package org.djutils.draw.curve;
   
   import java.util.ArrayList;
   import java.util.Iterator;
   import java.util.List;
   import java.util.Map.Entry;
   import java.util.NavigableMap;
   import java.util.NavigableSet;
   import java.util.Set;
  import java.util.SortedSet;
  import java.util.TreeMap;
  import java.util.TreeSet;
  
  import org.djutils.draw.Transform2d;
  import org.djutils.draw.function.ContinuousPiecewiseLinearFunction;
  import org.djutils.draw.line.PolyLine2d;
  import org.djutils.draw.line.Ray2d;
  import org.djutils.draw.point.DirectedPoint2d;
  import org.djutils.draw.point.Point2d;
  import org.djutils.exceptions.Throw;
  import org.djutils.math.functions.MathFunction.TupleSt;
  
  /**
   * Continuous definition of a cubic Bézier curves in 2d. This extends from the more general {@code Bezier} as certain
   * methods are applied to calculate e.g. the roots, that are specific to cubic Bézier curves. With such information this
   * class can also specify information to be a {@code Curve}.
   * <p>
   * Copyright (c) 2023-2025 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://www.tudelft.nl/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>
   * @see <a href="https://pomax.github.io/bezierinfo/">B&eacute;zier info</a>
   */
  public class BezierCubic2d extends Bezier2d implements Curve2d, OffsetCurve2d, Curvature
  {
  
      /** Angle below which segments are seen as straight. */
      private static final double STRAIGHT = Math.PI / 36000; // 1/100th of a degree
  
      /** Start point with direction. */
      private final DirectedPoint2d startPoint;
  
      /** End point with direction. */
      private final DirectedPoint2d endPoint;
  
      /** Length. */
      private final double length;
  
      /**
       * Create a cubic B&eacute;zier curve.
       * @param start start point.
       * @param control1 first intermediate shape point.
       * @param control2 second intermediate shape point.
       * @param end end point.
       * @throws NullPointerException when <code>start</code>, <code>control1</code>, <code>control2</code>, or <code>end</code>
       *             is <code>null</code>
       */
      public BezierCubic2d(final Point2d start, final Point2d control1, final Point2d control2, final Point2d end)
      {
          super(start, control1, control2, end);
          this.startPoint = new DirectedPoint2d(start, control1);
          this.endPoint = new DirectedPoint2d(end, control2.directionTo(end));
          this.length = super.getLength();
      }
  
      /**
       * Create a cubic B&eacute;zier curve.
       * @param points array containing four <code>Point2d</code> objects
       * @throws NullPointerException when <code>points</code> is <code>null</code>, or contains a <code>null</code> value
       * @throws IllegalArgumentException when length of <code>points</code> is not equal to <code>4</code>
       */
      public BezierCubic2d(final Point2d[] points)
      {
          this(checkArray(points)[0], points[1], points[2], points[3]);
      }
  
      /**
       * Verify that a Point2d[] contains exactly 4 elements.
       * @param points the array to check
       * @return the provided array
       * @throws NullPointerException when <code>points</code> is <code>null</code>, or contains a <code>null</code> value
       * @throws IllegalArgumentException when length of <code>points</code> is not equal to <code>4</code>
       */
      private static Point2d[] checkArray(final Point2d[] points)
      {
          Throw.when(points.length != 4, IllegalArgumentException.class, "points must contain exactly 4 Point2d objects");
          return points;
      }
  
      /**
       * Construct a BezierCubic2d from <code>start</code> to <code>end</code> with two generated control points. This constructor
       * creates two control points. The first of these is offset from <code>start</code> in the direction of the
       * <code>start</code> and at a distance from <code>start</code> equal to half the distance from <code>start</code> to
       * <code>end</code>. The second is placed in a similar relation to <code>end</code>.
       * @param start the start point and start direction of the B&eacute;zier curve
      * @param end the end point and end direction of the B&eacute;zier curve
      * @throws NullPointerException when <code>start</code>, or <code>end</code> is <code>null</code>
      * @throws IllegalArgumentException when <code>start</code> and <code>end</code> are at the same location
      */
     public BezierCubic2d(final Ray2d start, final Ray2d end)
     {
         this(start, end, 1.0);
     }
 
