SourceCode/PROMS/DotNetBar Source Code/DevComponents.DotNetBar.Charts/ConvexHull.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Text;

namespace DevComponents.DotNetBar.Charts
{
    // Monotone Chain Convex Hull.
    // It takes O(NlogN) in sorting & O(N) in actual convex hull calc for N points.

    public static class ConvexHull
    {
        #region GetConvexHull

        public static Point[] GetConvexHull(Point[] points)
        {
            int n = points.Length;

            Array.Sort(points, 0, n, new PointComparer());

            Point[] ans = new Point[2 * n];

            int k = 0;
            int start = 0;

            // Bottom hull

            Point lpt = Point.Empty;

            for (int i = 0; i < n; i++)
            {
                Point p = points[i];

                if (PtCompare(p, lpt) == 0)
                    continue;

                lpt = p;

                while (k - start >= 2)
                {
                    Point sp1 = PtSub(p, ans[k - 1]);
                    Point sp2 = PtSub(p, ans[k - 2]);

                    if (PtCross(sp1, sp2) <= 0)
                        break;

                    k--;
                }

                ans[k++] = p;
            }

            k--;

            // Top hull

            start = k;
            lpt = Point.Empty;

            for (int i = n - 1; i >= 0; i--)
            {
                Point p = points[i];

                if (PtCompare(p, lpt) == 0)
                    continue;

                lpt = p;

                while (k - start >= 2)
                {
                    Point sp1 = PtSub(p, ans[k - 1]);
                    Point sp2 = PtSub(p, ans[k - 2]);

                    if (PtCross(sp1, sp2) <= 0)
                        break;

                    k--;
                }

                ans[k++] = p;
            }

            k--;

            Point[] hullPoints = new Point[k + 1];
            Array.Copy(ans, hullPoints, k);

            hullPoints[k] = hullPoints[0];

            return (hullPoints);
        }

        #endregion

        #region PointComparer

        private class PointComparer : IComparer<Point>
        {
            public int Compare(Point pt1, Point pt2)
            {
                return (ConvexHull.PtCompare(pt1, pt2));
            }
        }

        #endregion

        #region PtCompare

        private static int PtCompare(Point pt1, Point pt2)
        {
            if (pt1.X == pt2.X)
                return (pt1.Y - pt2.Y);

            return (pt1.X - pt2.X);
        }

        #endregion

        #region PtCross

        private static int PtCross(Point pt1, Point pt2)
        {
            return (pt1.X * pt2.Y - pt1.Y * pt2.X);
        }

        #endregion

        #region PtSub

        private static Point PtSub(Point pt1, Point pt2)
        {
            return (new Point(pt1.X - pt2.X, pt1.Y - pt2.Y));
        }

        #endregion
    }
}