136 insertions, 0 deletions

diff --git a/src/main/java/org/apache/commons/math3/ode/nonstiff/ClassicalRungeKuttaFieldStepInterpolator.java b/src/main/java/org/apache/commons/math3/ode/nonstiff/ClassicalRungeKuttaFieldStepInterpolator.java
new file mode 100644
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+++ b/src/main/java/org/apache/commons/math3/ode/nonstiff/ClassicalRungeKuttaFieldStepInterpolator.java
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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;
+
+/**
+ * This class implements a step interpolator for the classical fourth
+ * order Runge-Kutta integrator.
+ *
+ * <p>This interpolator allows to compute dense output inside the last
+ * step computed. The interpolation equation is consistent with the
+ * integration scheme :
+ * <ul>
+ *   <li>Using reference point at step start:<br>
+ *   y(t<sub>n</sub> + &theta; h) = y (t<sub>n</sub>)
+ *                    + &theta; (h/6) [  (6 - 9 &theta; + 4 &theta;<sup>2</sup>) y'<sub>1</sub>
+ *                                     + (    6 &theta; - 4 &theta;<sup>2</sup>) (y'<sub>2</sub> + y'<sub>3</sub>)
+ *                                     + (   -3 &theta; + 4 &theta;<sup>2</sup>) y'<sub>4</sub>
+ *                                    ]
+ *   </li>
+ *   <li>Using reference point at step end:<br>
+ *   y(t<sub>n</sub> + &theta; h) = y (t<sub>n</sub> + h)
+ *                    + (1 - &theta;) (h/6) [ (-4 &theta;^2 + 5 &theta; - 1) y'<sub>1</sub>
+ *                                          +(4 &theta;^2 - 2 &theta; - 2) (y'<sub>2</sub> + y'<sub>3</sub>)
+ *                                          -(4 &theta;^2 +   &theta; + 1) y'<sub>4</sub>
+ *                                        ]
+ *   </li>
+ * </ul>
+ * </p>
+ *
+ * where &theta; belongs to [0 ; 1] and where y'<sub>1</sub> to y'<sub>4</sub> are the four
+ * evaluations of the derivatives already computed during the
+ * step.</p>
+ *
+ * @see ClassicalRungeKuttaFieldIntegrator
+ * @param <T> the type of the field elements
+ * @since 3.6
+ */
+
+class ClassicalRungeKuttaFieldStepInterpolator<T extends RealFieldElement<T>>
+    extends RungeKuttaFieldStepInterpolator<T> {
+
+    /** Simple constructor.
+     * @param field field to which the time and state vector elements belong
+     * @param forward integration direction indicator
+     * @param yDotK slopes at the intermediate points
+     * @param globalPreviousState start of the global step
+     * @param globalCurrentState end of the global step
+     * @param softPreviousState start of the restricted step
+     * @param softCurrentState end of the restricted step
+     * @param mapper equations mapper for the all equations
+     */
+    ClassicalRungeKuttaFieldStepInterpolator(final Field<T> field, final boolean forward,
+                                             final T[][] yDotK,
+                                             final FieldODEStateAndDerivative<T> globalPreviousState,
+                                             final FieldODEStateAndDerivative<T> globalCurrentState,
+                                             final FieldODEStateAndDerivative<T> softPreviousState,
+                                             final FieldODEStateAndDerivative<T> softCurrentState,
+                                             final FieldEquationsMapper<T> mapper) {
+        super(field, forward, yDotK,
+              globalPreviousState, globalCurrentState, softPreviousState, softCurrentState,
+              mapper);
+    }
+
+    /** {@inheritDoc} */
+    @Override
+    protected ClassicalRungeKuttaFieldStepInterpolator<T> create(final Field<T> newField, final boolean newForward, final T[][] newYDotK,
+                                                                 final FieldODEStateAndDerivative<T> newGlobalPreviousState,
+                                                                 final FieldODEStateAndDerivative<T> newGlobalCurrentState,
+                                                                 final FieldODEStateAndDerivative<T> newSoftPreviousState,
+                                                                 final FieldODEStateAndDerivative<T> newSoftCurrentState,
+                                                                 final FieldEquationsMapper<T> newMapper) {
+        return new ClassicalRungeKuttaFieldStepInterpolator<T>(newField, newForward, newYDotK,
+                                                               newGlobalPreviousState, newGlobalCurrentState,
+                                                               newSoftPreviousState, newSoftCurrentState,
+                                                               newMapper);
+    }
+
+    /** {@inheritDoc} */
+    @SuppressWarnings("unchecked")
+    @Override
+    protected FieldODEStateAndDerivative<T> computeInterpolatedStateAndDerivatives(final FieldEquationsMapper<T> mapper,
+                                                                                   final T time, final T theta,
+                                                                                   final T thetaH, final T oneMinusThetaH) {
+
+        final T one                       = time.getField().getOne();
+        final T oneMinusTheta             = one.subtract(theta);
+        final T oneMinus2Theta            = one.subtract(theta.multiply(2));
+        final T coeffDot1                 = oneMinusTheta.multiply(oneMinus2Theta);
+        final T coeffDot23                = theta.multiply(oneMinusTheta).multiply(2);
+        final T coeffDot4                 = theta.multiply(oneMinus2Theta).negate();
+        final T[] interpolatedState;
+        final T[] interpolatedDerivatives;
+
+        if (getGlobalPreviousState() != null && theta.getReal() <= 0.5) {
+            final T fourTheta2      = theta.multiply(theta).multiply(4);
+            final T s               = thetaH.divide(6.0);
+            final T coeff1          = s.multiply(fourTheta2.subtract(theta.multiply(9)).add(6));
+            final T coeff23         = s.multiply(theta.multiply(6).subtract(fourTheta2));
+            final T coeff4          = s.multiply(fourTheta2.subtract(theta.multiply(3)));
+            interpolatedState       = previousStateLinearCombination(coeff1, coeff23, coeff23, coeff4);
+            interpolatedDerivatives = derivativeLinearCombination(coeffDot1, coeffDot23, coeffDot23, coeffDot4);
+        } else {
+            final T fourTheta       = theta.multiply(4);
+            final T s               = oneMinusThetaH.divide(6);
+            final T coeff1          = s.multiply(theta.multiply(fourTheta.negate().add(5)).subtract(1));
+            final T coeff23         = s.multiply(theta.multiply(fourTheta.subtract(2)).subtract(2));
+            final T coeff4          = s.multiply(theta.multiply(fourTheta.negate().subtract(1)).subtract(1));
+            interpolatedState       = currentStateLinearCombination(coeff1, coeff23, coeff23, coeff4);
+            interpolatedDerivatives = derivativeLinearCombination(coeffDot1, coeffDot23, coeffDot23, coeffDot4);
+        }
+
+        return new FieldODEStateAndDerivative<T>(time, interpolatedState, interpolatedDerivatives);
+
+    }
+
+}