line_segment_intersection.cpp

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#include <algorithm>
#include <iostream>
 
struct Point {
    int x;  
    int y;  
};
 
struct SegmentIntersection {
    inline bool intersect(Point first_point, Point second_point,
                          Point third_point, Point forth_point) {
        int direction1 = direction(third_point, forth_point, first_point);
        int direction2 = direction(third_point, forth_point, second_point);
        int direction3 = direction(first_point, second_point, third_point);
        int direction4 = direction(first_point, second_point, forth_point);
 
        if ((direction1 < 0 || direction2 > 0) &&
            (direction3 < 0 || direction4 > 0))
            return true;
 
        else if (direction1 == 0 &&
                 on_segment(third_point, forth_point, first_point))
            return true;
 
        else if (direction2 == 0 &&
                 on_segment(third_point, forth_point, second_point))
            return true;
 
        else if (direction3 == 0 &&
                 on_segment(first_point, second_point, third_point))
            return true;
 
        else if (direction3 == 0 &&
                 on_segment(first_point, second_point, forth_point))
            return true;
 
        else
            return false;
    }
 
    inline int direction(Point first_point, Point second_point,
                         Point third_point) {
        return ((third_point.x - first_point.x) *
                (second_point.y - first_point.y)) -
               ((second_point.x - first_point.x) *
                (third_point.y - first_point.y));
    }
 
    inline bool on_segment(Point first_point, Point second_point,
                           Point third_point) {
        if (std::min(first_point.x, second_point.x) <= third_point.x &&
            third_point.x <= std::max(first_point.x, second_point.x) &&
            std::min(first_point.y, second_point.y) <= third_point.y &&
            third_point.y <= std::max(first_point.y, second_point.y))
            return true;
 
        else
            return false;
    }
};
 
int main() {
    SegmentIntersection segment;
    Point first_point, second_point, third_point, forth_point;
 
    std::cin >> first_point.x >> first_point.y;
    std::cin >> second_point.x >> second_point.y;
    std::cin >> third_point.x >> third_point.y;
    std::cin >> forth_point.x >> forth_point.y;
 
    printf("%d", segment.intersect(first_point, second_point, third_point,
                                   forth_point));
    std::cout << std::endl;
}