     /**
      * Construct a BezierCubic2d from <code>start</code> to <code>end</code> with two generated control points. The control
      * points are in placed along the direction of the <code>start</code>, resp. <code>end</code> points at a distance that
      * depends on the <code>shape</code> parameter.
      * @param start the start point and start direction of the B&eacute;zier curve
      * @param end the end point and end direction of the B&eacute;zier curve
      * @param shape 1 = control points at half the distance between start and end, &gt; 1 results in a pointier shape, &lt; 1
      *            results in a flatter shape, value should be above 0 and finite
      * @throws NullPointerException when <code>start</code>, or <code>end</code> is <code>null</code>
      * @throws IllegalArgumentException when <code>start</code> and <code>end</code> are at the same location,
      *             <code>shape &le; 0</code>, <code>shape</code> is <code>NaN</code>, or infinite
      */
     public BezierCubic2d(final Ray2d start, final Ray2d end, final double shape)
     {
         this(start, end, shape, false);
     }
 
     /**
      * Construct a BezierCubic2d from start to end with two generated control points.
      * @param start the start point and start direction of the B&eacute;zier curve
      * @param end the end point and end direction of the B&eacute;zier curve
      * @param shape the shape; higher values put the generated control points further away from end and result in a pointier
      *            B&eacute;zier curve
      * @param weighted whether weights will be applied
      * @throws NullPointerException when <code>start</code>, or <code>end</code> is <code>null</code>
      * @throws IllegalArgumentException when <code>start</code> and <code>end</code> are at the same location,
      *             <code>shape &le; 0</code>, <code>shape</code> is <code>NaN</code>, or infinite
      */
     public BezierCubic2d(final Ray2d start, final Ray2d end, final double shape, final boolean weighted)
     {
         this(createControlPoints(start, end, shape, weighted));
     }
 
     /** Unit vector for transformations in createControlPoints. */
     private static final Point2d UNIT_VECTOR2D = new Point2d(1, 0);
 
     /**
      * Create control points for a cubic B&eacute;zier curve defined by two Rays.
      * @param start the start point (and direction)
      * @param end the end point (and direction)
      * @param shape the shape; higher values put the generated control points further away from end and result in a pointier
      *            B&eacute;zier curve
      * @param weighted whether weights will be applied
      * @return an array of four Point2d elements: start, the first control point, the second control point, end.
      * @throws NullPointerException when <code>start</code>, or <code>end</code> is <code>null</code>
      * @throws IllegalArgumentException when <code>start</code> and <code>end</code> are at the same location,
      *             <code>shape &le; 0</code>, <code>shape</code> is <code>NaN</code>, or infinite
      */
     private static Point2d[] createControlPoints(final Ray2d start, final Ray2d end, final double shape, final boolean weighted)
     {
         Throw.whenNull(start, "start");
         Throw.whenNull(end, "end");
         Throw.when(start.distanceSquared(end) == 0, IllegalArgumentException.class,
                 "Cannot create control points if start and end points coincide");
         Throw.whenNaN(shape, "shape");
         Throw.when(shape <= 0 || Double.isInfinite(shape), IllegalArgumentException.class,
                 "shape must be a finite, positive value");
 
