static constexpr I sum_helper(I m, I n) {
if (m == n)
return m;
// 0 == m < n
else if (0 == m)
return n * (n+1) / 2;
// 0 < m < n
else if (0 < m)
return sum_helper(0, n) - sum_helper(0, m-1);
// m < 0 <= n
else if (0 <= n)
return sum_helper(0, n) - sum_helper(0, -m);
// m < n < 0
else
return -sum_helper(-n, -m);
}
static constexpr auto apply(range<T, from, to> const&) {
return integral_c<T, from == to ? 0 : sum_helper(from, to-1)>;
}
static int sum_helper(list_t list, int sum) {
if (list_isEmpty(list)) return sum;
return sum_helper(list_rest(list), sum + list_first(list));
}
int sum(list_t list) {
return sum_helper(list, 0);
}
