103 lines
2.6 KiB
C++
103 lines
2.6 KiB
C++
#include "mybase64.hpp"
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#include <array>
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#include <climits>
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#include <cstdint>
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#include <string_view>
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namespace mybase64 {
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namespace {
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constexpr std::array<char, 64> alphabet{
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'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
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'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
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'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
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'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/'
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};
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constexpr std::array<std::int8_t, 256> alphabet_values = []() constexpr {
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std::array<std::int8_t, 256> result;
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result.fill(-1);
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std::int8_t v = 0;
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for (auto const k : alphabet)
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{
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result[k] = v++;
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}
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return result;
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}();
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}
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static_assert(CHAR_BIT == 8);
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auto encode(std::string_view const input, char* output) -> void
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{
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auto cursor = std::begin(input);
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auto const end = std::end(input);
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while (end - cursor >= 3)
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{
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std::uint32_t buffer = std::uint8_t(*cursor++);
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buffer <<= 8;
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buffer |= std::uint8_t(*cursor++);
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buffer <<= 8;
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buffer |= std::uint8_t(*cursor++);
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*output++ = alphabet[(buffer >> 6 * 3) % 64];
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*output++ = alphabet[(buffer >> 6 * 2) % 64];
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*output++ = alphabet[(buffer >> 6 * 1) % 64];
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*output++ = alphabet[(buffer >> 6 * 0) % 64];
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}
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if (cursor < end)
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{
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std::uint32_t buffer = std::uint8_t(*cursor++) << 10;
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if (cursor < end)
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buffer |= std::uint8_t(*cursor) << 2;
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*output++ = alphabet[(buffer >> 12) % 64];
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*output++ = alphabet[(buffer >> 6) % 64];
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*output++ = cursor < end ? alphabet[(buffer % 64)] : '=';
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*output++ = '=';
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}
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*output = '\0';
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}
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auto decode(std::string_view const input, char* output) -> char*
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{
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std::uint32_t buffer = 1;
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for (auto const c : input)
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{
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if (auto const value = alphabet_values[uint8_t(c)]; -1 != value)
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{
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buffer = (buffer << 6) | value;
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if (buffer & 1 << 6 * 4)
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{
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*output++ = buffer >> 16;
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*output++ = buffer >> 8;
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*output++ = buffer >> 0;
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buffer = 1;
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}
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}
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}
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if (buffer & 1 << 6 * 3)
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{
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*output++ = buffer >> 10;
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*output++ = buffer >> 2;
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}
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else if (buffer & 1 << 6 * 2)
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{
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*output++ = buffer >> 4;
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}
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else if (buffer & 1 << 6 * 1)
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{
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return nullptr;
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}
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return output;
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}
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} // namespace
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