/* 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
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   */
  package org.orekit.utils;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.Precision;
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitIllegalStateException;
  import org.orekit.errors.OrekitMessages;
  import org.orekit.propagation.events.ParameterDrivenDateIntervalDetector;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.utils.TimeSpanMap.Span;
  import org.orekit.utils.TimeSpanMap.Transition;
  
  
  /** Class allowing to drive the value of a parameter.
   * <p>
   * This class is typically used as a bridge between an estimation
   * algorithm (typically orbit determination or optimizer) and an
   * internal parameter in a physical model that needs to be tuned,
   * or a bridge between a finite differences algorithm and an
   * internal parameter in a physical model that needs to be slightly
   * offset. The physical model will expose to the algorithm a
   * set of instances of this class so the algorithm can call the
   * {@link #setValue(double, AbsoluteDate)} method to update the
   * parameter value at a given date. Some parameters driver only have 1 value estimated/driven
   * over the all period (constructor by default). Some others have several
   * values estimated/driven on several periods/intervals. For example if the time period is 3 days
   * for a drag parameter estimated all days then 3 values would be estimated, one for
   * each time period. In order to allow several values to be estimated, the PDriver has
   * a name and a value {@link TimeSpanMap} as attribute. In order,
   * to cut the time span map there are 2 options :
   * </p>
   * <ul>
   * <li>Passive cut calling the {@link #addSpans(AbsoluteDate, AbsoluteDate, double)} method.
   * Given a start date, an end date and and a validity period (in sec)
   * for the driver, the {@link #addSpans} method will cut the interval of name and value time span map
   * from start date to date end in several interval of validity period duration. This method should not
   * be called on orbital drivers and must be called only once at beginning of the process (for example
   * beginning of orbit determination). <b>WARNING : In order to ensure convergence for orbit determination,
   * the start, end date and driver periodicity must be wisely chosen </b>. There must be enough measurements
   * on each interval or convergence won't reach or singular matrices will appear.  </li>
   * <li> Active cut calling the {@link #addSpanAtDate(AbsoluteDate)} method.
   * Given a date, the method will cut the value and name time span name, in order to have a new span starting at
   * the given date. Can be called several time to cut the time map as wished. <b>WARNING : In order to ensure
   * convergence for orbit determination, if the method is called several time, the start date must be wisely chosen </b>.
   * There must be enough measurements on each interval or convergence won't reach or singular matrices will appear.  </li>
   * </ul>
   * <p>
   * Several ways exist in order to get a ParameterDriver value at a certain
   * date for parameters having several values on several intervals.
   * </p>
   * <ul>
   * <li>First of all, the step estimation, that is to say, if a value wants
   * to be known at a certain date, the value returned is the one of span
   * beginning corresponding to the date. With this definition a value
   * will be kept constant all along the span duration and will be the value at span
   * start.</li>
   * <li> The continuous estimation, that is to say, when a value wants be to
   * known at a date t, the value returned would be a linear interpolation between
   * the value at the beginning of the span corresponding to date t and end this span
   * (which is also the beginning of next span). NOT IMPLEMENTED FOR NOW
   * </li>
   * </ul>
   * Each time the value is set, the physical model
   * will be notified as it will register a {@link ParameterObserver
   * ParameterObserver} for this purpose.
   * <p>
   * This design has two major goals. First, it allows an external
   * algorithm to drive internal parameters blindly, as it only
   * needs to get a list of instances of this class, without knowing
   * what they really drive. Second, it allows the physical model to
   * not expose directly setters methods for its parameters. In order
   * to be able to modify the parameter value, the algorithm
   * <em>must</em> retrieve a parameter driver.
   * </p>
   * @see ParameterObserver
   * @author Luc Maisonobe
  * @author Melina Vanel
  * @since 8.0
  */
 public class ParameterDriver {
 
     /** Name of the parameter.*/
     public static final String SPAN = "Span";
 
     /** Name of the parameter. */
     private String name;
 
     /** TimeSpan for period names.
      * @since 12.0
      */
     private TimeSpanMap<String> nameSpanMap;
 
     /** Reference value. */
     private double referenceValue;
 
