/* Copyright 2002-2025 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.time;
  
  import org.hipparchus.util.FastMath;
  import org.orekit.annotation.DefaultDataContext;
  import org.orekit.data.DataContext;
  import org.orekit.propagation.analytical.gnss.data.GNSSConstants;
  import org.orekit.utils.Constants;
  
  /**
   * Container for date in GLONASS form.
   * @author Bryan Cazabonne
   * @see AbsoluteDate
   * @see "GLONASS Interface Control Document v1.0, 2016"
   * @since 10.0
   */
  public class GLONASSDate implements TimeStamped {
  
      /** Constant for date computation. */
      private static final int C1 = 44195;
  
      /** Constant for date computation. */
      private static final int C2 = 45290;
  
      /** The number of the current day in a four year interval N<sub>a</sub>. */
      private final int na;
  
      /** The number of the current four year interval N<sub>4</sub>. */
      private final int n4;
  
      /** Number of seconds since N<sub>a</sub>. */
      private final double secInNa;
  
      /** Current Julian date JD0. */
      private final double jd0;
  
      /** Greenwich Mean Sidereal Time (rad). */
      private final double gmst;
  
      /** Corresponding date. */
      private final AbsoluteDate date;
  
      /** Build an instance corresponding to a GLONASS date.
       *
       * <p>This method uses the {@link DataContext#getDefault() default data context}.
       *
       * @param na the number of the current day in a four year interval
       * @param n4 the number of the current four year interval
       * @param secInNa the number of seconds since na start
       * @see #GLONASSDate(int, int, double, TimeScale)
       */
      @DefaultDataContext
      public GLONASSDate(final int na, final int n4, final double secInNa) {
          this(na, n4, secInNa, DataContext.getDefault().getTimeScales().getGLONASS());
      }
  
      /**
       * Build an instance corresponding to a GLONASS date.
       *
       * @param na      the number of the current day in a four year interval
       * @param n4      the number of the current four year interval
       * @param secInNa the number of seconds since na start
       * @param glonass time scale.
       * @since 10.1
       */
      public GLONASSDate(final int na,
                         final int n4,
                         final double secInNa,
                         final TimeScale glonass) {
          this.na      = na;
          this.n4      = n4;
          this.secInNa = secInNa;
          // Compute JD0
          final int ratio = FastMath.round((float) (na - 3) / (25 + C1 + C2));
          this.jd0  = 1461 * (n4 - 1) + na + 2450082.5 - ratio;
          // GMST
          this.gmst = computeGMST();
          this.date = computeDate(glonass);
      }
  
      /** Build an instance from an absolute date.
       *
       * <p>This method uses the {@link DataContext#getDefault() default data context}.
       *
      * @param date absolute date to consider
      * @see #GLONASSDate(AbsoluteDate, TimeScale)
      */
     @DefaultDataContext
     public GLONASSDate(final AbsoluteDate date) {
         this(date, DataContext.getDefault().getTimeScales().getGLONASS());
     }
 
     /**
      * Build an instance from an absolute date.
      *
      * @param date    absolute date to consider
      * @param glonass time scale.
      * @since 10.1
      */
     public GLONASSDate(final AbsoluteDate date, final TimeScale glonass) {
         final DateTimeComponents dateTime = date.getComponents(glonass);
         // N4
         final int year = dateTime.getDate().getYear();
         this.n4 = ((int) (year - 1996) / 4) + 1;
         // Na
         final int start = 1996 + 4 * (n4 - 1);
         final double duration = date.durationFrom(new AbsoluteDate(start, 1, 1, glonass));
         this.na = (int) (duration / 86400) + 1;
         this.secInNa = dateTime.getTime().getSecondsInLocalDay();
         // Compute JD0
         final int ratio = FastMath.round((float) (na - 3) / (25 + C1 + C2));
         this.jd0 = 1461 * (n4 - 1) + na + 2450082.5 - ratio;
         // GMST
         this.gmst = computeGMST();
         this.date = date;
     }
 
     @Override
     public AbsoluteDate getDate() {
         return date;
     }
 
     /** Get the number of seconds since N<sub>a</sub> start.
      * @return number of seconds since N<sub>a</sub> start
      */
     public double getSecInDay() {
         return secInNa;
     }
 
     /** Get the number of the current day in a four year interval.
      * @return the number of the current day in a four year interval
      */
     public int getDayNumber() {
         return na;
     }
 
     /** Get the number of the current four year interval.
      * @return the number of the current four year interval
      */
     public int getIntervalNumber() {
         return n4;
     }
 
     /** Get the current Julian date JD0.
      * @return the current date JD0
      */
     public double getJD0() {
         return jd0;
     }
 
     /** Get the Greenwich Mean Sidereal Time.
      * @return the Greenwich Mean Sidereal Time (rad)
      */
     public double getGMST() {
         return gmst;
     }
 
     /** Compute the Greenwich Mean Sidereal Time using the current Julian date JD0.
      * @return the Greenwich Mean Sidereal Time (rad)
      */
     private double computeGMST() {
         final double ref = 2451545.0;
         // Earth's rotation angle in radians
         final double era = 2. * GNSSConstants.GLONASS_PI *
                         (0.7790572732640 + 1.00273781191135448 * (jd0 - ref));
         // Time from Epoch 2000 (1st January, 00:00 UTC) till current Epoch in Julian centuries
         final double time = (jd0 - ref) / Constants.JULIAN_CENTURY;
         // Time to the power n
         final double time2 = time * time;
         final double time3 = time2 * time;
         final double time4 = time2 * time2;
         final double time5 = time2 * time3;
         // GMST computation
         final double gTime = era + 7.03270726e-8 + time * 2.23603658710194e-2 +
                         time2 * 6.7465784654e-6 - time3 * 2.1332e-12 - time4 * 1.452308e-10 - time5 * 1.784e-13;
         return gTime;
     }
 
     /** Compute the GLONASS date.
      * @return the date
      * @param glonass time scale.
      */
     private AbsoluteDate computeDate(final TimeScale glonass) {
         // Compute the number of Julian day for the current date
         final double jdn = jd0 + 0.5;
         // Coefficients
         final int a = (int) (jdn + 32044);
         final int b = (4 * a + 3) / 146097;
         final int c = a - (146097 * b) / 4;
         final int d = (4 * c + 3) / 1461;
         final int e = c - (1461 * d) / 4;
         final int m = (5 * e + 2) / 153;
         // Year, month and day
         final int day   = e - (153 * m + 2) / 5 + 1;
         final int month = m + 3 - 12 * (m / 10);
         final int year  = 100 * b + d - 4800 + m / 10;
         return new AbsoluteDate(new DateComponents(year, month, day),
                                 new TimeComponents(secInNa),
                                 glonass);
     }
 
 }