partition_point.c

int const* custom_partition_point(int const* first, int const* last) {
    while (last - first > 1) {
        int const* middle = first + (last - first) / 2;
        if (*(middle - 1) < *middle) {
            last = middle - 1;
        } else {
            first = middle;
        }
    }
    if (*first < *(first + 1)) {
        return first;
    }
    return first + 1;
}