src/base64.cpp

#include <base64.hpp>

namespace base64
{
bool base64_encode(const std::string &input, std::string &output)
{
    //  As is, this array is 64 bytes long, which should be greater than the max
    //  of 0b00111111 when indexed NOLINT calls below are to silence clang-tidy
    //  warnings about this
    constexpr std::array<char, 64> encoding_data = {
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
        'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
        'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/'};

    size_t input_length = input.size();

    // allocate space for output string
    output.clear();
    output.reserve(((input_length + 2) / 3) * 4);

    // for each 3-bytes sequence from the input, extract 4 6-bits sequences and
    // encode using
    // encoding_data lookup table.
    // if input do not contains enough chars to complete 3-byte sequence,use pad
    // char '='
    for (size_t i = 0; i < input_length; i++)
    {
        int base64code0 = 0;
        int base64code1 = 0;
        int base64code2 = 0;
        int base64code3 = 0;

        base64code0 = (input[i] >> 2) & 0x3f; // 1-byte 6 bits

        output += encoding_data[base64code0]; // NOLINT
        base64code1 = (input[i] << 4) & 0x3f; // 1-byte 2 bits +

        if (++i < input_length)
        {
            base64code1 |= (input[i] >> 4) & 0x0f; // 2-byte 4 bits
            output += encoding_data[base64code1];  // NOLINT
            base64code2 = (input[i] << 2) & 0x3f;  // 2-byte 4 bits +

            if (++i < input_length)
            {
                base64code2 |= (input[i] >> 6) & 0x03; // 3-byte 2 bits
                base64code3 = input[i] & 0x3f;         // 3-byte 6 bits
                output += encoding_data[base64code2];  // NOLINT
                output += encoding_data[base64code3];  // NOLINT
            }
            else
            {
                output += encoding_data[base64code2]; // NOLINT
                output += '=';
            }
        }
        else
        {
            output += encoding_data[base64code1]; // NOLINT
            output += '=';
            output += '=';
        }
    }

    return true;
}

bool base64_decode(const std::string &input, std::string &output)
{
    constexpr char nop = -1;
    // See note on encoding_data[] in above function
    constexpr std::array<char, 256> decoding_data = {
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, 62,  nop, nop, nop, 63,  52,  53,  54,  55,  56,  57,  58,  59,
        60,  61,  nop, nop, nop, nop, nop, nop, nop, 0,   1,   2,   3,   4,
        5,   6,   7,   8,   9,   10,  11,  12,  13,  14,  15,  16,  17,  18,
        19,  20,  21,  22,  23,  24,  25,  nop, nop, nop, nop, nop, nop, 26,
        27,  28,  29,  30,  31,  32,  33,  34,  35,  36,  37,  38,  39,  40,
        41,  42,  43,  44,  45,  46,  47,  48,  49,  50,  51,  nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop, nop,
        nop, nop, nop, nop};

    size_t input_length = input.size();

    // allocate space for output string
    output.clear();
    output.reserve(((input_length + 2) / 3) * 4);

    // for each 4-bytes sequence from the input, extract 4 6-bits sequences by
    // droping first two bits
    // and regenerate into 3 8-bits sequences

    for (size_t i = 0; i < input_length; i++)
    {
        char base64code0;
        char base64code1;
        char base64code2 = 0; // initialized to 0 to suppress warnings
        char base64code3;

        base64code0 = decoding_data[static_cast<size_t>(input[i])]; // NOLINT
        if (base64code0 == nop)
        { // non base64 character
            return false;
        }
        if (!(++i < input_length))
        { // we need at least two input bytes for first
          // byte output
            return false;
        }
        base64code1 = decoding_data[static_cast<size_t>(input[i])]; // NOLINT
        if (base64code1 == nop)
        { // non base64 character
            return false;
        }
        output +=
            static_cast<char>((base64code0 << 2) | ((base64code1 >> 4) & 0x3));

        if (++i < input_length)
        {
            char c = input[i];
            if (c == '=')
            { // padding , end of input
                return (base64code1 & 0x0f) == 0;
            }
            base64code2 =
                decoding_data[static_cast<size_t>(input[i])]; // NOLINT
            if (base64code2 == nop)
            { // non base64 character
                return false;
            }
            output += static_cast<char>(((base64code1 << 4) & 0xf0) |
                                        ((base64code2 >> 2) & 0x0f));
        }

        if (++i < input_length)
        {
            char c = input[i];
            if (c == '=')
            { // padding , end of input
                return (base64code2 & 0x03) == 0;
            }
            base64code3 =
                decoding_data[static_cast<size_t>(input[i])]; // NOLINT
            if (base64code3 == nop)
            { // non base64 character
                return false;
            }
            output +=
                static_cast<char>((((base64code2 << 6) & 0xc0) | base64code3));
        }
    }

    return true;
}
} // namespace base64