Find the index of the large integer

We will use binary search to find the index of the large integer.

# """
# This is ArrayReader's API interface.
# You should not implement it, or speculate about its implementation
# """
#class ArrayReader(object):
#    # Compares the sum of arr[l..r] with the sum of arr[x..y]
#    # return 1 if sum(arr[l..r]) > sum(arr[x..y])
#    # return 0 if sum(arr[l..r]) == sum(arr[x..y])
#    # return -1 if sum(arr[l..r]) < sum(arr[x..y])
#    def compareSub(self, l: int, r: int, x: int, y: int) -> int:
#
#    # Returns the length of the array
#    def length(self) -> int:
#

class Solution:
    def getIndex(self, reader: 'ArrayReader') -> int:
        length = reader.length()
        start = 0

        while length > 1:
            middle = start + (length//2 - 1)
            comparison = reader.compareSub(start, middle, middle + 1, middle + 1 + (middle - start))

            if comparison <= 0:
                length -= middle - start + 1
                start = middle + 1
            else:
                length = middle - start + 1

        return start

Time complexity: O(log(n))
Space complexity: O(1)