/* Copyright 2002-2015 CS Systèmes d'Information
    * Licensed to CS Systèmes d'Information (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.propagation.semianalytical.dsst.utilities;
  
  import java.util.ArrayList;
  
  import org.apache.commons.math3.analysis.interpolation.HermiteInterpolator;
  import org.apache.commons.math3.util.FastMath;
  import org.orekit.time.AbsoluteDate;
  
  /** Interpolated short periodics coefficients.
   * <p>
   * Representation of a coefficient that need to be interpolated over time.
   * </p><p>
   * The short periodics coefficients can be interpolated for faster computation.
   * This class stores computed values of the coefficients through the method
   * {@link #addGridPoint} and gives an interpolated result through the method
   * {@link #value}.
   * </p>
   * @author Nicolas Bernard
   *
   */
  public class ShortPeriodicsInterpolatedCoefficient {
  
      /**Values of the already computed coefficients.*/
      private ArrayList<Double> values;
  
      /**Grid points.*/
      private ArrayList<AbsoluteDate> abscissae;
  
      /**Number of points used in the interpolation.*/
      private int interpolationPoints;
  
      /**Index of the latest closest neighbor.*/
      private int latestClosestNeighbor;
  
      /**Simple constructor.
       * @param interpolationPoints number of points used in the interpolation
       */
      public ShortPeriodicsInterpolatedCoefficient(final int interpolationPoints) {
          this.interpolationPoints = interpolationPoints;
          this.abscissae = new ArrayList<AbsoluteDate>();
          this.values = new ArrayList<Double>();
          this.latestClosestNeighbor = 0;
      }
  
      /**Compute the value of the coefficient.
       * @param date date at which the coefficient should be computed
       * @return value of the coefficient
       */
      public double value(final AbsoluteDate date) {
          //Get the closest points from the input date
          final int[] neighbors = getNeighborsIndices(date);
  
          //Creation and set up of the interpolator
          final HermiteInterpolator interpolator = new HermiteInterpolator();
          for (int i : neighbors) {
              final double abscissa = abscissae.get(i).durationFrom(date);
              final double value = values.get(i);
  
              interpolator.addSamplePoint(abscissa, new double[]{value});
          }
  
          //interpolation
          return interpolator.value(0.0)[0];
      }
  
      /**Find the closest available points from the specified date.
       * @param date date of interest
       * @return indices corresponding to the closest points on the time scale
       */
      private int[] getNeighborsIndices(final AbsoluteDate date) {
          final int sizeofNeighborhood = FastMath.min(interpolationPoints, abscissae.size());
          final int[] neighborsIndices = new int[sizeofNeighborhood];
  
          //If the size of the complete sample is less than
          //the desired number of interpolation points,
          //then the entire sample is considered as the neighborhood
          if (interpolationPoints >= abscissae.size()) {
              for (int i = 0; i < sizeofNeighborhood; i++) {
                  neighborsIndices[i] = i;
              }
          }
  
          //Else the neighborsIndices array is completed step-by-step,
         //starting with its closest neighbor
         else {
             final int closestNeighbor = getClosestNeighbor(date);
 
             neighborsIndices[0] = closestNeighbor;
 
             int i = 1;
             int lowerNeighbor = closestNeighbor - 1;
             int upperNeighbor = closestNeighbor + 1;
 
             while (i < interpolationPoints) {
                 if (lowerNeighbor < 0) { //This means that we have reached the earliest date
                     neighborsIndices[i] = upperNeighbor;
                     upperNeighbor++;
                 }
                 else if (upperNeighbor >= abscissae.size()) { //This means that we have reached the latest date
                     neighborsIndices[i] = lowerNeighbor;
                     lowerNeighbor--;
                 }
                 else { //the choice is made between the two next neighbors
                     final double lowerNeighborDistance = FastMath.abs(abscissae.get(lowerNeighbor).durationFrom(date));
                     final double upperNeighborDistance = FastMath.abs(abscissae.get(upperNeighbor).durationFrom(date));
 
                     if (lowerNeighborDistance <= upperNeighborDistance) {
                         neighborsIndices[i] = lowerNeighbor;
                         lowerNeighbor--;
                     }
                     else {
                         neighborsIndices[i] = upperNeighbor;
                         upperNeighbor++;
                     }
                 }
 
                 i++;
             }
         }
 
         return neighborsIndices;
     }
 
     /**Find the closest point from a specific date amongst the available points.
      * @param date date of interest
      * @return index of the closest abscissa from the date of interest
      */
     private int getClosestNeighbor(final AbsoluteDate date) {
         //the starting point is the latest result of a call to this method.
         //Indeed, as this class is meant to be called during an integration process
         //with an input date evolving often continuously in time, there is a high
         //probability that the result will be the same as for last call of
         //this method.
         int closestNeighbor = latestClosestNeighbor;
 
         //case where the date is before the available points
         if (date.compareTo(abscissae.get(0)) <= 0) {
             closestNeighbor = 0;
         }
         //case where the date is after the available points
         else if (date.compareTo(abscissae.get(abscissae.size() - 1)) >= 0) {
             closestNeighbor = abscissae.size() - 1;
         }
         //general case: one is looking for the two consecutives entries that surround the input date
         //then one choose the closest one
         else {
             int lowerBorder = latestClosestNeighbor;
             int upperBorder = latestClosestNeighbor;
 
             final int searchDirection = date.compareTo(abscissae.get(latestClosestNeighbor));
             if (searchDirection > 0) {
                 upperBorder++;
                 while (date.compareTo(abscissae.get(upperBorder)) > 0) {
                     upperBorder++;
                     lowerBorder++;
                 }
             }
             else {
                 lowerBorder--;
                 while (date.compareTo(abscissae.get(lowerBorder)) < 0) {
                     upperBorder--;
                     lowerBorder--;
                 }
             }
 
             final double lowerDistance = FastMath.abs(date.durationFrom(abscissae.get(lowerBorder)));
             final double upperDistance = FastMath.abs(date.durationFrom(abscissae.get(upperBorder)));
 
             closestNeighbor = (lowerDistance < upperDistance) ? lowerBorder : upperBorder;
         }
 
         //The result is stored in order to speed up the next call to the function
         //Indeed, it is highly likely that the requested result will be the same
         this.latestClosestNeighbor = closestNeighbor;
         return closestNeighbor;
     }
 
     /** Clear the recorded values from the interpolation grid.
      */
     public void clearHistory() {
         abscissae.clear();
         values.clear();
     }
 
     /** Add a point to the interpolation grid.
      * @param date abscissa of the point
      * @param value value of the element
      */
     public void addGridPoint(final AbsoluteDate date, final double value) {
         //If the grid is empty, the value is directly added to both arrays
         if (abscissae.isEmpty()) {
             abscissae.add(date);
             values.add(value);
         }
         //If the grid already contains this point, only its value is changed
         else if (abscissae.contains(date)) {
             values.set(abscissae.indexOf(date), value);
         }
         //If the grid does not contain this point, the position of the point
         //in the grid is computed first
         else {
             final int closestNeighbor = getClosestNeighbor(date);
             final int index = (date.compareTo(abscissae.get(closestNeighbor)) < 0) ? closestNeighbor : closestNeighbor + 1;
             abscissae.add(index, date);
             values.add(index, value);
         }
     }
 }