     /** Scaling factor. */
     private double scale;
 
     /** Minimum value. */
     private double minValue;
 
     /** Maximum value. */
     private double maxValue;
 
     /** Reference date.
      * @since 9.0
      */
     private AbsoluteDate referenceDate;
 
     /** Flag to choose estimation method. If estimationContinuous
      * is true then when a value wants to be known an interpolation
      * is performed between given date span start and end (start of
      * next span) otherwise the value returned is the value of span start
      * @since 12.0
      */
     private boolean isEstimationContinuous;
 
     /** Value time span map.
      * @since 12.0
      */
     private TimeSpanMap<Double> valueSpanMap;
 
     /** Selection status.
      * <p>
      * Selection is used for estimated parameters in orbit determination,
      * or to compute the Jacobian matrix in partial derivatives computation.
      * </p>
      */
     private boolean selected;
 
     /** Observers observing this driver. */
     private final List<ParameterObserver> observers;
 
     /** Create a new instance from another parameterDriver informations
      * for example (useful for {@link ParameterDriversList.DelegatingDriver}))
      * At construction, the parameter new is configured as <em>not</em> selected,
      * the reference date is set to {@code null}. validityPeriod, namesSpanMap and
      * valueSpanMap.
      * @param name general name of the parameter
      * @param namesSpanMap name time span map. WARNING, number of Span must be coherent with
      * validityPeriod and valueSpanMap (same number of Span with same transitions
      * dates)
      * @param valuesSpanMap values time span map
      * @param referenceValue reference value of the parameter
      * @param scale scaling factor to convert the parameters value to
      * non-dimensional (typically set to the expected standard deviation of the
      * parameter), it must be non-zero
      * @param minValue minimum value allowed
      * @param maxValue maximum value allowed
      * @since 12.0
      */
     public ParameterDriver(final String name, final TimeSpanMap<String> namesSpanMap,
                            final TimeSpanMap<Double> valuesSpanMap, final double referenceValue,
                            final double scale, final double minValue, final double maxValue) {
         if (FastMath.abs(scale) <= Precision.SAFE_MIN) {
             throw new OrekitException(OrekitMessages.TOO_SMALL_SCALE_FOR_PARAMETER,
                                       name, scale);
         }
         this.name                   = name;
         this.nameSpanMap            = namesSpanMap;
         this.referenceValue         = referenceValue;
         this.scale                  = scale;
         this.minValue               = minValue;
         this.maxValue               = maxValue;
         this.referenceDate          = null;
         this.valueSpanMap           = valuesSpanMap;
         this.selected               = false;
         this.observers              = new ArrayList<>();
         this.isEstimationContinuous = false;
     }
 
     /** Simple constructor.
      * <p>
      * At construction, the parameter is configured as <em>not</em> selected,
      * the reference date is set to {@code null}, the value is set to the
      * {@code referenceValue}, the validity period is set to 0 so by default
      * the parameterDriver will be estimated on only 1 interval from -INF to
      * +INF. To change the validity period the
      * {@link ParameterDriver#addSpans(AbsoluteDate, AbsoluteDate, double)}
      * method must be called.
      * </p>
      * @param name name of the parameter
      * @param referenceValue reference value of the parameter
      * @param scale scaling factor to convert the parameters value to
      * non-dimensional (typically set to the expected standard deviation of the
      * parameter), it must be non-zero
      * @param minValue minimum value allowed
      * @param maxValue maximum value allowed
      */
     public ParameterDriver(final String name,
                            final double referenceValue, final double scale,
                            final double minValue, final double maxValue) {
         if (FastMath.abs(scale) <= Precision.SAFE_MIN) {
             throw new OrekitException(OrekitMessages.TOO_SMALL_SCALE_FOR_PARAMETER,
                                       name, scale);
         }
         this.name                   = name;
         this.nameSpanMap            = new TimeSpanMap<>(SPAN + name + 0);
         this.referenceValue         = referenceValue;
         this.scale                  = scale;
         this.minValue               = minValue;
         this.maxValue               = maxValue;
         this.referenceDate          = null;
         // at construction the parameter driver
         // will be consider with only 1 estimated value over the all orbit
         // determination
         this.valueSpanMap           = new TimeSpanMap<>(referenceValue);
         this.selected               = false;
         this.observers              = new ArrayList<>();
         this.isEstimationContinuous = false;
     }
 
