/* Copyright 2002-2023 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 java.io.Serializable;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Comparator;
  import java.util.List;
  
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.util.FastMath;
  import org.orekit.annotation.DefaultDataContext;
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitInternalError;
  import org.orekit.utils.Constants;
  
  /** Coordinated Universal Time.
   * <p>UTC is related to TAI using step adjustments from time to time
   * according to IERS (International Earth Rotation Service) rules. Before 1972,
   * these adjustments were piecewise linear offsets. Since 1972, these adjustments
   * are piecewise constant offsets, which require introduction of leap seconds.</p>
   * <p>Leap seconds are always inserted as additional seconds at the last minute
   * of the day, pushing the next day forward. Such minutes are therefore more
   * than 60 seconds long. In theory, there may be seconds removal instead of seconds
   * insertion, but up to now (2010) it has never been used. As an example, when a
   * one second leap was introduced at the end of 2005, the UTC time sequence was
   * 2005-12-31T23:59:59 UTC, followed by 2005-12-31T23:59:60 UTC, followed by
   * 2006-01-01T00:00:00 UTC.</p>
   * <p>This is intended to be accessed thanks to {@link TimeScales},
   * so there is no public constructor.</p>
   * @author Luc Maisonobe
   * @see AbsoluteDate
   */
  public class UTCScale implements TimeScale {
  
      /** Serializable UID. */
      private static final long serialVersionUID = 20230302L;
  
      /** International Atomic Scale. */
      private final TimeScale tai;
  
      /** base UTC-TAI offsets (may lack the pre-1975 offsets). */
      private final Collection<? extends OffsetModel> baseOffsets;
  
      /** UTC-TAI offsets. */
      private final UTCTAIOffset[] offsets;
  
      /** Package private constructor for the factory.
       * Used to create the prototype instance of this class that is used to
       * clone all subsequent instances of {@link UTCScale}. Initializes the offset
       * table that is shared among all instances.
       * @param tai TAI time scale this UTC time scale references.
       * @param baseOffsets UTC-TAI base offsets (may lack the pre-1975 offsets)
       */
      UTCScale(final TimeScale tai, final Collection<? extends OffsetModel> baseOffsets) {
  
          this.tai         = tai;
          this.baseOffsets = baseOffsets;
  
          // copy input so the original list is unmodified
          final List<OffsetModel> offsetModels = new ArrayList<>(baseOffsets);
          offsetModels.sort(Comparator.comparing(OffsetModel::getStart));
          if (offsetModels.get(0).getStart().getYear() > 1968) {
              // the pre-1972 linear offsets are missing, add them manually
              // excerpt from UTC-TAI.history file:
              //  1961  Jan.  1 - 1961  Aug.  1     1.422 818 0s + (MJD - 37 300) x 0.001 296s
              //        Aug.  1 - 1962  Jan.  1     1.372 818 0s +        ""
              //  1962  Jan.  1 - 1963  Nov.  1     1.845 858 0s + (MJD - 37 665) x 0.001 123 2s
              //  1963  Nov.  1 - 1964  Jan.  1     1.945 858 0s +        ""
              //  1964  Jan.  1 -       April 1     3.240 130 0s + (MJD - 38 761) x 0.001 296s
              //        April 1 -       Sept. 1     3.340 130 0s +        ""
              //        Sept. 1 - 1965  Jan.  1     3.440 130 0s +        ""
              //  1965  Jan.  1 -       March 1     3.540 130 0s +        ""
              //        March 1 -       Jul.  1     3.640 130 0s +        ""
              //        Jul.  1 -       Sept. 1     3.740 130 0s +        ""
              //        Sept. 1 - 1966  Jan.  1     3.840 130 0s +        ""
              //  1966  Jan.  1 - 1968  Feb.  1     4.313 170 0s + (MJD - 39 126) x 0.002 592s
              //  1968  Feb.  1 - 1972  Jan.  1     4.213 170 0s +        ""
              offsetModels.add( 0, new OffsetModel(new DateComponents(1961,  1, 1), 37300, 1.4228180, 0.0012960));
              offsetModels.add( 1, new OffsetModel(new DateComponents(1961,  8, 1), 37300, 1.3728180, 0.0012960));
              offsetModels.add( 2, new OffsetModel(new DateComponents(1962,  1, 1), 37665, 1.8458580, 0.0011232));
              offsetModels.add( 3, new OffsetModel(new DateComponents(1963, 11, 1), 37665, 1.9458580, 0.0011232));
              offsetModels.add( 4, new OffsetModel(new DateComponents(1964,  1, 1), 38761, 3.2401300, 0.0012960));
              offsetModels.add( 5, new OffsetModel(new DateComponents(1964,  4, 1), 38761, 3.3401300, 0.0012960));
             offsetModels.add( 6, new OffsetModel(new DateComponents(1964,  9, 1), 38761, 3.4401300, 0.0012960));
             offsetModels.add( 7, new OffsetModel(new DateComponents(1965,  1, 1), 38761, 3.5401300, 0.0012960));
             offsetModels.add( 8, new OffsetModel(new DateComponents(1965,  3, 1), 38761, 3.6401300, 0.0012960));
             offsetModels.add( 9, new OffsetModel(new DateComponents(1965,  7, 1), 38761, 3.7401300, 0.0012960));
             offsetModels.add(10, new OffsetModel(new DateComponents(1965,  9, 1), 38761, 3.8401300, 0.0012960));
             offsetModels.add(11, new OffsetModel(new DateComponents(1966,  1, 1), 39126, 4.3131700, 0.0025920));
             offsetModels.add(12, new OffsetModel(new DateComponents(1968,  2, 1), 39126, 4.2131700, 0.0025920));
         }
 