         Point2d control1;
         Point2d control2;
         if (weighted)
         {
             // each control point is 'w' * the distance between the end-points away from the respective end point
             // 'w' is a weight given by the distance from the end point to the extended line of the other end point
             double distance = shape * start.distance(end);
             double dStart = start.distance(end.projectOrthogonalExtended(start));
             double dEnd = end.distance(start.projectOrthogonalExtended(end));
             double wStart = dStart / (dStart + dEnd);
             double wEnd = dEnd / (dStart + dEnd);
             control1 = new Transform2d().translate(start).rotation(start.dirZ).scale(distance * wStart, distance * wStart)
                     .transform(UNIT_VECTOR2D);
             // - (minus) as the angle is where the line leaves, i.e. from shape point to end
             control2 = new Transform2d().translate(end).rotation(end.dirZ + Math.PI).scale(distance * wEnd, distance * wEnd)
                     .transform(UNIT_VECTOR2D);
         }
         else
         {
             // each control point is at shape times half the distance between the end-points away from the respective end point
             double distance = shape * start.distance(end) / 2.0;
             control1 = start.getLocation(distance);
             // new Transform2d().translate(start).rotation(start.phi).scale(distance, distance).transform(UNIT_VECTOR2D);
             control2 = end.getLocationExtended(-distance);
             // new Transform2d().translate(end).rotation(end.phi + Math.PI).scale(distance, distance).transform(UNIT_VECTOR2D);
         }
         return new Point2d[] {start, control1, control2, end};
     }
 
     @Override
     public Point2d getPoint(final double t)
     {
         return new Point2d(B3(t, this.x), B3(t, this.y));
     }
 
     /**
      * Calculate the cubic B&eacute;zier point with B(t) = (1 - t)<sup>3</sup>P<sub>0</sub> + 3t(1 - t)<sup>2</sup>
      * P<sub>1</sub> + 3t<sup>2</sup> (1 - t) P<sub>2</sub> + t<sup>3</sup> P<sub>3</sub>.
      * @param t the fraction
      * @param p the four control values of this dimension of this curve
      * @return the cubic B&eacute;zier value B(t)
      */
     @SuppressWarnings("checkstyle:methodname")
     private static double B3(final double t, final double[] p)
     {
         double t2 = t * t;
         double t3 = t2 * t;
         double m = (1.0 - t);
         double m2 = m * m;
         double m3 = m2 * m;
         return m3 * p[0] + 3.0 * t * m2 * p[1] + 3.0 * t2 * m * p[2] + t3 * p[3];
     }
 
     /**
      * Start curvature of this BezierCubic2d.
      * @return start curvature of this BezierCubic2d
      */
     @Override
     public double getStartCurvature()
     {
         return curvature(0.0);
     }
 
     /**
      * End curvature of this BezierCubic2d.
      * @return end curvature of this BezierCubic2d
      */
     @Override
     public double getEndCurvature()
     {
         return curvature(1.0);
     }
 
     /**
      * Returns the root t values, where each of the sub-components derivative for x and y are 0.0.
      * @return set of root t values, sorted and in the range (0, 1).
      */
     private SortedSet<Double> getRoots()
     {
         TreeSet<Double> roots = new TreeSet<>();
         for (double[] dimension : new double[][] {this.x, this.y})
         {
             // Coefficients of the derivative in this dimension (for quadaratic B&eacure;zier)
             double a = 3.0 * (-dimension[0] + 3.0 * dimension[1] - 3.0 * dimension[2] + dimension[3]);
             double b = 6.0 * (dimension[0] - 2.0 * dimension[1] + dimension[2]);
             double c = 3.0 * (dimension[1] - dimension[0]);
             // ABC formula
             double discriminant = b * b - 4.0 * a * c;
             if (discriminant > 0)
             {
                 double sqrtDiscriminant = Math.sqrt(discriminant);
                 double a2 = 2.0 * a;
                 roots.add((-b + sqrtDiscriminant) / a2);
                 roots.add((-b - sqrtDiscriminant) / a2);
             }
         }
         // Only roots in range (0.0 ... 1.0) are valid and useful
         return roots.subSet(0.0, false, 1.0, false);
     }
 
     /**
      * Returns the inflection t values; these are the points where curvature changes sign.
      * @return set of inflection t values, sorted and in the range (0, 1)
      */
     private SortedSet<Double> getInflections()
     {
         // Align: translate so first point is (0, 0), rotate so last point is on x=axis (y = 0)
         Point2d[] aligned = new Point2d[4];
         double ang = -Math.atan2(getY(3) - getY(0), getX(3) - getX(0));
         double cosAng = Math.cos(ang);
         double sinAng = Math.sin(ang);
         for (int i = 0; i < 4; i++)
         {
             aligned[i] = new Point2d(cosAng * (getX(i) - getX(0)) - sinAng * (getY(i) - getY(0)),
                     sinAng * (getX(i) - getX(0)) + cosAng * (getY(i) - getY(0)));
         }
 
