std::string EncodeBase64(const char* data, size_t length)
{
    // calculate extra bytes needed to get a multiple of 3
    size_t extraBytes = 3 - length % 3;
    // number of base64 bytes
    size_t encodedBytes = 4 * (length + extraBytes) / 3;
    std::string encoded_string(encodedBytes, '=');
    // read blocks of 3 bytes
    for (size_t ib3 = 0; ib3 < length / 3; ib3++)
    {
        const size_t iByte = ib3 * 3;
        const size_t iEncodedByte = ib3 * 4;
        const char* currData = &data[iByte];
        EncodeByteBlock(currData, encoded_string, iEncodedByte);
    }
    // if size of data is not a multiple of 3,
    // also encode the final bytes (and add zeros where needed)
    if (extraBytes > 0)
    {
        char finalBytes[4] = { 0,0,0,0 };
        memcpy(&finalBytes[0], &data[length - length % 3], length % 3);
        const size_t iEncodedByte = encodedBytes - 4;
        EncodeByteBlock(&finalBytes[0], encoded_string, iEncodedByte);
        // add '=' at the end
        for (size_t i = 0; i < 4 * extraBytes / 3; i++)
            encoded_string[encodedBytes - i - 1] = '=';
    }
    return encoded_string;
}

// calculate extra bytes needed to get a multiple of 3
size_t extraBytes = 3 - length % 3;

size_t extraBytes = length % 3;

size_t extraBytes = 3 - length % 3;

size_t extraBytes = 3 - [0..2];

if (extraBytes > 0)

// calculate extra bytes needed to get a multiple of 3
size_t extraBytes = 3 - length % 3; // 3-6%3 = 3
// number of base64 bytes
size_t encodedBytes = 4 * (length + extraBytes) / 3; // 4*(6+3)/3 = 12
std::string encoded_string(encodedBytes, '=');