/* Copyright 2002-2026 CS GROUP
    * Licensed to CS GROUP (CS) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
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    * CS 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
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  package org.orekit.frames;
  
  import java.util.HashMap;
  import java.util.Map;
  
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.Field;
  import org.hipparchus.geometry.euclidean.threed.FieldRotation;
  import org.hipparchus.geometry.euclidean.threed.Rotation;
  import org.hipparchus.geometry.euclidean.threed.RotationConvention;
  import org.hipparchus.geometry.euclidean.threed.RotationOrder;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.time.TimeScalarFunction;
  import org.orekit.time.TimeScales;
  import org.orekit.time.TimeVectorFunction;
  import org.orekit.utils.IERSConventions;
  
  /** Mean Equator, Mean Equinox Frame.
   * <p>This frame handles precession effects according to to selected IERS conventions.</p>
   * <p>Its parent frame is the GCRF frame.
   * <p>It is sometimes called Mean of Date (MoD) frame.
   * @author Pascal Parraud
   */
  class MODProvider implements FieldBasedTransformProvider {
  
      /** Function computing the precession angles. */
      private final TimeVectorFunction precessionFunction;
  
      /** Constant rotation between ecliptic and equator poles at J2000.0. */
      private final Rotation r4;
  
      /** Constant rotations between ecliptic and equator poles at J2000.0. */
      private final Map<Field<? extends CalculusFieldElement<?>>, FieldRotation<? extends CalculusFieldElement<?>>> fieldR4;
  
      /** Simple constructor.
       * @param conventions IERS conventions to apply
       * @param timeScales used to define this frame.
       */
      MODProvider(final IERSConventions conventions, final TimeScales timeScales) {
          this.precessionFunction = conventions.getPrecessionFunction(timeScales);
          final TimeScalarFunction epsilonAFunction =
                  conventions.getMeanObliquityFunction(timeScales);
          final AbsoluteDate date0 = conventions.getNutationReferenceEpoch(timeScales);
          final double epsilon0 = epsilonAFunction.value(date0);
          r4 = new Rotation(Vector3D.PLUS_I, epsilon0, RotationConvention.FRAME_TRANSFORM);
          fieldR4 = new HashMap<>();
      }
  
      /** Compute the complete precession rotation.
       * @param date current date
       * @param <T> type of the field elements
       * @return complete precession rotation
       */
      public <T extends CalculusFieldElement<T>> FieldRotation<T> getRotation(final FieldAbsoluteDate<T> date) {
  
          // compute the precession angles phiA, omegaA, chiA
          final T[] angles = precessionFunction.value(date);
  
          // complete precession
          return getR4(date.getField()).compose(
                          new FieldRotation<>(RotationOrder.ZXZ, RotationConvention.FRAME_TRANSFORM,
                                              angles[0].negate(), angles[1].negate(), angles[2]),
                          RotationConvention.FRAME_TRANSFORM);
  
      }
  
      /** Get the constant rotation converted to the specified field.
       * @param field field to which the elements belong
       * @param <T> type of the field elements
       * @return constant rotation converted to the specified field
       */
      @SuppressWarnings("unchecked")
      private <T extends CalculusFieldElement<T>> FieldRotation<T> getR4(final Field<T> field) {
          synchronized (fieldR4) {
              return (FieldRotation<T>) fieldR4.computeIfAbsent(field, f -> new FieldRotation<>(field, r4));
          }
      }
  
  }