     /** Get current name span map of the parameterDriver, cut in interval
      * in accordance with value span map and validity period.
      * @return current name span map
      * @since 12.0
      */
     public TimeSpanMap<String> getNamesSpanMap() {
         return nameSpanMap;
     }
 
     /** Get value time span map for parameterDriver.
      * @return value time span map
      * @since 12.0
      */
     public TimeSpanMap<Double> getValueSpanMap() {
         return valueSpanMap;
     }
 
     /** Set current parameter value span map to match another driver. In order to keep
      * consistency, the validity period and name span map are updated.
      * @param driver for which the value span map wants to be copied for the
      * current driver
      * @since 12.0
      */
     public void setValueSpanMap(final ParameterDriver driver) {
         final TimeSpanMap<Double> previousValueSpanMap = driver.getValueSpanMap();
         valueSpanMap   = driver.getValueSpanMap();
         nameSpanMap    = driver.getNamesSpanMap();
         for (final ParameterObserver observer : observers) {
             observer.valueSpanMapChanged(previousValueSpanMap, this);
         }
     }
 
     /** Get the number of values to estimate that is to say the number.
      * of Span present in valueSpanMap
      * @return int the number of values to estimate
      * @since 12.0
      */
     public int getNbOfValues() {
         return valueSpanMap.getSpansNumber();
     }
 
     /** Get the dates of the transitions {@link TimeSpanMap}.
      * @return dates of the transitions {@link TimeSpanMap}
      * @since 12.0
      */
     public AbsoluteDate[] getTransitionDates() {
 
         // Get all transitions
         final List<AbsoluteDate> listDates = new ArrayList<>();
 
         // Extract all the transitions' dates
         for (Transition<Double> transition = getValueSpanMap().getFirstSpan().getEndTransition(); transition != null; transition = transition.next()) {
             listDates.add(transition.getDate());
         }
         // Return the array of transition dates
         return listDates.toArray(new AbsoluteDate[0]);
     }
 
     /** Get all values of the valueSpanMap in the chronological order.
      * @return double[] containing values of the valueSpanMap in the chronological order
      */
     public double[] getValues() {
         final double[] chronologicalValues = new double[getNbOfValues()];
         Span<Double> currentSpan = valueSpanMap.getFirstSpan();
         for (int i = 0; i < getNbOfValues() - 1; i++) {
             chronologicalValues[i] = currentSpan.getData();
             currentSpan = currentSpan.next();
         }
         chronologicalValues[getNbOfValues() - 1 ] = currentSpan.getData();
         return chronologicalValues;
     }
 
 
     /** Add an observer for this driver.
      * <p>
      * The observer {@link ParameterObserver#valueSpanMapChanged(TimeSpanMap, ParameterDriver)
      * valueSpanMapChanged} method is called once automatically when the
      * observer is added, and then called at each value change.
      * </p>
      * @param observer observer to add
           * while being updated
      */
     public void addObserver(final ParameterObserver observer) {
         observers.add(observer);
         observer.valueSpanMapChanged(getValueSpanMap(), this);
     }
 
     /** Remove an observer.
      * @param observer observer to remove
      * @since 9.1
      */
     public void removeObserver(final ParameterObserver observer) {
         for (final Iterator<ParameterObserver> iterator = observers.iterator(); iterator.hasNext();) {
             if (iterator.next() == observer) {
                 iterator.remove();
                 return;
             }
         }
     }
 
     /** Replace an observer.
      * @param oldObserver observer to replace
      * @param newObserver new observer to use
      * @since 10.1
      */
     public void replaceObserver(final ParameterObserver oldObserver, final ParameterObserver newObserver) {
         for (int i = 0; i < observers.size(); ++i) {
             if (observers.get(i) == oldObserver) {
                 observers.set(i, newObserver);
             }
         }
     }
 
     /** Get the observers for this driver.
      * @return an unmodifiable view of the observers for this driver
      * @since 9.1
      */
     public List<ParameterObserver> getObservers() {
         return Collections.unmodifiableList(observers);
     }
 