         // Inflection as curvature = 0, using:
         // curvature = x'(t)*y''(t) + y'(t)*x''(t) = 0
         // (this is highly simplified due to the alignment, removing many terms)
         double a = aligned[2].x * aligned[1].y;
         double b = aligned[3].x * aligned[1].y;
         double c = aligned[1].x * aligned[2].y;
         double d = aligned[3].x * aligned[2].y;
 
         double x = -3.0 * a + 2.0 * b + 3.0 * c - d;
         double y = 3.0 * a - b - 3.0 * c;
         double z = c - a;
 
         // ABC formula (on x, y, z)
         TreeSet<Double> inflections = new TreeSet<>();
         if (Math.abs(x) < 1.0e-6)
         {
             if (Math.abs(y) >= 1.0e-12)
             {
                 inflections.add(-z / y);
             }
         }
         else
         {
             double det = y * y - 4.0 * x * z;
             double sq = Math.sqrt(det);
             double d2 = 2 * x;
             if (det >= 0.0 && Math.abs(d2) >= 1e-12)
             {
                 inflections.add(-(y + sq) / d2);
                 inflections.add((sq - y) / d2);
             }
         }
 
         // Only inflections in range (0.0 ... 1.0) are valid and useful
         return inflections.subSet(0.0, false, 1.0, false);
     }
 
     /**
      * Returns the offset t values.
      * @param fractions length fractions at which offsets are defined.
      * @return set of offset t values, sorted and only those in the range <code>(0, 1)</code>
      */
     private SortedSet<Double> getOffsetT(final Set<Double> fractions)
     {
         TreeSet<Double> result = new TreeSet<>();
         for (Double f : fractions)
         {
             if (f > 0.0 && f < 1.0)
             {
                 result.add(getT(f * getLength()));
             }
         }
         return result;
     }
 
     /**
      * Returns the offset t values.
      * @param fractions yields the fractions at which offsets are defined.
      * @return set of offset t values, sorted and only those in the range <code>(0, 1)</code>
      */
     private SortedSet<Double> getOffsetT(final Iterable<TupleSt> fractions)
     {
         TreeSet<Double> result = new TreeSet<>();
         for (TupleSt tuple : fractions)
         {
             double f = tuple.s();
             if (f > 0.0 && f < 1.0)
             {
                 result.add(getT(f * getLength()));
             }
         }
         return result;
     }
 
     /**
      * Returns the t value at the provided length along the B&eacute;zier curve. This method uses an iterative approach with a
      * precision of 1e-6.
      * @param position position along the B&eacute;zier curve.
      * @return t value at the provided length along the B&eacute;zier curve.
      */
     @Override
     public double getT(final double position)
     {
         if (0.0 == position)
         {
             return 0.0;
         }
         if (this.length == position)
         {
             return 1.0;
         }
         // start at 0.0 and 1.0, cut in half, see which half to use next
         double t0 = 0.0;
         double t2 = 1.0;
         double t1 = 0.5;
         while (t2 > t0 + 1.0e-6)
         {
             t1 = (t2 + t0) / 2.0;
             SplitBeziers parts = split(t1);
             double len1 = parts.first.getLength();
             if (len1 < position)
             {
                 t0 = t1;
             }
             else
             {
                 t2 = t1;
             }
         }
         return t1;
     }
 
     /**
      * Wrapper for two BezierCubic2d objects.
      * @param first the part before the split point
      * @param remainder the part after the split point
      */
     private record SplitBeziers(BezierCubic2d first, BezierCubic2d remainder)
     {
     };
 
