LazySeg

import std.algorithm : max;
///区間max, 区間加算
auto seg = LazySeg!(int, int,
    (a, b) => max(a, b), (a, b) => a+b, (a, b) => a+b, 0, 0)([2, 1, 4]);

//[2, 1, 4]
seg[0] = 2; seg[1] = 1; seg[2] = 4;
assert(seg[0..3].sum == 4);

//[2, 1, 5]
seg[2] = 5;
assert(seg[0..2].sum == 2);
assert(seg[0..3].sum == 5);

//[12, 11, 5]
seg[0..2] += 10;
assert(seg[0..3].sum == 12);