package org.djutils.draw.line;
   
   import java.util.ArrayList;
   import java.util.Collection;
   import java.util.Collections;
   import java.util.Comparator;
   import java.util.Iterator;
   import java.util.List;
   
  import org.djutils.draw.DrawRuntimeException;
  import org.djutils.draw.Drawable2d;
  import org.djutils.draw.bounds.Bounds2d;
  import org.djutils.draw.point.Point2d;
  import org.djutils.exceptions.Throw;
  
  /**
   * ConvexHull.java. Compute the convex hull of a collection of Point2d or Drawable2d. All implementations here return a
   * Polygon2d object. If the convex hull of the input would be a single point, the implementations will throw a
   * DrawRuntimeException because a single point does not make a valid Polygon2d.
   * <p>
   * Copyright (c) 2020-2025 Delft University of Technology, PO Box 5, 2600 AA, Delft, the Netherlands. All rights reserved. <br>
   * BSD-style license. See <a href="https://djutils.org/docs/current/djutils/licenses.html">DJUTILS License</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>
   */
  public final class ConvexHull
  {
      /**
       * Do not instantiate.
       */
      private ConvexHull()
      {
          // Do not instantiate
      }
  
      /**
       * Compute the convex hull of a collection of Point2d objects.
       * @param iterator iterator that shall return all the points for which the convex hull is to be
       *            computed
       * @return the convex hull of the points
       */
      public static Polygon2d convexHull(final Iterator<Point2d> iterator)
      {
          List<Point2d> list = new ArrayList<>();
          iterator.forEachRemaining(list::add);
          return convexHullAlshamrani(list);
      }
  
      /**
       * Compute the convex hull of one or more Drawable2d objects.
       * @param drawable2d the Drawable2d objects
       * @return the convex hull of the Drawable2d objects
       * @throws NullPointerException when any of the drawable2d object is <code>null</code>
       * @throws IllegalArgumentException when zero arguments are provided
       */
      public static Polygon2d convexHull(final Drawable2d... drawable2d)
      {
          return convexHull(Bounds2d.pointsOf(drawable2d));
      }
  
      /**
       * Construct a Bounds2d for a Collection of Drawable2d objects.
       * @param drawableCollection the collection
       * @return the convex hull of the Drawable2d objects
       * @throws NullPointerException when the <code>drawableCollection</code> is <code>null</code>, or contains a
       *             <code>null</code> value
       * @throws IllegalArgumentException when the <code>drawableCollection</code> is empty
       */
      public static Polygon2d convexHull(final Collection<Drawable2d> drawableCollection)
      {
          Throw.whenNull(drawableCollection, "drawableCollection");
          Throw.when(drawableCollection.isEmpty(), DrawRuntimeException.class, "drawableCollection may not be empty");
          return convexHull(Bounds2d.pointsOf(drawableCollection));
      }
  
      /**
       * Compute the convex hull of a list of Point2d objects. The input list will not be modified.
       * @param list the list of Point2d objects
       * @return the convex hull of the points
       */
      public static Polygon2d convexHull(final List<Point2d> list)
      {
          return convexHullAlshamrani(list);
      }
  
      /**
       * Return whether moving from a through b to c, the turn at b is counter-clockwise.
       * @param a point a
       * @param b point b
       * @param c point c
       * @return <code>true</code> if the turn at b is counter clockwise; <code>false</code> if there is not turn; or it
       *         is clockwise
       */
      private static boolean ccw(final Point2d a, final Point2d b, final Point2d c)
      {
          return ((b.x - a.x) * (c.y - a.y)) > ((b.y - a.y) * (c.x - a.x));
      }
 
     /**
      * Repeatedly remove the last point if not counter clockwise with new point; then add the new point. If the new point is
      * equal to the last point in the list; do nothing.
      * @param list the list of points
      * @param newPoint the point that will be added.
      */
     private static void cleanAndAppend(final List<Point2d> list, final Point2d newPoint)
     {
         Point2d last = list.get(list.size() - 1);
         if (last.x == newPoint.x && last.y == newPoint.y)
         {
             return;
         }
         while (list.size() >= 2 && !ccw(list.get(list.size() - 2), list.get(list.size() - 1), newPoint))
         {
             list.remove(list.size() - 1);
         }
         list.add(newPoint);
     }
 
