sum method

@override T sum({ required int index_to, })

Returns the sum up to the given index index_to.

Example: If the tree is built with 1, 2, 3, 4, 5, then sum(4) will return the sum of 1, 2, 3, 4, 5 which is 15.

Implementation

@override
T sum({
  required final int index_to,
}) {
  int i = index_to;
  assert(i > -1 && array.length > i + 1, "");
  T sum = algebra.identity();
  i++;
  while (i > 0) {
    sum = algebra.operate(sum, array[i]);
    i -= i & -i;
  }
  if (i == array.length - 1) {
    return algebra.operate(sum, array[i]);
  } else {
    return sum;
  }
}