package org.djutils.draw.curve;
   
   import org.djutils.draw.curve.Flattener.FlattableCurve;
   import org.djutils.draw.line.PolyLine2d;
   import org.djutils.draw.point.DirectedPoint2d;
   import org.djutils.draw.point.Point2d;
   
   /**
    * This interface narrows down the interface of continuous curves for 2d 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 Curve2d extends Curve<DirectedPoint2d, Double, Point2d, Flattener2d, PolyLine2d>, FlattableCurve<Point2d, Double>
  {
  
      @Override
      default DirectedPoint2d getStartPoint()
      {
          return new DirectedPoint2d(getPoint(0.0), getDirection(0.0));
      }
  
      @Override
      default DirectedPoint2d getEndPoint()
      {
          return new DirectedPoint2d(getPoint(1.0), getDirection(1.0));
      }
  
      @Override
      default Double getStartDirection()
      {
          return getStartPoint().dirZ;
      }
  
      @Override
      default Double getEndDirection()
      {
          return getEndPoint().dirZ;
      }
  
      @Override
      default Double getDirection(final double fraction)
      {
          Point2d 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);
      }
      
  }