線分の交差判定

検索で引っかかったものがコーナーケースに引っかかるようだったので

int cross_product(point_t a, point_t b, point_t c) {
    return (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
}

int is_intersect(point_t a, point_t b, point_t c, point_t d) {
    int cp1 = cross_product(a, b, c);
    int cp2 = cross_product(a, b, d);
    int cp3 = cross_product(c, d, a);
    int cp4 = cross_product(c, d, b);

    if (((cp1 > 0 && cp2 < 0) || (cp1 < 0 && cp2 > 0)) && ((cp3 > 0 && cp4 < 0) || (cp3 < 0 && cp4 > 0))) {
        return 1;
    }
    if (cp1 == 0 && cp2 == 0 && cp3 == 0 && cp4 == 0) {
        if ((a.x <= c.x && c.x <= b.x) || (a.x <= d.x && d.x <= b.x) || (c.x <= a.x && a.x <= d.x) || (c.x <= b.x && b.x <= d.x)) {
            return 1;
        }
    }
    return 0;
}