CCW

#include <iostream>
#include <algorithm>
using namespace std;
struct line {
    pair<int,int> a, b;
};
int ccw(pair<int, int> a, pair<int, int> b, pair<int, int> c) {
    int op = a.first * b.second + b.first * c.second + c.first * a.second - (a.second * b.first + b.second * c.first + c.second * a.first);
    return op > 0 ? 1 : !op ? 0 : -1;
}
bool isIntersect(line l1, line l2) {
    if(l1.a == l2.a || l1.a == l2.b || l1.b == l2.a || l1.b == l2.b)
        return true;
    int ab = ccw(l1.a, l1.b, l2.a) * ccw(l1.a, l1.b, l2.b);
    int cd = ccw(l2.a, l2.b, l1.a) * ccw(l2.a, l2.b, l1.b);
    if(!ab && !cd) {
        if(l1.a > l1.b) swap(l1.a, l1.b);
        if(l2.a > l2.b) swap(l2.a, l2.b);
        return l2.a <= l1.b && l1.a <= l2.b;
    }
    return ab <= 0 && cd <= 0;
}
int main() {
    line line1 = line{.a.first = 1, .a.second = 1, .b.first = 3, .b.second = 3};
    line line2 = line{.a.first = 1, .a.second = 2, .b.first = 3, .b.second = 4};
    line line3 = line{.a.first = 1, .a.second = 2, .b.first = 3, .b.second = 3};
    printf("%s\n", isIntersect(line1, line2) ? "cross" : "non-cross");
    printf("%s\n", isIntersect(line1, line3) ? "cross" : "non-cross");
    printf("%s\n", isIntersect(line2, line3) ? "cross" : "non-cross");
}