package org.djutils.math.functions;
   
   import java.util.SortedSet;
   import java.util.TreeSet;
   
   import org.djutils.exceptions.Throw;
   
   /**
    * Nan; MathFunction that returns NaN.
   * <p>
   * Copyright (c) 2024-2025 Delft University of Technology, Jaffalaan 5, 2628 BX Delft, the Netherlands. All rights reserved. See
   * for project information <a href="https://djutils.org" target="_blank"> https://djutils.org</a>. The DJUTILS project is
   * distributed under a three-clause BSD-style license, which can be found at
   * <a href="https://djutils.org/docs/license.html" target="_blank"> https://djutils.org/docs/license.html</a>.
   * </p>
   * @author <a href="https://github.com/averbraeck">Alexander Verbraeck</a>
   * @author <a href="https://github.com/peter-knoppers">Peter Knoppers</a>
   * @author <a href="https://github.com/wjschakel">Wouter Schakel</a>
   */
  public final class Nan implements MathFunction
  {
      /** MathFunction that always return NaN (Not A Number). */
      public static final Nan NAN = new Nan();
  
      /**
       * Utility class; do not instantiate.
       */
      private Nan()
      {
          // Do not instantiate
      }
  
      @Override
      public Double apply(final Double x)
      {
          return Double.NaN;
      }
  
      @Override
      public MathFunction getDerivative()
      {
          return this; // same NaN value, same domain
      }
  
      @Override
      public MathFunction scaleBy(final double factor)
      {
          return this;
      }
  
      @Override
      public int sortPriority()
      {
          return 3;
      }
  
      @Override
      public int compareWithinSubType(final MathFunction other)
      {
          Throw.when(!(other instanceof Nan), IllegalArgumentException.class, "other is of wrong type");
          return 0;
      }
  
      @Override
      public KnotReport getKnotReport(final Interval<?> interval)
      {
          return interval.low() < interval.high() ? KnotReport.KNOWN_INFINITE : KnotReport.KNOWN_FINITE;
      }
  
      @Override
      public SortedSet<Double> getKnots(final Interval<?> interval)
      {
          if (interval.low() == interval.high())
          {
              SortedSet<Double> result =  new TreeSet<Double>();
              result.add(interval.low());
              return result;
          }
          throw new UnsupportedOperationException("there are Infinitely many knots in " + interval);
      }
  
      @Override
      public String toString()
      {
          return "NaN";
      }
  
  }