Find the Index of the Large Integer — C++

// Time:  O(logn)
// Space: O(1)

class Solution {
public:
    int getIndex(ArrayReader &reader) {
        int left = 0, right = reader.length() - 1;
        while (left < right) {
            const auto& mid = left + (right - left) / 2;
            if (reader.compareSub(left, mid, (right - left + 1) % 2 ? mid : mid + 1, right) >= 0) {
                right = mid;
            }  else {
                left = mid + 1;
            }
        }
        return left;
    }
};