     /**
      * Splits the B&eacute;zier in two B&eacute;zier curves of the same order.
      * @param t t value along the B&eacute;zier curve to apply the split
      * @return the B&eacute;zier curve before t, and the B&eacute;zier curve after t
      * @throws IllegalArgumentException when <code>t &lt; 0.0</code>, or <code>t &gt; 1.0</code>
      */
     public SplitBeziers split(final double t)
     {
         Throw.when(t < 0.0 || t > 1.0, IllegalArgumentException.class, "t value should be in the range [0.0 ... 1.0].");
         // System.out.println("Splitting at " + t + ": " + this);
         List<Point2d> p1 = new ArrayList<>();
         List<Point2d> p2 = new ArrayList<>();
         List<Point2d> all = new ArrayList<>();
         for (int i = 0; i < size(); i++)
         {
             all.add(new Point2d(getX(i), getY(i)));
         }
         split0(t, all, p1, p2);
         SplitBeziers result = new SplitBeziers(new BezierCubic2d(p1.get(0), p1.get(1), p1.get(2), p1.get(3)),
                 new BezierCubic2d(p2.get(3), p2.get(2), p2.get(1), p2.get(0)));
         return result;
     }
 
     /**
      * Performs the iterative algorithm of Casteljau to derive the split B&eacute;zier curves.
      * @param t t value along the B&eacute;zier to apply the split.
      * @param p shape points of B&eacute;zier still to split.
      * @param p1 shape points of first part, accumulated in the recursion.
      * @param p2 shape points of second part, accumulated in the recursion.
      */
     private void split0(final double t, final List<Point2d> p, final List<Point2d> p1, final List<Point2d> p2)
     {
         if (p.size() == 1)
         {
             p1.add(p.get(0));
             p2.add(p.get(0));
         }
         else
         {
             List<Point2d> pNew = new ArrayList<>();
             for (int i = 0; i < p.size() - 1; i++)
             {
                 if (i == 0)
                 {
                     p1.add(p.get(i));
                 }
                 if (i == p.size() - 2)
                 {
                     p2.add(p.get(i + 1));
                 }
                 double t1 = 1.0 - t;
                 pNew.add(new Point2d(t1 * p.get(i).x + t * p.get(i + 1).x, t1 * p.get(i).y + t * p.get(i + 1).y));
             }
             split0(t, pNew, p1, p2);
         }
     }
 
     /** The derivative B&eacute;zier used (and cashed) to compute the direction at some t value. */
     private Bezier2d derivative = null;
 
     @Override
     public PolyLine2d toPolyLine(final Flattener2d flattener)
     {
         Throw.whenNull(flattener, "Flattener may not be null");
         if (null == this.derivative)
         {
             this.derivative = derivative();
         }
         return flattener.flatten(this);
     }
 
     /**
      * A B&eacute;zier curve does not have a trivial offset. Hence, we split the B&eacute;zier along points of 3 types. 1)
      * roots, where the derivative of either the x-component or y-component is 0, such that we obtain C-shaped scalable
      * segments, 2) inflections, where the curvature changes sign and the offset and offset angle need to flip sign, and 3)
      * offset fractions so that the intended offset segments can be adhered to. Note that C-shaped segments can be scaled
      * similar to a circle arc, whereas S-shaped segments have no trivial scaling and thus must be split.
      */
     private NavigableMap<Double, BezierCubic2d> segments;
 
     /** The offset data for which the (current) segments were created. */
     private ContinuousPiecewiseLinearFunction ofForSegments = null;
 
     /**
      * Check if the current segment map matches the provided offset data. If not; rebuild the segments to match.
      * @param of offset function
      */
     private void updateSegments(final ContinuousPiecewiseLinearFunction of)
     {
         Throw.whenNull(of, "Offsets may not be null");
         if (of.equals(this.ofForSegments))
         {
             return;
         }
         // Detect straight line
         double ang1 = Math.atan2(getY(1) - getY(0), getX(1) - getX(0));
         double ang2 = Math.atan2(getY(3) - getY(1), getX(3) - getX(1));
         double ang3 = Math.atan2(getY(3) - getY(2), getX(3) - getX(2));
         boolean straight =
                 Math.abs(ang1 - ang2) < STRAIGHT && Math.abs(ang2 - ang3) < STRAIGHT && Math.abs(ang3 - ang1) < STRAIGHT;
 
         this.segments = new TreeMap<>();
 
