package org.djutils.draw.curve;
   
   import org.djutils.draw.Direction3d;
   import org.djutils.draw.curve.Flattener.FlattableCurve;
   import org.djutils.draw.line.PolyLine3d;
   import org.djutils.draw.point.DirectedPoint3d;
   import org.djutils.draw.point.Point3d;
   
   /**
   * This interface narrows down the interface of continuous curves for 3d use.
   * <p>
   * Copyright (c) 2023-2025 Delft University of Technology, PO Box 5, 2600 AA, Delft, the Netherlands. All rights reserved. <br>
   * BSD-style license. See <a href="https://opentrafficsim.org/docs/license.html">OpenTrafficSim License</a>.
   * </p>
   * @author <a href="https://www.tudelft.nl/averbraeck">Alexander Verbraeck</a>
   * @author <a href="https://www.tudelft.nl/pknoppers">Peter Knoppers</a>
   * @author <a href="https://github.com/wjschakel">Wouter Schakel</a>
   */
  interface Curve3d
          extends Curve<DirectedPoint3d, Direction3d, Point3d, Flattener3d, PolyLine3d>, FlattableCurve<Point3d, Direction3d>
  {
  
      @Override
      default DirectedPoint3d getStartPoint()
      {
          return new DirectedPoint3d(getPoint(0.0), getDirection(0.0));
      }
  
      @Override
      default DirectedPoint3d getEndPoint()
      {
          return new DirectedPoint3d(getPoint(1.0), getDirection(1.0));
      }
  
      @Override
      default Direction3d getStartDirection()
      {
          return getStartPoint().getDir();
      }
  
      @Override
      default Direction3d getEndDirection()
      {
          return getEndPoint().getDir();
      }
  
      @Override
      default Direction3d getDirection(final double fraction)
      {
          Point3d p1, p2;
          if (fraction < 0.5) // to prevent going above 1.0
          {
              p1 = getPoint(fraction);
              p2 = getPoint(fraction + 1e-6);
          }
          else
          {
              p1 = getPoint(fraction - 1e-6);
              p2 = getPoint(fraction);
          }
          return p1.directionTo(p2);
      }
  
  }