判断线段相交

 1 float cross(const Point &point0, const Point &point1)
 2 {
 3     return point0.first * point1.second - point0.second * point1.first;
 4 }
 5 
 6 bool isLineIntersect(const Point &point0, const Point &point1, const Point &point2, const Point &point3)
 7 {
 8     auto value0 = cross(
 9         std::make_pair(point1.first - point0.first, point1.second - point0.second),
10         std::make_pair(point3.first - point0.first, point3.second - point0.second));
11     auto value1 = cross(
12         std::make_pair(point1.first - point0.first, point1.second - point0.second),
13         std::make_pair(point2.first - point0.first, point2.second - point0.second));
14     auto value2 = cross(
15         std::make_pair(point3.first - point2.first, point3.second - point2.second),
16         std::make_pair(point0.first - point2.first, point0.second - point2.second));
17     auto value3 = cross(
18         std::make_pair(point3.first - point2.first, point3.second - point2.second),
19         std::make_pair(point1.first - point2.first, point1.second - point2.second));
20     return value0 < 0 != value1 < 0 && value2 < 0 != value3 < 0;
21 }