         // Gather all points to split segments, and their types (Root, Inflection, or Kink in offsets)
         NavigableMap<Double, Boundary> splits0 = new TreeMap<>(); // splits0 & splits because splits0 must be effectively final
         if (!straight)
         {
             getRoots().forEach((t) -> splits0.put(t, Boundary.ROOT));
             getInflections().forEach((t) -> splits0.put(t, Boundary.INFLECTION));
         }
         getOffsetT(of).forEach((t) -> splits0.put(t, Boundary.KNOT));
         NavigableMap<Double, Boundary> splits = splits0.subMap(1e-6, false, 1.0 - 1e-6, false);
 
         // Initialize loop variables
         // Work on a copy of the offset fractions, so we can remove each we use.
         // Skip t == 0.0 while collecting the split points -on- this B&eacute;zier
         NavigableSet<Double> fCrossSectionRemain = new TreeSet<>();
         for (TupleSt tuple : of)
         {
             if (tuple.s() > 0.0)
             {
                 fCrossSectionRemain.add(tuple.s());
             }
         }
         double lengthTotal = getLength();
         BezierCubic2d currentBezier = this;
         // System.out.println("Current bezier is " + this);
         double lengthSoFar = 0.0;
         // curvature and angle sign, flips at each inflection, start based on initial curve
         double sig = Math.signum((getY(1) - getY(0)) * (getX(2) - getX(0)) - (getX(1) - getX(0)) * (getY(2) - getY(0)));
 
         Iterator<Double> typeIterator = splits.navigableKeySet().iterator();
         double tStart = 0.0;
         if (splits.isEmpty())
         {
             this.segments.put(tStart, currentBezier.offset(of, lengthSoFar, lengthTotal, sig, true));
         }
         while (typeIterator.hasNext())
         {
             double tInFull = typeIterator.next();
             Boundary type = splits.get(tInFull);
             double t;
             // Note: as we split the B&eacute;zier curve and work with the remainder in each loop, the resulting t value is not
             // the same as on the full B&eacute;zier curve. Therefore we need to refind the roots, or inflections, or at least
             // one cross-section.
             if (type == Boundary.ROOT)
             {
                 t = currentBezier.getRoots().first();
             }
             else if (type == Boundary.INFLECTION)
             {
                 t = currentBezier.getInflections().first();
             }
             else
             {
                 NavigableSet<Double> fCrossSection = new TreeSet<>();
                 double fSoFar = lengthSoFar / lengthTotal;
                 double fFirst = fCrossSectionRemain.pollFirst(); // fraction in total B&eacute;zier curve
                 fCrossSection.add((fFirst - fSoFar) / (1.0 - fSoFar)); // add fraction in remaining B&eacute;zier curve
                 SortedSet<Double> offsets = currentBezier.getOffsetT(fCrossSection);
                 t = offsets.first();
             }
             if (t < 1e-10)
             {
                 continue;
             }
 
             // Split B&eacute;zier curve, and add offset of first part
             SplitBeziers parts = currentBezier.split(t);
             BezierCubic2d segment = parts.first.offset(of, lengthSoFar, lengthTotal, sig, false);
             // System.out.println("Offset segment at " + tStart + ": " + segment);
             this.segments.put(tStart, segment);
 
             // Update loop variables
             lengthSoFar += parts.first.getLength();
             if (type == Boundary.INFLECTION)
             {
                 sig = -sig;
             }
             tStart = tInFull;
 