     /** Get parameter driver general name.
      * @return name
      */
     public String getName() {
         return name;
     }
 
     /** Get name of the parameter span for a specific date.
      * @param date date at which the name of the span wants to be known
      * @return name data of the name time span map at date
      */
     public String getNameSpan(final AbsoluteDate date) {
         return nameSpanMap.get(date);
     }
 
     /** Change the general name of this parameter driver.
      * @param name new name
      */
     public void setName(final String name) {
         final String previousName = this.name;
         this.name = name;
         for (final ParameterObserver observer : observers) {
             observer.nameChanged(previousName, this);
         }
         // the names time span map must also be updated with the new name
         if (nameSpanMap.getSpansNumber() > 1) {
             Span<String> currentNameSpan = nameSpanMap.getFirstSpan();
             nameSpanMap.addValidBefore(SPAN + name + 0, currentNameSpan.getEnd(), false);
             for (int spanNumber = 1; spanNumber < nameSpanMap.getSpansNumber(); ++spanNumber) {
                 currentNameSpan = nameSpanMap.getSpan(currentNameSpan.getEnd());
                 nameSpanMap.addValidAfter(SPAN + name + spanNumber, currentNameSpan.getStart(), false);
             }
         } else {
             nameSpanMap = new TimeSpanMap<>(SPAN + name + 0);
         }
     }
 
     /** Cut values and names time span map given orbit determination start and end and driver
      * periodicity.
      * <p>
      * For example for a drag coefficient the validity period would be
      * 1 days = 86400sec. To be called after constructor to cut the temporal axis with
      * the wanted parameter driver temporality for estimations on the wanted interval.
      * </p>
      * <p>
      * Must be called only once at the beginning of orbit
      * determination for example. If called several times, will throw exception. If parameter
      * estimations intervals must be changed then a new ParameterDriver must be created or the
      * function {@link #addSpanAtDate} should be used.
      * </p>
      * <p>
      * This function should not be called on {@link DateDriver} and
      * any of {@link ParameterDrivenDateIntervalDetector} attribute, because there is no sense to
      * estimate several values for dateDriver.
      * </p>
      * <p>
      * The choice of {@code orbitDeterminationStartDate}, {@code orbitDeterminationEndDate} and
      * {@code validityPeriodForDriver} in a case of orbit determination must be done carefully,
      * indeed, enough measurement should be available for each time interval or
      * the orbit determination won't converge.
      * </p>
      * @param orbitDeterminationStartDate start date for which the parameter driver
      * starts to be estimated.
      * @param orbitDeterminationEndDate end date for which the parameter driver
      * stops to be estimated.
      * @param validityPeriodForDriver validity period for which the parameter value
      * is effective (for example 1 day for drag coefficient). Warning, validityPeriod
      * should not be too short or the orbit determination won't converge.
      * @since 12.0
      */
     public void addSpans(final AbsoluteDate orbitDeterminationStartDate,
                          final AbsoluteDate orbitDeterminationEndDate,
                          final double validityPeriodForDriver) {
 
         // by convention 0 is when the parameter needs to be drived only on 1
         // interval from -INF to +INF time period
         if (getNbOfValues() != 1) {
             // throw exception if called several time, must be called only once at the beginning of orbit
             // determination, if the periods wants to be changed a new parameter must be created
             throw new OrekitIllegalStateException(OrekitMessages.PARAMETER_PERIODS_HAS_ALREADY_BEEN_SET, name);
         } else {
 
             int spanNumber = 1;
             AbsoluteDate currentDate = orbitDeterminationStartDate.shiftedBy(validityPeriodForDriver);
             //splitting the names and values span map accordingly with start and end of orbit determination
             //and validity period. A security is added to avoid having to few measurements point for a span
             //in order to assure orbit determination convergence
             while (currentDate.isBefore(orbitDeterminationEndDate) && orbitDeterminationEndDate.durationFrom(currentDate) > validityPeriodForDriver / 3.0) {
                 valueSpanMap.addValidAfter(getValue(currentDate), currentDate, false);
                 nameSpanMap.addValidAfter(SPAN + getName() + spanNumber++, currentDate, false);
                 currentDate = currentDate.shiftedBy(validityPeriodForDriver);
             }
         }
     }
 