         // create cache
         this.offsets = new UTCTAIOffset[offsetModels.size()];
 
         UTCTAIOffset previous = null;
 
         // link the offsets together
         for (int i = 0; i < offsetModels.size(); ++i) {
 
             final OffsetModel    o      = offsetModels.get(i);
             final DateComponents date   = o.getStart();
             final int            mjdRef = o.getMJDRef();
             final double         offset = o.getOffset();
             final double         slope  = o.getSlope();
 
             // start of the leap
             final double previousOffset    = (previous == null) ? 0.0 : previous.getOffset(date, TimeComponents.H00);
             final AbsoluteDate leapStart   = new AbsoluteDate(date, tai).shiftedBy(previousOffset);
 
             // end of the leap
             final double startOffset       = offset + slope * (date.getMJD() - mjdRef);
             final AbsoluteDate leapEnd     = new AbsoluteDate(date, tai).shiftedBy(startOffset);
 
             // leap computed at leap start and in UTC scale
             final double normalizedSlope   = slope / Constants.JULIAN_DAY;
             final double leap              = leapEnd.durationFrom(leapStart) / (1 + normalizedSlope);
 
             final AbsoluteDate reference = AbsoluteDate.createMJDDate(mjdRef, 0, tai)
                     .shiftedBy(offset);
             previous = new UTCTAIOffset(leapStart, date.getMJD(), leap, offset, mjdRef,
                     normalizedSlope, reference);
             this.offsets[i] = previous;
 
         }
 
     }
 
     /** Get the base offsets.
      * @return base offsets (may lack the pre-1975 offsets)
      * @since 12.0
      */
     public Collection<? extends OffsetModel> getBaseOffsets() {
         return baseOffsets;
     }
 
     /**
      * Returns the UTC-TAI offsets underlying this UTC scale.
      * <p>
      * Modifications to the returned list will not affect this UTC scale instance.
      * @return new non-null modifiable list of UTC-TAI offsets time-sorted from
      *         earliest to latest
      */
     public List<UTCTAIOffset> getUTCTAIOffsets() {
         final List<UTCTAIOffset> offsetList = new ArrayList<>(offsets.length);
         for (int i = 0; i < offsets.length; ++i) {
             offsetList.add(offsets[i]);
         }
         return offsetList;
     }
 
     /** {@inheritDoc} */
     @Override
     public double offsetFromTAI(final AbsoluteDate date) {
         final int offsetIndex = findOffsetIndex(date);
         if (offsetIndex < 0) {
             // the date is before the first known leap
             return 0;
         } else {
             return -offsets[offsetIndex].getOffset(date);
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> T offsetFromTAI(final FieldAbsoluteDate<T> date) {
         final int offsetIndex = findOffsetIndex(date.toAbsoluteDate());
         if (offsetIndex < 0) {
             // the date is before the first known leap
             return date.getField().getZero();
         } else {
             return offsets[offsetIndex].getOffset(date).negate();
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public double offsetToTAI(final DateComponents date,
                               final TimeComponents time) {
 
         // take offset from local time into account, but ignoring seconds,
         // so when we parse an hour like 23:59:60.5 during leap seconds introduction,
         // we do not jump to next day
         final int minuteInDay = time.getHour() * 60 + time.getMinute() - time.getMinutesFromUTC();
         final int correction  = minuteInDay < 0 ? (minuteInDay - 1439) / 1440 : minuteInDay / 1440;
 
         // find close neighbors, assuming date in TAI, i.e a date earlier than real UTC date
         final int mjd = date.getMJD() + correction;
         final UTCTAIOffset offset = findOffset(mjd);
         if (offset == null) {
             // the date is before the first known leap
             return 0;
         } else {
             return offset.getOffset(date, time);
         }
 
     }
 
     /** {@inheritDoc} */
     public String getName() {
         return "UTC";
     }
 
     /** {@inheritDoc} */
     public String toString() {
         return getName();
     }
 
     /** Get the date of the first known leap second.
      * @return date of the first known leap second
      */
     public AbsoluteDate getFirstKnownLeapSecond() {
         return offsets[0].getDate();
     }
 
     /** Get the date of the last known leap second.
      * @return date of the last known leap second
      */
     public AbsoluteDate getLastKnownLeapSecond() {
         return offsets[offsets.length - 1].getDate();
     }
 
