A Base64 Encoding Implementation using C++ Template Metaprogramming

tmp-base64.cc

#include <iostream>
#include <type_traits>

template <char ...chars>
using TString = std::integer_sequence<char, chars...>;

template <typename T, T ...chars>
constexpr TString<chars...> operator""_tstr() { return { }; }

template <typename>
char const FromTString[] = { 0 };

template <char ..._Bytes>
char const FromTString<TString<_Bytes...>>[] = { _Bytes..., 0 };

template <char _First, typename>
struct TStringPrepend {} ;

template <char _First, char ..._Bytes>
struct TStringPrepend<_First, TString<_Bytes...>> {
    using type = TString<_First, _Bytes...>;
};

constexpr char Base64Lookup(int idx) {
    return "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[idx];
}

template <int _Bits, int _Data, char _First, char ..._Rest>
struct Base64Inner;

template <int _Bits, int _Data>
struct Base64Last;

template <int _Bits, int _Encoded, int _NextBits, int _Next, char _First, char ..._Rest>
struct Base64EncodeNextType {
    using type = typename TStringPrepend<_Encoded, typename Base64Inner<_NextBits, _Next, _Rest...>::StringType>::type;
};

template <int _Encoded, int _Next, int _NextBits, char _First, char ..._Rest>
struct Base64EncodeNextType<6, _Encoded, _NextBits, _Next, _First, _Rest...> {
    using type = typename TStringPrepend<_Encoded, typename Base64Inner<0, 0, _First, _Rest...>::StringType>::type;
};

template <int _Bits, int _Data, char _First, char ..._Rest>
struct Base64Inner {
    static constexpr int EncodeBits = 6 - _Bits;
    static constexpr int ShiftBits = 8 - EncodeBits;
    static constexpr int EncodedByte = Base64Lookup((_Data << EncodeBits) | (((unsigned char)_First) >> ShiftBits));
    static constexpr int StagedByte = _First & ((1 << ShiftBits) - 1);
    using StringType = typename Base64EncodeNextType<_Bits, EncodedByte, ShiftBits, StagedByte, _First, _Rest...>::type;
};

template <int _Data, char _Only>
struct Base64Inner<6, _Data, _Only> {
    using StringType = typename TStringPrepend<Base64Lookup(_Data), typename Base64Inner<0, 0, _Only>::StringType>::type;
};

template <int _Bits, int _Data, char _Only>
struct Base64Inner<_Bits, _Data, _Only> {
    static constexpr int EncodeBits = 6 - _Bits;
    static constexpr int ShiftBits = 8 - EncodeBits;
    static constexpr int EncodedByte = Base64Lookup((_Data << EncodeBits) | (((unsigned char)_Only) >> ShiftBits));
    static constexpr int StagedByte = _Only & ((1 << ShiftBits) - 1);
    using StringType = typename TStringPrepend<EncodedByte, typename Base64Last<ShiftBits, StagedByte>::StringType>::type;
};

template <int _Bits, int _Data>
struct Base64Last {};

template <int _Data>
struct Base64Last<2, _Data> {
    using StringType = TString<Base64Lookup(_Data), '=', '='>;
};

template <int _Data>
struct Base64Last<4, _Data> {
    using StringType = TString<Base64Lookup(_Data), '='>;
};

template <int _Data>
struct Base64Last<6, _Data> {
    using StringType = TString<Base64Lookup(_Data)>;
};

template <char ..._Bytes>
using Base64Impl = typename Base64Inner<0, 0, _Bytes...>::StringType;

template <typename>
struct Base64 {};

template <char ..._Bytes>
struct Base64<TString<_Bytes...>> {
    using StringType = Base64Impl<_Bytes...>;
};

int main() {
    const char *encoded = FromTString<Base64<decltype("偶像好帅"_tstr)>::StringType>;
    std::cout << encoded << std::endl;
    return 0;
}