     /** Create a new span in values and names time span map given a start date.
      * <b> One must be aware of the importance of choosing wise dates if this function is called
      * several times to create several span at wanted times. Indeed, if orbit determination is performed
      * it might not converge or find singular matrix if the spans are too short and contains to few measurements.
      * Must be called before any computation (for example before
      * orbit determination).</b>
      * @param spanStartDate wanted start date for parameter value interval
      * starts to be estimated.
      * @since 12.0
      */
     public void addSpanAtDate(final AbsoluteDate spanStartDate) {
 
         // Split value span map with new interval having for start date spanStartDate and end
         // date next span start date of +INF if no span is present after
         valueSpanMap.addValidAfter(getValue(spanStartDate), spanStartDate, false);
         nameSpanMap.addValidAfter(name, spanStartDate, false);
         // Rename spans recursively
         Span<String> currentNameSpan = nameSpanMap.getFirstSpan();
         nameSpanMap.addValidBefore(SPAN + name + 0, currentNameSpan.getEnd(), false);
 
         for (int spanNumber = 1; spanNumber < nameSpanMap.getSpansNumber(); spanNumber++) {
             currentNameSpan = nameSpanMap.getSpan(currentNameSpan.getEnd());
             nameSpanMap.addValidAfter(SPAN + name + spanNumber, currentNameSpan.getStart(), false);
         }
     }
 
     /** Get reference parameter value.
      * @return reference parameter value
      */
     public double getReferenceValue() {
         return referenceValue;
     }
 
     /** Set reference parameter value.
      * @since 9.3
      * @param referenceValue the reference value to set.
      */
     public void setReferenceValue(final double referenceValue) {
         final double previousReferenceValue = this.referenceValue;
         this.referenceValue = referenceValue;
         for (final ParameterObserver observer : observers) {
             observer.referenceValueChanged(previousReferenceValue, this);
         }
     }
 
     /** Get minimum parameter value.
      * @return minimum parameter value
      */
     public double getMinValue() {
         return minValue;
     }
 
     /** Set minimum parameter value.
      * @since 9.3
      * @param minValue the minimum value to set.
      */
     public void setMinValue(final double minValue) {
         final double previousMinValue = this.minValue;
         this.minValue = minValue;
         for (final ParameterObserver observer : observers) {
             observer.minValueChanged(previousMinValue, this);
         }
         // Check if all values are still not out of min/max range
         for (Span<Double> span = valueSpanMap.getFirstSpan(); span != null; span = span.next()) {
             setValue(getValue(span.getStart()), span.getStart());
         }
     }
 
     /** Get maximum parameter value.
      * @return maximum parameter value
      */
     public double getMaxValue() {
         return maxValue;
     }
 
     /** Set maximum parameter value.
      * @since 9.3
      * @param maxValue the maximum value to set.
      */
     public void setMaxValue(final double maxValue) {
         final double previousMaxValue = this.maxValue;
         this.maxValue = maxValue;
         for (final ParameterObserver observer : observers) {
             observer.maxValueChanged(previousMaxValue, this);
         }
         // Check if all values are still not out of min/max range
         for (Span<Double> span = valueSpanMap.getFirstSpan(); span != null; span = span.next()) {
             setValue(getValue(span.getStart()), span.getStart());
         }
     }
 
     /** Get scale.
      * @return scale
      */
     public double getScale() {
         return scale;
     }
 
     /** Set scale.
      * @since 9.3
      * @param scale the scale to set.
      */
     public void setScale(final double scale) {
         final double previousScale = this.scale;
         this.scale = scale;
         for (final ParameterObserver observer : observers) {
             observer.scaleChanged(previousScale, this);
         }
     }
 
     /** Get normalized value at specific date.
      * <p>
      * The normalized value is a non-dimensional value
      * suitable for use as part of a vector in an optimization
      * process. It is computed as {@code (current - reference)/scale}.
      * </p>
      * @param date date for which the normalized value wants to be known
      * @return normalized value
      */
     public double getNormalizedValue(final AbsoluteDate date) {
         return (getValue(date) - getReferenceValue()) / scale;
     }
 