     /** {@inheritDoc} */
     @Override
     public boolean insideLeap(final AbsoluteDate date) {
         final int offsetIndex = findOffsetIndex(date);
         if (offsetIndex < 0) {
             // the date is before the first known leap
             return false;
         } else {
             return date.compareTo(offsets[offsetIndex].getValidityStart()) < 0;
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> boolean insideLeap(final FieldAbsoluteDate<T> date) {
         return insideLeap(date.toAbsoluteDate());
     }
 
     /** {@inheritDoc} */
     @Override
     public int minuteDuration(final AbsoluteDate date) {
         final int offsetIndex = findOffsetIndex(date);
         final UTCTAIOffset offset;
         if (offsetIndex >= 0 &&
                 date.compareTo(offsets[offsetIndex].getValidityStart()) < 0) {
             // the date is during the leap itself
             offset = offsets[offsetIndex];
         } else if (offsetIndex + 1 < offsets.length &&
             offsets[offsetIndex + 1].getDate().durationFrom(date) <= 60.0) {
             // the date is after a leap, but it may be just before the next one
             // the next leap will start in one minute, it will extend the current minute
             offset = offsets[offsetIndex + 1];
         } else {
             offset = null;
         }
         if (offset != null) {
             // since this method returns an int we can't return the precise duration in
             // all cases, but we can bound it. Some leaps are more than 1s. See #694
             return 60 + (int) FastMath.ceil(offset.getLeap());
         }
         // no leap is expected within the next minute
         return 60;
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> int minuteDuration(final FieldAbsoluteDate<T> date) {
         return minuteDuration(date.toAbsoluteDate());
     }
 
     /** {@inheritDoc} */
     @Override
     public double getLeap(final AbsoluteDate date) {
         final int offsetIndex = findOffsetIndex(date);
         if (offsetIndex < 0) {
             // the date is before the first known leap
             return 0;
         } else {
             return offsets[offsetIndex].getLeap();
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> T getLeap(final FieldAbsoluteDate<T> date) {
         return date.getField().getZero().add(getLeap(date.toAbsoluteDate()));
     }
 
     /** Find the index of the offset valid at some date.
      * @param date date at which offset is requested
      * @return index of the offset valid at this date, or -1 if date is before first offset.
      */
     private int findOffsetIndex(final AbsoluteDate date) {
         int inf = 0;
         int sup = offsets.length;
         while (sup - inf > 1) {
             final int middle = (inf + sup) >>> 1;
             if (date.compareTo(offsets[middle].getDate()) < 0) {
                 sup = middle;
             } else {
                 inf = middle;
             }
         }
         if (sup == offsets.length) {
             // the date is after the last known leap second
             return offsets.length - 1;
         } else if (date.compareTo(offsets[inf].getDate()) < 0) {
             // the date is before the first known leap
             return -1;
         } else {
             return inf;
         }
     }
 
     /** Find the offset valid at some date.
      * @param mjd Modified Julian Day of the date at which offset is requested
      * @return offset valid at this date, or null if date is before first offset.
      */
     private UTCTAIOffset findOffset(final int mjd) {
         int inf = 0;
         int sup = offsets.length;
         while (sup - inf > 1) {
             final int middle = (inf + sup) >>> 1;
             if (mjd < offsets[middle].getMJD()) {
                 sup = middle;
             } else {
                 inf = middle;
             }
         }
         if (sup == offsets.length) {
             // the date is after the last known leap second
             return offsets[offsets.length - 1];
         } else if (mjd < offsets[inf].getMJD()) {
             // the date is before the first known leap
             return null;
         } else {
             return offsets[inf];
         }
     }
 
     /** Replace the instance with a data transfer object for serialization.
      * @return data transfer object that will be serialized
      */
     @DefaultDataContext
     private Object writeReplace() {
         return new DataTransferObject(tai, baseOffsets);
     }
 
     /** Internal class used only for serialization. */
     @DefaultDataContext
     private static class DataTransferObject implements Serializable {
 
         /** Serializable UID. */
         private static final long serialVersionUID = 20230302L;
 
         /** International Atomic Scale. */
         private final TimeScale tai;
 
         /** base UTC-TAI offsets (may lack the pre-1975 offsets). */
         private final Collection<? extends OffsetModel> baseOffsets;
 
         /** Simple constructor.
          * @param tai TAI time scale this UTC time scale references.
          * @param baseOffsets UTC-TAI base offsets (may lack the pre-1975 offsets)
          */
         DataTransferObject(final TimeScale tai, final Collection<? extends OffsetModel> baseOffsets) {
             this.tai         = tai;
             this.baseOffsets = baseOffsets;
         }
 
         /** Replace the deserialized data transfer object with a {@link UTCScale}.
          * @return replacement {@link UTCScale}
          */
         private Object readResolve() {
             try {
                 return new UTCScale(tai, baseOffsets);
             } catch (OrekitException oe) {
                 throw new OrekitInternalError(oe);
             }
         }
 
     }
 
 }