/* Copyright 2002-2021 CS GROUP
    * Licensed to CS GROUP (CS) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * 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
   * limitations under the License.
   */
  package org.orekit.forces.gravity;
  
  import java.util.List;
  
  import org.hipparchus.util.FastMath;
  import org.orekit.data.BodiesElements;
  import org.orekit.data.FundamentalNutationArguments;
  import org.orekit.forces.gravity.potential.NormalizedSphericalHarmonicsProvider;
  import org.orekit.forces.gravity.potential.OceanTidesWave;
  import org.orekit.forces.gravity.potential.TideSystem;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.TimeVectorFunction;
  
  /** Gravity field corresponding to ocean tides.
   * <p>
   * This ocean tides force model implementation corresponds to the method described
   * in <a href="http://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html">
   * IERS conventions (2010)</a>, chapter 6, section 6.3.
   * </p>
   * <p>
   * Note that this class is <em>not</em> thread-safe, and that tides computation
   * are computer intensive if repeated. So this class is really expected to
   * be wrapped within a {@link
   * org.orekit.forces.gravity.potential.CachedNormalizedSphericalHarmonicsProvider}
   * that will both ensure thread safety and improve performances using caching.
   * </p>
   * @see OceanTides
   * @author Luc Maisonobe
   * @since 6.1
   */
  class OceanTidesField implements NormalizedSphericalHarmonicsProvider {
  
      /** Maximum degree. */
      private final int degree;
  
      /** Maximum order. */
      private final int order;
  
      /** Central body reference radius. */
      private final double ae;
  
      /** Central body attraction coefficient. */
      private final double mu;
  
      /** Tides model. */
      private final List<OceanTidesWave> waves;
  
      /** Object computing the fundamental arguments. */
      private final FundamentalNutationArguments arguments;
  
      /** Function computing pole tide terms (ΔC₂₁, ΔS₂₁). */
      private final TimeVectorFunction poleTideFunction;
  
      /** Simple constructor.
       * @param ae central body reference radius
       * @param mu central body attraction coefficient
       * @param waves ocean tides waves
       * @param arguments object computing the fundamental arguments
       * @param poleTideFunction function computing ocean pole tide terms (ΔC₂₁, ΔS₂₁), may be null
       */
      OceanTidesField(final double ae, final double mu,
                             final List<OceanTidesWave> waves,
                             final FundamentalNutationArguments arguments,
                             final TimeVectorFunction poleTideFunction) {
  
          // store mode parameters
          this.ae  = ae;
          this.mu  = mu;
  
          // waves
          this.waves = waves;
          int m = 0;
          int n = 0;
          for (final OceanTidesWave wave : waves) {
              m = FastMath.max(m, wave.getMaxDegree());
              n = FastMath.max(n, wave.getMaxOrder());
          }
          degree = m;
          order  = n;
  
          this.arguments = arguments;
  
          // pole tide
         this.poleTideFunction = poleTideFunction;
 
     }
 
     /** {@inheritDoc} */
     @Override
     public int getMaxDegree() {
         return degree;
     }
 
     /** {@inheritDoc} */
     @Override
     public int getMaxOrder() {
         return order;
     }
 
     /** {@inheritDoc} */
     @Override
     public double getMu() {
         return mu;
     }
 
     /** {@inheritDoc} */
     @Override
     public double getAe() {
         return ae;
     }
 
     /** {@inheritDoc} */
     @Override
     public AbsoluteDate getReferenceDate() {
         return AbsoluteDate.ARBITRARY_EPOCH;
     }
 
     /** {@inheritDoc} */
     @Override
     public double getOffset(final AbsoluteDate date) {
         return date.durationFrom(getReferenceDate());
     }
 
     /** {@inheritDoc} */
     @Override
     public TideSystem getTideSystem() {
         // not really used here, but for consistency we can state that either
         // we add the permanent tide or it was already in the central attraction
         return TideSystem.ZERO_TIDE;
     }
 
     /** {@inheritDoc} */
     @Override
     public NormalizedSphericalHarmonics onDate(final AbsoluteDate date) {
 
         // computed Cnm and Snm coefficients
         final int        rows = degree + 1;
         final double[][] cnm  = new double[rows][];
         final double[][] snm  = new double[rows][];
         for (int i = 0; i <= degree; ++i) {
             final int m = FastMath.min(i, order) + 1;
             cnm[i] = new double[m];
             snm[i] = new double[m];
         }
 
         final BodiesElements bodiesElements = arguments.evaluateAll(date);
         for (final OceanTidesWave wave : waves) {
             wave.addContribution(bodiesElements, cnm, snm);
         }
 
         if (poleTideFunction != null && degree > 1 && order > 0) {
             // add pole tide
             poleTide(date, cnm, snm);
         }
 
         return new TideHarmonics(date, cnm, snm);
 
     }
 
     /** Update coefficients applying pole tide.
      * @param date current date
      * @param cnm the Cnm coefficients. Modified in place.
      * @param snm the Snm coefficients. Modified in place.
      */
     private void poleTide(final AbsoluteDate date, final double[][] cnm, final double[][] snm) {
         final double[] deltaCS = poleTideFunction.value(date);
         cnm[2][1] += deltaCS[0]; // ΔC₂₁
         snm[2][1] += deltaCS[1]; // ΔS₂₁
     }
 
     /** The Tidal geopotential evaluated on a specific date. */
     private static class TideHarmonics implements NormalizedSphericalHarmonics {
 
         /** evaluation date. */
         private final AbsoluteDate date;
 
         /** Cached cnm. */
         private final double[][] cnm;
 
         /** Cached snm. */
         private final double[][] snm;
 
         /** Construct the tidal harmonics on the given date.
          *
          * @param date of evaluation
          * @param cnm the Cnm coefficients. Not copied.
          * @param snm the Snm coeffiecients. Not copied.
          */
         private TideHarmonics(final AbsoluteDate date,
                               final double[][] cnm,
                               final double[][] snm) {
             this.date = date;
             this.cnm = cnm;
             this.snm = snm;
         }
 
         /** {@inheritDoc} */
         @Override
         public AbsoluteDate getDate() {
             return date;
         }
 
         /** {@inheritDoc} */
         @Override
         public double getNormalizedCnm(final int n, final int m) {
             return cnm[n][m];
         }
 
         /** {@inheritDoc} */
         @Override
         public double getNormalizedSnm(final int n, final int m) {
             return snm[n][m];
         }
 
     }
 
 }