/* Copyright 2022-2026 Thales Alenia Space
    * 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.clocks;
  
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  import java.util.function.DoubleFunction;
  
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.Field;
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.analysis.differentiation.GradientField;
  import org.hipparchus.analysis.polynomials.PolynomialFunction;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.MathArrays;
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitMessages;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.time.TimeInterval;
  import org.orekit.utils.drivers.ParameterDriver;
  
  /** Polynomial clock model.
   *
   * @author Luc Maisonobe
   * @since 12.1
   *
   */
  public class PolynomialClockModel implements ClockModel {
  
      /**
       * Clock offset scaling factor.
       * <p>
       * We use a power of 2 to avoid numeric noise introduction
       * in the multiplications/divisions sequences.
       * </p>
       */
      private static final double CLOCK_OFFSET_SCALE = FastMath.scalb(1.0, -10);
  
      /** Prefix for parameters names. */
      private final String prefix;
  
      /** List of terms. */
      private final List<ParameterDriver> terms;
  
      /** Cached field-based models.
       * @since 14.0
       */
      private final Map<Field<? extends CalculusFieldElement<?>>, PolynomialFieldClockModel<?>> fieldModels;
  
      /** Simple constructor.
       * @param referenceDate reference date (may be null)
       * @param prefix        prefix for the parameter names
       */
      public PolynomialClockModel(final AbsoluteDate referenceDate,
                                  final String prefix) {
          this(referenceDate, prefix, 0.0);
      }
  
      /** Simple constructor.
       * @param referenceDate reference date (may be null if there is only one term)
       * @param prefix        prefix for the parameter names
       * @param terms         the polynomial terms in order.
       */
      public PolynomialClockModel(final AbsoluteDate referenceDate, final String prefix,
                                  final double... terms) {
  
          this.prefix = prefix;
  
          // set up a safe reference date
          final AbsoluteDate safeReferenceDate;
          if (referenceDate == null) {
              if (terms.length == 1) {
                  // we accept null reference date for constant polynomials
                  // we just change it to arbitrary to avoid null pointer exceptions
                  safeReferenceDate = AbsoluteDate.ARBITRARY_EPOCH;
              } else {
                  throw new OrekitException(OrekitMessages.NO_REFERENCE_DATE_FOR_PARAMETER,
                                            prefix + getAcceptedTermSuffix(0));
              }
          } else {
              safeReferenceDate = referenceDate;
         }
 
         final List<ParameterDriver> convertedTerms = new ArrayList<>();
         for (int i = 0; i < terms.length; ++i) {
             final String name = prefix + getAcceptedTermSuffix(i);
             final ParameterDriver parameterTerm =
                 new ParameterDriver(name, 0.0, CLOCK_OFFSET_SCALE,
                                     Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY,
                                     TimeInterval.UNLIMITED);
             parameterTerm.setValue(terms[i]);
             parameterTerm.setReferenceDate(safeReferenceDate);
             convertedTerms.add(parameterTerm);
         }
         this.terms = convertedTerms;
 
         this.fieldModels = new HashMap<>();
 
     }
 
     /**
      * Simple constructor.
      * <p>
      * The reference date for computing the polynom is the reference
      * date of the first parameter driver.
      * </p>
      * @param terms the parameter driver terms (they must have names that end
      *              with the {@link #getAcceptedTermSuffix(int) accepted suffixes})
      */
     public PolynomialClockModel(final ParameterDriver... terms) {
 
         // extract prefix
         if (terms.length == 0) {
             prefix = "";
         } else {
             final String firstName   = terms[0].getName();
             final String firstSuffix = getAcceptedTermSuffix(0);
             if (firstName.endsWith(firstSuffix)) {
                 prefix = firstName.substring(0, firstName.length() - firstSuffix.length());
             } else {
                 // there will be an error triggered in the following loop
                 prefix = firstName;
             }
         }
 
         int idx = 0;
         for (final ParameterDriver term : terms) {
             final String acceptedName = prefix + getAcceptedTermSuffix(idx);
             if (!term.getName().equals(acceptedName)) {
                 throw new OrekitException(OrekitMessages.UNSUPPORTED_PARAMETER_NAME,
                                           term.getName(), acceptedName);
             }
             idx++;
         }
         this.terms  = Arrays.asList(terms);
 
         this.fieldModels = new HashMap<>();
 
     }
 
     /**
      * Simple constructor.
      *
      * @param terms the parameter driver terms
      */
     public PolynomialClockModel(final List<ParameterDriver> terms) {
         this(terms.toArray(new ParameterDriver[0]));
     }
 
     /** {@inheritDoc}
      * <p>
      * Validity is extracted from the first parameter driver.
      * </p>
      */
     @Override
     public AbsoluteDate getValidityStart() {
         return terms.getFirst().getValidity().getStartDate();
     }
 
     /** {@inheritDoc}
      * <p>
      * Validity is extracted from the first parameter driver.
      * </p>
      */
     @Override
     public AbsoluteDate getValidityEnd() {
         return terms.getFirst().getValidity().getEndDate();
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return terms;
     }
 