             // Append last segment, or loop again with remainder
             if (!typeIterator.hasNext())
             {
                 BezierCubic2d lastSegment = parts.remainder.offset(of, lengthSoFar, lengthTotal, sig, true);
                 // System.out.println("Offset last segment at " + tStart + ": " + lastSegment);
                 this.segments.put(tStart, lastSegment);
             }
             else
             {
                 currentBezier = parts.remainder;
             }
         }
         this.segments.put(1.0, null); // so we can interpolate t values along segments
         this.ofForSegments = of;
     }
 
     @Override
     public Point2d getPoint(final double fraction, final ContinuousPiecewiseLinearFunction of)
     {
         updateSegments(of);
         Entry<Double, BezierCubic2d> entry;
         double nextT;
         if (fraction == 1.0)
         {
             entry = BezierCubic2d.this.segments.lowerEntry(fraction);
             nextT = fraction;
         }
         else
         {
             entry = BezierCubic2d.this.segments.floorEntry(fraction);
             nextT = BezierCubic2d.this.segments.higherKey(fraction);
         }
         double t = (fraction - entry.getKey()) / (nextT - entry.getKey());
         return entry.getValue().getPoint(t);
     }
 
     @Override
     public double getDirection(final double fraction, final ContinuousPiecewiseLinearFunction of)
     {
         updateSegments(of);
         Entry<Double, BezierCubic2d> entry = BezierCubic2d.this.segments.floorEntry(fraction);
         if (entry.getValue() == null)
         {
             // end of line
             entry = BezierCubic2d.this.segments.lowerEntry(fraction);
             Point2d derivativeBezier = entry.getValue().derivative().getPoint(1.0);
             return Math.atan2(derivativeBezier.y, derivativeBezier.x);
         }
         Double nextT = BezierCubic2d.this.segments.higherKey(fraction);
         if (nextT == null)
         {
             nextT = 1.0;
         }
         double t = (fraction - entry.getKey()) / (nextT - entry.getKey());
         Point2d derivativeBezier = entry.getValue().derivative().getPoint(t);
         return Math.atan2(derivativeBezier.y, derivativeBezier.x);
     }
 
     @Override
     public PolyLine2d toPolyLine(final OffsetFlattener2d flattener, final ContinuousPiecewiseLinearFunction of)
     {
         Throw.whenNull(of, "fld");
         Throw.whenNull(flattener, "flattener");
 
         return flattener.flatten(this, of);
     }
 
     /**
      * Creates the offset B&eacute;zier curve of a B&eacute;zier segment. These segments are part of the offset procedure.
      * @param offsets offsets as defined for the entire B&eacute;zier
      * @param lengthSoFar cumulative length of all previously split off segments
      * @param lengthTotal length of full B&eacute;zier
      * @param sig sign of offset and offset slope
      * @param last must be <code>true</code> for the last B&eacute;zier segment; <code>false</code> for all other segments
      * @return offset B&eacute;zier
      */
     private BezierCubic2d offset(final ContinuousPiecewiseLinearFunction offsets, final double lengthSoFar,
             final double lengthTotal, final double sig, final boolean last)
     {
         double offsetStart = sig * offsets.get(lengthSoFar / lengthTotal);
         double offsetEnd = sig * offsets.get((lengthSoFar + getLength()) / lengthTotal);
 
         Point2d p1 = new Point2d(getX(0) - (getY(1) - getY(0)), getY(0) + (getX(1) - getX(0)));
         Point2d p2 = new Point2d(getX(3) - (getY(2) - getY(3)), getY(3) + (getX(2) - getX(3)));
         Point2d center = Point2d.intersectionOfLines(new Point2d(getX(0), getY(0)), p1, p2, new Point2d(getX(3), getY(3)));
 
         if (center == null)
         {
             // Start and end have same direction, offset first two by same amount, and last two by same amount to maintain
             // directions
             Point2d[] newBezierPoints = new Point2d[4];
             double ang = Math.atan2(p1.y - getY(0), p1.x - getX(0));
             double dxStart = Math.cos(ang) * offsetStart;
             double dyStart = -Math.sin(ang) * offsetStart;
             newBezierPoints[0] = new Point2d(getX(0) + dxStart, getY(0) + dyStart);
             newBezierPoints[1] = new Point2d(getX(1) + dxStart, getY(1) + dyStart);
             double dxEnd = Math.cos(ang) * offsetEnd;
             double dyEnd = -Math.sin(ang) * offsetEnd;
             newBezierPoints[2] = new Point2d(getX(2) + dxEnd, getY(2) + dyEnd);
             newBezierPoints[3] = new Point2d(getX(3) + dxEnd, getY(3) + dyEnd);
             return new BezierCubic2d(newBezierPoints[0], newBezierPoints[1], newBezierPoints[2], newBezierPoints[3]);
         }
 