     /** Get normalized value. Only useable on ParameterDriver
      * which have only 1 span on their TimeSpanMap value (that is
      * to say for which the setPeriod method wasn't called) otherwise
      * it will throw an exception.
      * <p>
      * The normalized value is a non-dimensional value
      * suitable for use as part of a vector in an optimization
      * process. It is computed as {@code (current - reference)/scale}.
      * </p>
      * @return normalized value
      */
     public double getNormalizedValue() {
         return (getValue() - getReferenceValue()) / scale;
     }
 
     /** Set normalized value at specific date.
      * <p>
      * The normalized value is a non-dimensional value
      * suitable for use as part of a vector in an optimization
      * process. It is computed as {@code (current - reference)/scale}.
      * </p>
      * @param date date for which the normalized value wants to be set
      * @param normalized value
      */
     public void setNormalizedValue(final double normalized, final AbsoluteDate date) {
         setValue(getReferenceValue() + scale * normalized, date);
     }
 
     /** Set normalized value at specific date. Only useable on ParameterDriver
      * which have only 1 span on their TimeSpanMap value (that is
      * to say for which the setPeriod method wasn't called) otherwise
      * it will throw an exception.
      * <p>
      * The normalized value is a non-dimensional value
      * suitable for use as part of a vector in an optimization
      * process. It is computed as {@code (current - reference)/scale}.
      * </p>
      * @param normalized value
      */
     public void setNormalizedValue(final double normalized) {
         setValue(getReferenceValue() + scale * normalized);
     }
 
     /** Get current reference date.
      * @return current reference date (null if it was never set)
      * @since 9.0
      */
     public AbsoluteDate getReferenceDate() {
         return referenceDate;
     }
 
     /** Set reference date.
      * @param newReferenceDate new reference date
      * @since 9.0
      */
     public void setReferenceDate(final AbsoluteDate newReferenceDate) {
         final AbsoluteDate previousReferenceDate = getReferenceDate();
         referenceDate = newReferenceDate;
         for (final ParameterObserver observer : observers) {
             observer.referenceDateChanged(previousReferenceDate, this);
         }
     }
 
     /** Get current parameter value. Only usable on ParameterDriver
      * which have only 1 span on their TimeSpanMap value (that is
      * to say for which the setPeriod method wasn't called)
      * @return current parameter value
      */
     public double getValue() {
         if (getNbOfValues() > 1) {
             throw new OrekitIllegalStateException(OrekitMessages.PARAMETER_WITH_SEVERAL_ESTIMATED_VALUES, name, "getValue(date)");
         }
         // Attention voir si qlqchose est retourné si une exception est levée
         return valueSpanMap.getFirstSpan().getData();
     }
 
     /** Get current parameter value at specific date, depending on isContinuousEstimation
      * value, the value returned will be obtained by step estimation or continuous estimation.
      * @param date date for which the value wants to be known. Only if
      * parameter driver has 1 value estimated over the all orbit determination
      * period (not validity period intervals for estimation), the date value can
      * be <em>{@code null}</em> and then the only estimated value will be
      * returned, in this case the date can also be whatever the value returned would
      * be the same. Moreover in this particular case one can also call the {@link #getValue()}.
      * @return current parameter value at date date, or for the all period if
      * no validity period (= 1 value estimated over the all orbit determination
      * period)
      */
     public double getValue(final AbsoluteDate date) {
         return isEstimationContinuous ? getValueContinuousEstimation(date) : getValueStepEstimation(date);
     }
 
     /** Get current parameter value at specific date with step estimation.
      * @param date date for which the value wants to be known. Only if
      * parameter driver has 1 value estimated over the all orbit determination
      * period (not validity period intervals for estimation), the date value can
      * be <em>{@code null}</em> and then the only estimated value will be
      * returned, in this case the date can also be whatever the value returned would
      * be the same. Moreover in this particular case one can also call the {@link #getValue()}.
      * @return current parameter value at date date, or for the all period if
      * no validity period (= 1 value estimated over the all orbit determination
      * period)
      */
     public double getValueStepEstimation(final AbsoluteDate date) {
         return getNbOfValues() == 1 ? valueSpanMap.getFirstSpan().getData() : valueSpanMap.get(date);
     }
 
