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c++ - C++ 中的编译时 Base64 解码

问题描述

是否可以在编译时将 base64 编码数据解码为二进制数据？

我想到了一些看起来像这样的东西：

constexpr auto decoded = decodeBase64<"SGVsbG8=">();

或者

constexpr auto decoded = decodeBase64("SGVsbG8=");

我对结果类型没有特殊要求decoded。

标签： c++c++17constexprcompile-time

解决方案

我发现很难用谷歌搜索 constexpr base64 解码器，所以我在这里调整了一个： https ://gist.github.com/tomykaira/f0fd86b6c73063283afe550bc5d77594

由于这是 MIT 许可的，（叹气），请务必在源文件中的某处拍打它：

/**
 * 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.
 */

要从constexpr函数返回字符串，您需要返回一个 char 数组。因为你不能返回一个数组 or std::string，所以 anstd::array是最好的选择。但是有一个问题——由于标准的疏忽，直到 C++17 的[]操作符std::array是非常量的。您可以通过继承和添加构造函数来解决这个问题：

template <size_t N>
struct fixed_string : std::array<char, N> {
    constexpr fixed_string(const char (&input)[N]) : fixed_string(input, std::make_index_sequence<N>{}) {}
    template <size_t... Is>
    constexpr fixed_string(const char (&input)[N], std::index_sequence<Is...>) : std::array<char, N>{ input[Is]... } {}
};

将解码器更改为使用它而不是std::string，它似乎可以作为 constexpr 工作。需要 C++14，因为 C++11 的 constexpr 函数只能有一个 return 语句：

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");

    char out[(((N-1) >> 2) * 3) + 1] {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 fixed_string<(((N-1) >> 2) * 3) + 1>(out);
}

用法：

constexpr auto x = decode("aGVsbG8gd29ybGQ=");
/*...*/
printf(x.data()); // hello world

演示：https ://godbolt.org/z/HFdk6Z

更新以解决来自 Marek R 和 Frank 的有用反馈

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