         // move 1st and 4th point their respective offsets away from the center
         Point2d[] newBezierPoints = new Point2d[4];
         double off = offsetStart;
         for (int i = 0; i < 4; i = i + 3)
         {
             double dy = getY(i) - center.y;
             double dx = getX(i) - center.x;
             double ang = Math.atan2(dy, dx);
             double len = Math.hypot(dx, dy) + off;
             newBezierPoints[i] = new Point2d(center.x + len * Math.cos(ang), center.y + len * Math.sin(ang));
             off = offsetEnd;
         }
 
         // find tangent unit vectors that account for slope in offset
         double ang = sig * Math.atan((offsetEnd - offsetStart) / getLength());
         double cosAng = Math.cos(ang);
         double sinAng = Math.sin(ang);
         double dx = getX(1) - getX(0);
         double dy = getY(1) - getY(0);
         double dx1;
         double dy1;
         if (0.0 == lengthSoFar)
         {
             // force same start angle
             dx1 = dx;
             dy1 = dy;
         }
         else
         {
             // shift angle by 'ang'
             dx1 = cosAng * dx - sinAng * dy;
             dy1 = sinAng * dx + cosAng * dy;
         }
         dx = getX(2) - getX(3);
         dy = getY(2) - getY(3);
         double dx2;
         double dy2;
         if (last)
         {
             // force same end angle
             dx2 = dx;
             dy2 = dy;
         }
         else
         {
             // shift angle by 'ang'
             dx2 = cosAng * dx - sinAng * dy;
             dy2 = sinAng * dx + cosAng * dy;
         }
 
         // control points 2 and 3 as intersections between tangent unit vectors and line through center and original point 2 and
         // 3 in original B&eacute;zier
         Point2d cp2 = new Point2d(newBezierPoints[0].x + dx1, newBezierPoints[0].y + dy1);
         newBezierPoints[1] = Point2d.intersectionOfLines(newBezierPoints[0], cp2, center, new Point2d(getX(1), getY(1)));
         Point2d cp3 = new Point2d(newBezierPoints[3].x + dx2, newBezierPoints[3].y + dy2);
         newBezierPoints[2] = Point2d.intersectionOfLines(newBezierPoints[3], cp3, center, new Point2d(getX(2), getY(2)));
 
         // create offset B&eacute;zier
         return new BezierCubic2d(newBezierPoints[0], newBezierPoints[1], newBezierPoints[2], newBezierPoints[3]);
     }
 
     @Override
     public double getLength()
     {
         return this.length;
     }
 
     @Override
     public String toString()
     {
         return "BezierCubic2d [startPoint=" + this.startPoint + ", endPoint=" + this.endPoint + ", controlPoint1="
                 + new Point2d(getX(1), getY(1)) + ", controlPoint2=" + new Point2d(getX(2), getY(2)) + ", length=" + this.length
                 + ", startDirZ=" + getStartPoint().dirZ + " (" + getPoint(0).directionTo(getPoint(1)) + "), endir="
                 + getEndPoint().dirZ + " (" + getPoint(2).directionTo(getPoint(3)) + ")]";
     }
 
     /**
      * The three types of discontinuities/boundaries of a B&eacute;zier curve.
      */
     private enum Boundary
     {
         /** Root of B&eacute;zier curve. */
         ROOT,
 
         /** Inflection point of B&eacute;zier curve. */
         INFLECTION,
 
         /** Knot in offsets. */
         KNOT
     }
 
 }