     /** Get current parameter value at specific date with continuous estimation.
      * @param date date for which the value wants to be known. Only if
      * parameter driver has 1 value estimated over the all orbit determination
      * period (not validity period intervals for estimation), the date value can
      * be <em>{@code null}</em> and then the only estimated value will be
      * returned, in this case the date can also be whatever the value returned would
      * be the same. Moreover in this particular case one can also call the {@link #getValue()}.
      * @return current parameter value at date date, or for the all period if
      * no validity period (= 1 value estimated over the all orbit determination
      * period)
      * @since 12.0
      */
     public double getValueContinuousEstimation(final AbsoluteDate date) {
         //TODO
         throw new UnsupportedOperationException();
     }
 
     /** Get the value as a gradient at special date.
      * @param freeParameters total number of free parameters in the gradient
      * @param indices indices of the differentiation parameters in derivatives computations
      * @return value with derivatives, will throw exception if called on a PDriver having
      * several values driven
      * @since 10.2
      */
     public Gradient getValue(final int freeParameters, final Map<String, Integer> indices) {
         Integer index = null;
         for (Span<String> span = nameSpanMap.getFirstSpan(); span != null; span = span.next()) {
             index = indices.get(span.getData());
             if (index != null) {
                 break;
             }
         }
         return (index == null) ? Gradient.constant(freeParameters, getValue()) : Gradient.variable(freeParameters, index, getValue());
     }
 
     /** Get the value as a gradient at special date.
      * @param freeParameters total number of free parameters in the gradient
      * @param indices indices of the differentiation parameters in derivatives computations,
      * must be span name and not driver name
      * @param date date for which the value wants to be known. Only if
      * parameter driver has 1 value estimated over the all orbit determination
      * period (not validity period intervals for estimation), the date value can
      * be <em>{@code null}</em> and then the only estimated value will be
      * returned
      * @return value with derivatives
      * @since 10.2
      */
     public Gradient getValue(final int freeParameters, final Map<String, Integer> indices, final AbsoluteDate date) {
         Integer index = null;
         for (Span<String> span = nameSpanMap.getFirstSpan(); span != null; span = span.next()) {
             index = indices.get(span.getData());
             if (index != null) {
                 break;
             }
         }
         return (index == null) ? Gradient.constant(freeParameters, getValue(date)) : Gradient.variable(freeParameters, index, getValue(date));
     }
 
     /** Set parameter value at specific date.
      * <p>
      * If {@code newValue} is below {@link #getMinValue()}, it will
      * be silently set to {@link #getMinValue()}. If {@code newValue} is
      * above {@link #getMaxValue()}, it will be silently set to {@link
      * #getMaxValue()}.
      * </p>
      * @param date date for which the value wants to be set. Only if
      * parameter driver has 1 value estimated over the all orbit determination
      * period (not validity period intervals for estimation), the date value can
      * be <em>{@code null}</em>
      * @param newValue new value to set
      */
     public void setValue(final double newValue, final AbsoluteDate date) {
 
         double previousValue = Double.NaN;
         AbsoluteDate referenceDateSpan = AbsoluteDate.ARBITRARY_EPOCH;
 
         // if valid for infinity (only 1 value estimation for the orbit determination )
         if (getNbOfValues() == 1) {
             previousValue = this.getValue(referenceDateSpan);
             this.valueSpanMap = new TimeSpanMap<>(FastMath.max(minValue, FastMath.min(maxValue, newValue)));
         // if needs to be estimated per time range / validity period
 