     /**
      * Implementation of the convex hull algorithm by Reham Alshamrani c.s.; see
      * <a href="https://www.sciencedirect.com/science/article/pii/S1877050920304750">A Preprocessing Technique for Fast Convex
      * Hull Computation</a>.
      * @param list list of the points (will not be modified)
      * @return the convex hull of the points
      * @throws NullPointerException when the <code>list</code> is <code>null</code>
      * @throws IllegalArgumentException when the <code>list</code> contains too few points
      */
     public static Polygon2d convexHullAlshamrani(final List<Point2d> list)
     {
         // Find the four extreme points
         Throw.whenNull(list, "list");
         Throw.when(list.size() < 1, IllegalArgumentException.class, "Too few points in list");
         Point2d minX = list.get(0); // Initialize to the first point in list to avoid checking for null on each iteration
         Point2d minY = list.get(0);
         Point2d maxX = list.get(0);
         Point2d maxY = list.get(0);
         for (Point2d point : list)
         {
             if (minX.x > point.x || minX.x == point.x && minX.y > point.y)
             {
                 minX = point;
             }
             if (minY.y > point.y || minY.y == point.y && minY.x < point.x)
             {
                 minY = point;
             }
             if (maxX.x < point.x || maxX.x == point.x && maxX.y > point.y)
             {
                 maxX = point;
             }
             if (maxY.y < point.y || maxY.y == point.y && maxY.x < point.x)
             {
                 maxY = point;
             }
         }
         // Filter and group the points into priority queues that order by x value (tie breaker is y value)
         // Alshamrani does not show how he tests that a point is outside each edge of the four extreme points. We use ccw.
         // Alshamrani poorly documents the ordering method for the four queues when the primary component values are the same.
         // Testing has shown that sorting a filled ArrayList is faster than putting the same elements one by one in a TreeSet.
         Comparator<Point2d> forwardComparator = new Comparator<Point2d>()
         {
             @Override
             public int compare(final Point2d point1, final Point2d point2)
             {
                 if (point1.x == point2.x)
                 {
                     return (int) Math.signum(point1.y - point2.y);
                 }
                 return (int) Math.signum(point1.x - point2.x);
             }
         };
         Comparator<Point2d> reverseComparator = new Comparator<Point2d>()
         {
             @Override
             public int compare(final Point2d point1, final Point2d point2)
             {
                 if (point1.x == point2.x)
                 {
                     return (int) Math.signum(point1.y - point2.y);
                 }
                 return (int) Math.signum(point2.x - point1.x);
             }
         };
         List<Point2d> lowerLeft = new ArrayList<>();
         List<Point2d> lowerRight = new ArrayList<>();
         List<Point2d> upperRight = new ArrayList<>();
         List<Point2d> upperLeft = new ArrayList<>();
 
         for (Point2d point : list)
         {
             if (point.x <= minY.x && point.y <= minX.y)
             {
                 if (ccw(minX, point, minY))
                 {
                     lowerLeft.add(point);
                 }
             }
             else if (point.x >= minY.x && point.y <= maxX.y)
             {
                 if (ccw(minY, point, maxX))
                 {
                     lowerRight.add(point);
                 }
             }
             else if (point.x >= maxY.x && point.y >= maxX.y)
             {
                 if (ccw(maxX, point, maxY))
                 {
                     upperRight.add(point);
                 }
             }
             else if (point.x <= maxY.x && point.y >= minX.y)
             {
                 if (ccw(maxY, point, minX))
                 {
                     upperLeft.add(point);
                 }
             }
         }
         Collections.sort(lowerLeft, forwardComparator);
         Collections.sort(lowerRight, forwardComparator);
         Collections.sort(upperRight, reverseComparator);
         Collections.sort(upperLeft, reverseComparator);
         // Construct the convex hull
         List<Point2d> result = new ArrayList<>();
         result.add(minX);
         for (Point2d point : lowerLeft)
         {
             cleanAndAppend(result, point);
         }
         cleanAndAppend(result, minY);
         for (Point2d point : lowerRight)
         {
             cleanAndAppend(result, point);
         }
         cleanAndAppend(result, maxX);
         for (Point2d point : upperRight)
         {
             cleanAndAppend(result, point);
         }
         cleanAndAppend(result, maxY);
         for (Point2d point : upperLeft)
         {
             cleanAndAppend(result, point);
         }
         return new Polygon2d(result);
     }
 
     /**
      * Implementation of Andrew's Monotone Chain convex hull algorithm. This implementation (sorts) modifies the provided list
      * of points!
      * @param list list of the points (will be modified)
      * @return the convex hull of the points
      * @throws NullPointerException when the <code>list</code> is <code>null</code>
      * @throws IllegalArgumentException when the <code>list</code> contains too few points
      */
     public static Polygon2d convexHullMonotone(final List<Point2d> list)
     {
         Collections.sort(list, new Comparator<Point2d>()
         {
             @Override
             public int compare(final Point2d point1, final Point2d point2)
             {
                 if (point1.x == point2.x)
                 {
                     return (int) Math.signum(point1.y - point2.y);
                 }
                 return (int) Math.signum(point1.x - point2.x);
             }
         });
         // That sort operation was O(N log (N)); the remainder is O(N)
         List<Point2d> result = new ArrayList<>();
         // Lower hull
         for (Point2d p : list)
         {
             while (result.size() >= 2 && !ccw(result.get(result.size() - 2), result.get(result.size() - 1), p))
             {
                 result.remove(result.size() - 1);
             }
             result.add(p);
         }
         // Upper hull
         int lowLimit = result.size() + 1;
         for (int i = list.size() - 1; i >= 0; i--)
         {
             Point2d p = list.get(i);
             while (result.size() >= lowLimit && !ccw(result.get(result.size() - 2), result.get(result.size() - 1), p))
             {
                 result.remove(result.size() - 1);
             }
             result.add(p);
         }
         if (result.size() > 0)
         {
             result.remove(result.size() - 1);
         }
         // else; zero points; the constructor of the Polygon2d will throw an IllegalArgumentException
         return new Polygon2d(result);
     }
 
 }