#include <array>
#include <fstream>
#include <iterator>
#include <string>

constexpr char base64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int main(int argc, char *argv[]) {
    std::string
        filename("a.bin"),
        textfile("a.txt");

    if (argc > 1) {
        filename.assign(argv[1]);
    }

    if (argc > 2) {
        textfile.assign(argv[2]);
    }

    std::fstream
        file(filename, std::ios::binary | std::ios::in),
        text(textfile, std::ios::out);

    std::istreambuf_iterator<char>
        iter_cur(file),
        iter_end;

    std::array<unsigned char, 0x1000> raw;
    std::array<unsigned char, 0x2000> b64;
    while (iter_cur != iter_end) {
        raw.fill(0);
        b64.fill(0);

        auto beg = raw.begin();
        auto cur = beg;
        auto end = raw.end();

        while (cur != end && iter_cur != iter_end) {
            *cur++ = *iter_cur++;
        }

        auto delta = cur - beg;
        auto repeat = delta / 3 + (delta % 3 != 0);
        auto bin_ptr = beg;
        auto b64_ptr = b64.begin();

        for (auto a = 0; a < repeat; ++ a, bin_ptr += 3, b64_ptr += 4) {
            int hex[4] = {
                (bin_ptr[0] & 0b11111100) >> 2,
                ((bin_ptr[0] & 0b00000011) << 4) +
                    ((bin_ptr[1] & 0b11110000) >> 4),
                ((bin_ptr[1] & 0b00001111) << 2) +
                    ((bin_ptr[2] & 0b11000000) >> 6),
                (bin_ptr[2] & 0b00111111),
            };

            for (int i = 0; i < 4; ++ i) {
                b64_ptr[i] = base64[hex[i]];
            }
        }

        for (int i = 1; i < delta % 3; ++ i) {
            b64_ptr[-i] = '=';
        }

        text << b64.data();
    }

    text.flush();

    return 0;
}