C++20 constexpr single header base64 decode/encoder.

base64.h

#ifndef _MACARON_BASE64_H_
#define _MACARON_BASE64_H_

/**
 * The MIT License (MIT)
 * Copyright (c) 2016 tomykaira
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

#include <cstddef>
#include <cstdint>
#include <array>
#include <utility>

namespace macaron {

template <size_t N>
constexpr const std::array<char, ((((N-1) >> 2) * 3) + 1)> decode(const char(&input)[N]) {
    constexpr unsigned char kDecodingTable[] = {
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63,
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64,
        64,  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, 64, 64, 64, 64, 64,
        64, 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, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64
    };

    static_assert(((N-1) & 3) == 0, "Input data size is not a multiple of 4");

    std::array<char, (((N-1) >> 2) * 3) + 1> out {0};

    size_t out_len = (N-1) / 4 * 3;
    if (input[(N-1) - 1] == '=') out_len--;
    if (input[(N-1) - 2] == '=') out_len--;

    for (size_t i = 0, j = 0; i < N-1;) {
      uint32_t a = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
      uint32_t b = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
      uint32_t c = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
      uint32_t d = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];

      uint32_t triple = (a << 3 * 6) + (b << 2 * 6) + (c << 1 * 6) + (d << 0 * 6);

      if (j < out_len) out[j++] = (triple >> 2 * 8) & 0xFF;
      if (j < out_len) out[j++] = (triple >> 1 * 8) & 0xFF;
      if (j < out_len) out[j++] = (triple >> 0 * 8) & 0xFF;
    }
    return out;
}

}

#endif /* _MACARON_BASE64_H_ */

It is this one but modified to take advantage of the constexpr [] operator of std::array in C++20 to remove the fixed_string template.

Demo here.

I was able to compile a simple test program with C++17:

#include <iostream>
#include "base64.h"

int main() {
    constexpr auto decoded = macaron::decode("aGVsbG8gd29ybGQ=");
    std::cout << decoded.data() << std::endl;
    return 0;
}

$ g++ --std=c++17 -O3 main.cpp
$ ./a.out
hello world
$ strings ./a.out # Base64 not found, "hello world" found 

According to cppreference.com, std::array::operator[] has supported constexpr fully since C++17.