void clip_polygon_by_half_plane(
const Polygon2d& P,
const vec2& q1,
const vec2& q2,
Polygon2d& result,
bool invert
) {
result.clear() ;
if(P.size() == 0) {
return ;
}
if(P.size() == 1) {
if(point_is_in_half_plane(P[0], q1, q2, invert)) {
result.push_back(P[0]) ;
}
return ;
}
vec2 prev_p = P[P.size() - 1] ;
Sign prev_status = point_is_in_half_plane(
prev_p, q1, q2, invert
) ;
for(unsigned int i=0; i<P.size(); i++) {
vec2 p = P[i] ;
Sign status = point_is_in_half_plane(
p, q1, q2, invert
) ;
if(
status != prev_status &&
status != ZERO &&
prev_status != ZERO
) {
vec2 intersect ;
if(intersect_segments(prev_p, p, q1, q2, intersect)) {
result.push_back(intersect) ;
} else {
}
}
switch(status) {
case NEGATIVE:
break ;
case ZERO:
result.push_back(p) ;
break ;
case POSITIVE:
result.push_back(p) ;
break ;
}
prev_p = p ;
prev_status = status ;
}
}
void convex_hull(const Polygon2d& PP, Polygon2d& result) {
result.clear() ;
int n = PP.size() ;
vec2* P = new vec2[n+1] ;
{ for(int i=0; i<n; i++) {
P[i] = PP[i] ;
}}
int u = make_chain(P, n, cmpl);
P[n] = P[0];
int ch = u+make_chain(P+u, n-u+1, cmph);
{for(int i=0; i<ch; i++) {
result.push_back(P[i]) ;
}}
delete[] P ;
}