[CSharpTips]判断两条线段是否相交

using System; using System.Collections.Generic; using System.Windows; using System.Linq; using System.Text; using System.Threading.Tasks;   namespace lineTest {     class Program     {         public struct Point         {             public double X;             public double Y;              public Point(double x, double y)             {                 X = x;                 Y = y;             }         }          static void Main(string[] args)         {             Point a = new Point(0, 0);             Point b = new Point(100, 100);             Point c = new Point(100,0);             Point d = new Point(50,49);             var result = IsIntersect(a, b, c, d);         }          public static Point? GetIntersection(Point lineAStart, Point lineAEnd, Point lineBStart, Point lineBEnd)         {             double x1 = lineAStart.X, y1 = lineAStart.Y;             double x2 = lineAEnd.X, y2 = lineAEnd.Y;              double x3 = lineBStart.X, y3 = lineBStart.Y;             double x4 = lineBEnd.X, y4 = lineBEnd.Y;              //equations of the form x=c (two vertical lines)             if (x1 == x2 && x3 == x4 && x1 == x3)             {                 return null;             }              //equations of the form y=c (two horizontal lines)             if (y1 == y2 && y3 == y4 && y1 == y3)             {                 return null;             }              //equations of the form x=c (two vertical lines)             if (x1 == x2 && x3 == x4)             {                 return null;             }              //equations of the form y=c (two horizontal lines)             if (y1 == y2 && y3 == y4)             {                 return null;             }             double x, y;              if (x1 == x2)             {                 double m2 = (y4 - y3) / (x4 - x3);                 double c2 = -m2 * x3 + y3;                  x = x1;                 y = c2 + m2 * x1;             }             else if (x3 == x4)             {                 double m1 = (y2 - y1) / (x2 - x1);                 double c1 = -m1 * x1 + y1;                  x = x3;                 y = c1 + m1 * x3;             }             else             {                 //compute slope of line 1 (m1) and c2                 double m1 = (y2 - y1) / (x2 - x1);                 double c1 = -m1 * x1 + y1;                  //compute slope of line 2 (m2) and c2                 double m2 = (y4 - y3) / (x4 - x3);                 double c2 = -m2 * x3 + y3;                  x = (c1 - c2) / (m2 - m1);                 y = c2 + m2 * x;             }              if (IsInsideLine(lineAStart, lineAEnd, x, y) &&                 IsInsideLine(lineBStart, lineBEnd, x, y))             {                 return new Point(x, y);             }              //return default null (no intersection)             return null;         }         private static bool IsInsideLine(Point start, Point end, double x, double y)         {             return ((x >= start.X && x <= end.X)                 || (x >= end.Y && x <= start.Y))                 && ((y >= start.Y && y <= end.Y)                     || (y >= end.Y && y <= start.Y));         }          public static bool IsIntersect(Point p1, Point p2, Point q1, Point q2)         {             //排斥试验，判断p1p2在q1q2为对角线的矩形区之外             if (Math.Max(p1.X, p2.X) < Math.Min(q1.X, q2.X))             {//P1P2中最大的X比Q1Q2中的最小X还要小，说明P1P2在Q1Q2的最左点的左侧，不可能相交。                 return false;             }              if (Math.Min(p1.X, p2.X) > Math.Max(q1.X, q2.X))             {//P1P2中最小的X比Q1Q2中的最大X还要大，说明P1P2在Q1Q2的最右点的右侧，不可能相交。                 return false;             }              if (Math.Max(p1.Y, p2.Y) < Math.Min(q1.Y, q2.Y))             {//P1P2中最大的Y比Q1Q2中的最小Y还要小，说明P1P2在Q1Q2的最低点的下方，不可能相交。                 return false;             }              if (Math.Min(p1.Y, p2.Y) > Math.Max(q1.Y, q2.Y))             {//P1P2中最小的Y比Q1Q2中的最大Y还要大，说明P1P2在Q1Q2的最高点的上方，不可能相交。                 return false;             }              //跨立试验             var crossP1P2Q1 = VectorCross(p1, p2, q1);             var crossP1Q2P2 = VectorCross(p1, q2, p2);             var crossQ1Q2P1 = VectorCross(q1, q2, p1);             var crossQ1P2Q2 = VectorCross(q1, p2, q2);              bool isIntersect = (crossP1P2Q1 * crossP1Q2P2 >= 0) && (crossQ1Q2P1 * crossQ1P2Q2 >= 0);             return isIntersect;         }          private static double VectorCross(Point p1, Point p2, Point p3)         {             Vector vectorP1 = new Vector(p1.X, p1.Y);             Vector vectorP2 = new Vector(p2.X, p2.Y);             Vector vectorP3 = new Vector(p3.X, p3.Y);             Vector vectorP1P2 = Vector.Subtract(vectorP2, vectorP1);             Vector vectorP1P3 = Vector.Subtract(vectorP3, vectorP1);             return Vector.CrossProduct(vectorP1P2, vectorP1P3);         }     } }