         // if several value intervals
         } else {
             final Span<Double> valueSpan = valueSpanMap.getSpan(date);
             previousValue = valueSpan.getData();
             referenceDateSpan = valueSpan.getStart();
             // if the Span considered is from past infinity to valueSpanEndDate it is
             // impossible to addValidAfter past infinity because it is creating a new span that
             // is why the below trick was set up
             if (referenceDateSpan.equals(AbsoluteDate.PAST_INFINITY)) {
                 referenceDateSpan = valueSpan.getEnd();
                 this.valueSpanMap.addValidBefore(FastMath.max(minValue, FastMath.min(maxValue, newValue)),
                                                  referenceDateSpan, false);
             } else {
                 this.valueSpanMap.addValidAfter(FastMath.max(minValue, FastMath.min(maxValue, newValue)),
                                                 referenceDateSpan, false);
             }
         }
 
         for (final ParameterObserver observer : observers) {
             observer.valueChanged(previousValue, this, date);
         }
     }
 
 
     /** Set parameter value. Only usable on ParameterDriver
      * which have only 1 span on their TimeSpanMap value (that is
      * to say for which the setPeriod method wasn't called)
      * <p>
      * If {@code newValue} is below {@link #getMinValue()}, it will
      * be silently set to {@link #getMinValue()}. If {@code newValue} is
      * above {@link #getMaxValue()}, it will be silently set to {@link
      * #getMaxValue()}.
      * </p>
      * @param newValue new value to set
      */
     public void setValue(final double newValue) {
         if (getNbOfValues() == 1) {
             final AbsoluteDate referenceDateSpan = AbsoluteDate.ARBITRARY_EPOCH;
             final double previousValue = this.getValue(referenceDateSpan);
             this.valueSpanMap = new TimeSpanMap<>(FastMath.max(minValue, FastMath.min(maxValue, newValue)));
             for (final ParameterObserver observer : observers) {
                 observer.valueChanged(previousValue, this, referenceDateSpan);
             }
         } else {
             throw new OrekitIllegalStateException(OrekitMessages.PARAMETER_WITH_SEVERAL_ESTIMATED_VALUES, name, "setValue(date)");
         }
     }
 
     /** Configure a parameter selection status.
      * <p>
      * Selection is used for estimated parameters in orbit determination,
      * or to compute the Jacobian matrix in partial derivatives computation.
      * </p>
      * @param selected if true the parameter is selected,
      * otherwise it will be fixed
      */
     public void setSelected(final boolean selected) {
         final boolean previousSelection = isSelected();
         this.selected = selected;
         for (final ParameterObserver observer : observers) {
             observer.selectionChanged(previousSelection, this);
         }
     }
 
     /** Check if parameter is selected.
      * <p>
      * Selection is used for estimated parameters in orbit determination,
      * or to compute the Jacobian matrix in partial derivatives computation.
      * </p>
      * @return true if parameter is selected, false if it is not
      */
     public boolean isSelected() {
         return selected;
     }
 
     /** Set parameter estimation to continuous, by default step estimation.
      * <p> Continuous estimation : when a value wants to be known at date
      * t, the value returned will be an interpolation between start value
      * of the span corresponding to date t and end value (which corresponds
      * to the start of the next span).
      * </p>
      * <p> Step estimation : when a value wants to be
      * known at date t, the value returned will be the value of the beginning
      * of span corresponding to date t, step estimation.
      * </p>
      * @param continuous if true the parameter will be estimated
      * with continuous estimation, if false with step estimation.
      */
     public void setContinuousEstimation(final boolean continuous) {
         final boolean previousEstimation = isContinuousEstimation();
         this.isEstimationContinuous = continuous;
         for (final ParameterObserver observer : observers) {
             observer.estimationTypeChanged(previousEstimation, this);
         }
     }
 
     /** Check if parameter estimation is continuous, that is to say when
      * a value wants to be known at date t, the value returned
      * will be an interpolation between start value on span corresponding
      * for date t and end value (which corresponds to the start of the next
      * span), continuous estimation. Or not continuous, that is to say when a value wants to be
      * known at date t, the value returned will be the value of the start
      * of span corresponding to date t, step estimation.
      * @return true if continuous estimation/definition, false if step estimation/definition
      * @since 12.0
      */
     public boolean isContinuousEstimation() {
         return isEstimationContinuous;
     }
 
     /** Get a text representation of the parameter.
      * @return text representation of the parameter, in the form name = value.
      */
     public String toString() {
         return name + " = " + valueSpanMap.get(AbsoluteDate.ARBITRARY_EPOCH);
     }
 
 }