➜ cat test.cpp
#include <iostream>
#include <cstdint>


// Is translation from 2-position to 26-position notation
//
// X - bit count in source
// Y - position count in result
//
// sizeof(T) = log(base=2, number=X)
// X = power(2, sizeof(T))
// Y = log(base=26, number=X)
// Y = log(base=26 number=power(2, sizeof(T)))
// Y = sizeof(T) * log(base=26, number=2)
//
// python:
//  >>> math.log(2, 26)
//  0.21274605355336315
//
// Y ~ = sizeof(T) / 4


template<typename T>
struct Size
{
    enum { value = sizeof(T) / 4 };
};

template<typename T>
uint8_t toExcel(T input, uint8_t (*result)[Size<T>::value])
{
    uint8_t position = 0;
    do
    {
        *result[position++] = input % 26;
        input = input / 26;
    }
    while (input > 0);
    return position;
}


int main(int, char*[])
{
    uint64_t input;
    uint8_t result[Size<uint64_t>::value];
    while (std::cin >> input)
    {
        uint8_t result_length = toExcel(input, &result);
        std::string result_string;
        while(result_length > 0)
        {
            result_string += 'A' + result[--result_length];
        }
        std::cout << input << "\t" << result_string << std::endl;
    }
    return 0;
}
otsarev@tsarev-desktop:~ ─────────────────────────────────────────────────────────────────────────────────
➜ g++ ./test.cpp --std=c++11
otsarev@tsarev-desktop:~ ─────────────────────────────────────────────────────────────────────────────────
➜ echo "0 1 2 3 25 26 27 28 51 52 53 12312" | ./a.out
0	A
1	B
2	C
3	D
25	Z
26	AA
27	AB
28	AC
51	AZ
52	AA
53	AB
12312	FAO