检测面对象是否存在自相交

在GIS中，如果一个面存在自相交的情况，那么他是一个存在错误拓扑关系的面，后续影响分析，如何判断呢？封装了一个checkCross的方法，作为判断。

using SuperMap.Data;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SuperMap.SampleCode.Mapping
{
   public static class gemoetryz
   {
       private static int dblcmp(double a, double b)
       {
           if (Math.Abs(a - b) <= 0.00006)
               return 0;
           if (a > b)
               return 1;
           else
               return -1;
       }
       private static double dot(double x1, double y1, double x2, double y2)
       {
           return x1 * x2 + y1 * y2;
       }
       private static int point_on_line(Point2D a, Point2D b, Point2D c)
       {
           double res = dot(b.X - a.X, b.Y - a.Y, c.X - a.X, c.Y - a.Y);//[self dot:b.x - a.x y1:b.y - a.y x2:c.x - a.x y2:c.y - a.y]
           return dblcmp(res, 0);
       }
       private static double cross(double x1, double y1, double x2, double y2)
       {
           return x1 * y2 - x2 * y1;
       }
       private static double ab_cross_ac(Point2D a, Point2D b, Point2D c)
       {
           return cross(b.X - a.X, b.Y - a.Y, c.X - a.X, c.Y - a.Y);
       }
       private static int ab_cross_cd(Point2D a, Point2D b, Point2D c, Point2D d)
       {
           double s1, s2, s3, s4;
           int d1, d2, d3, d4;
           Point2D p = new Point2D();
           d1 = dblcmp(s1 = ab_cross_ac(a, b, c), 0);
           d2 = dblcmp(s2 = ab_cross_ac(a, b, d), 0);
           d3 = dblcmp(s3 = ab_cross_ac(c, d, a), 0);
           d4 = dblcmp(s4 = ab_cross_ac(c, d, b), 0);
           if ((d1 ^ d2) == -2 && (d3 ^ d4) == -2)
           {
               p.X = (float)((c.X * s2 - d.X * s1) / (s2 - s1));
               p.Y = (float)((c.Y * s2 - d.Y * s1) / (s2 - s1));
               return 1;
           }
           if(d1==0&&point_on_line(c,a,b)<=0)
           {
               p=c;
               return 0;
           }
           if(d2==0&&point_on_line(d,a,b)<=0)
           {
               p=d;
               return 0;
           }
           if(d3==0&&point_on_line(a,c,d)<=0)
           {
               p=a;
               return 0;
           }
           if(d4==0&&point_on_line(b,c,d)<=0)
           {
               p=a;
               return 0;
           }
           //如果不相交
           return -1;
       }

       /// <summary>
       /// 判断是否面有自相交
       /// </summary>
       /// <param name="region"></param>
       /// <returns></returns>
       public static Boolean checkCross(GeoRegion region)
       {
           Boolean isCorss=false;
           Point2Ds points=region[0];
           int count = points.Count;


           for(int i=0;i<count-1;i++)
           {
               for(int j=i+1;j<count;j++)
               {
                   Point2D point3=new Point2D();
                   if(j==count-1)
                   {
                       point3 = points[0];
                   }
                   else
                   {
                       point3=points[j+1];
                   }
                   Point2D point=points[i];
                   Point2D point1=points[i+1];
                   Point2D point2=points[j];
                   if(ab_cross_cd(point,point1,point2,point3)==1)
                   {
                       return true;
                   }
               }
           }
           return isCorss;
       }

   }
}