     /** Add a parameter driver term in a given index to the list of parameters.
      * If parameters prior to the one requested don't exist, it will create empty ones.
      * This allows adding a velocity, acceleration, or above without explicitly defining the terms below.
      *
      * @param index the index at which to add the parameter driver
      * @param driver the parameter driver to add
      */
     public void addParameterDriver(final int index, final ParameterDriver driver) {
         final List<ParameterDriver> parameters = getParametersDrivers();
         if (parameters.size() < index) {
             // Recursively add empty parameters to fill gaps
             addParameterDriver(index - 1, null);
             // After filling gaps, add the driver at the target index
             addParameterDriver(index, driver);
         } else if (parameters.size() == index) {
             if (driver != null) {
                 // Add the driver at the correct index
                 parameters.add(driver);
             } else {
                 // Create empty parameter with correct name for this index
                 final ParameterDriver empty = new ParameterDriver(prefix + getAcceptedTermSuffix(index),
                                                                   0.0, CLOCK_OFFSET_SCALE,
                                                                   Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY,
                                                                   TimeInterval.UNLIMITED);
                 empty.setReferenceDate(AbsoluteDate.ARBITRARY_EPOCH);
                 parameters.add(empty);
             }
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public ClockOffset getOffset(final AbsoluteDate date) {
         final double[] result = new double[3];
         if (terms.isEmpty()) {
             return new ClockOffset(date, result);
         }
 
         final AbsoluteDate referenceDate = terms.getFirst().getReferenceDate();
         final double dt;
         if (referenceDate == null) {
             if (terms.size() == 1) {
                 // this is a constant polynomial, we accept a missing reference date
                 dt = 0;
             } else {
                 throw new OrekitException(OrekitMessages.NO_REFERENCE_DATE_FOR_PARAMETER,
                                           terms.getFirst().getName());
             }
         } else {
             dt = date.durationFrom(referenceDate);
         }
 
         final double[] convertedTerms = terms.
                                         stream().
                                         map(ParameterDriver::getValue).
                                         mapToDouble(Double::doubleValue).
                                         toArray();
         // Turn the terms into a polynomial function
         PolynomialFunction function = new PolynomialFunction(convertedTerms);
         // Loop over all of the terms in order
         for (int ii = 0; ii < 3; ii++) {
             result[ii] = function.value(dt);
             function = function.polynomialDerivative();
         }
         return new ClockOffset(date, result);
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> PolynomialFieldClockModel<T> toField(final DoubleFunction<T> converter) {
         // build aggregated models may be costly, so we cache the results
         return (PolynomialFieldClockModel<T>) fieldModels.computeIfAbsent(converter.apply(0.0).getField(),
                                                                           f -> buildFieldModel(converter));
     }
 
     /**
      * Build a field model.
      * @param <T> type of the field elements
      * @param converter converter to field elements
      * @return field version of the instance
      * @since 14.0
      */
     private <T extends CalculusFieldElement<T>> PolynomialFieldClockModel<T> buildFieldModel(final DoubleFunction<T> converter) {
 
         final Field<T> field = converter.apply(0.0).getField();
 
         // handle special case
         if (terms.isEmpty()) {
             return new PolynomialFieldClockModel<>(FieldAbsoluteDate.getArbitraryEpoch(field), "");
         }
 
         // convert reference date
         if (terms.size() > 1 && terms.getFirst().getReferenceDate() == null) {
             throw new OrekitException(OrekitMessages.NO_REFERENCE_DATE_FOR_PARAMETER,
                                       terms.getFirst().getName());
         }
         final FieldAbsoluteDate<T> fieldDate =
             new FieldAbsoluteDate<>(field, terms.getFirst().getReferenceDate());
 
         // convert polynomial coefficients
         final T[] fieldTerms = MathArrays.buildArray(field, terms.size());
         for (int i = 0; i < terms.size(); ++i) {
             fieldTerms[i] = converter.apply(terms.get(i).getValue());
         }
 
         return new PolynomialFieldClockModel<>(fieldDate, prefix, fieldTerms);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public PolynomialFieldClockModel<Gradient> toGradient(final int freeParameters, final Map<String, Integer> indices) {
 
         final GradientField field = GradientField.getField(freeParameters);
 
         // handle special case
         if (terms.isEmpty()) {
             return new PolynomialFieldClockModel<>(FieldAbsoluteDate.getArbitraryEpoch(field), "");
         }
 
         // convert reference date
         if (terms.size() > 1 && terms.getFirst().getReferenceDate() == null) {
             throw new OrekitException(OrekitMessages.NO_REFERENCE_DATE_FOR_PARAMETER,
                                       terms.getFirst().getName());
         }
         final FieldAbsoluteDate<Gradient> fieldDate =
             new FieldAbsoluteDate<>(field, terms.getFirst().getReferenceDate());
 
         // convert polynomial coefficients
         final Gradient[] fieldTerms = new Gradient[terms.size()];
         for (int i = 0; i < terms.size(); ++i) {
             fieldTerms[i] = terms.get(i).getValue(freeParameters, indices);
         }
 
         return new PolynomialFieldClockModel<>(fieldDate, prefix, fieldTerms);
 
     }
 
 }