Convex hull на массивах

point a[N], u[N], d[N];
int as = 0, us = 0, ds = 0;

void convex_hull() {
    assert(us == 0);
    while (ds) {
        a[as++] = d[--ds];
    }
    assert(ds == 0);
    sort(a, a + as);
    as = unique(a, a + as) - a;
    if (as < 3) {
        copy_n(a, as, d);
        ds = as;
        as = 0;
        return;
    }
    for (int i = 0; i < as; ++i) {
        if (i == 0 || i == as - 1 || a[0].rt(a[i], a[as - 1])) {
            while (us >= 2 && !u[us - 2].rt(u[us - 1], a[i])) {
                --us;
            }
            u[us++] = a[i];
        }
        if (i == 0 || i == as - 1 || a[0].lt(a[i], a[as - 1])) {
            while (ds >= 2 && !d[ds - 2].lt(d[ds - 1], a[i])) {
                --ds;
            }
            d[ds++] = a[i];
        }
    }
    for (int i = us - 2; i >= 1; --i) {
        d[ds++] = u[i];
    }
    us = 0;
    as = 0;
}