Base64.cc

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#include "Base64.hh"
#include "narrow.hh"
#include "ranges.hh"
#include "xrange.hh"
#include <algorithm>
#include <array>
#include <cassert>
 
namespace Base64 {
 
using openmsx::MemBuffer;
 
[[nodiscard]] static constexpr char encode(uint8_t c)
{
        constexpr std::array<char, 64> base64_chars = {
                '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','+','/',
        };
        assert(c < 64);
        return base64_chars[c];
}
 
[[nodiscard]] static constexpr uint8_t decode(uint8_t c)
{
        if        ('A' <= c && c <= 'Z') {
                return c - 'A';
        } else if ('a' <= c && c <= 'z') {
                return c - 'a' + 26;
        } else if ('0' <= c && c <= '9') {
                return c - '0' + 52;
        } else if (c == '+') {
                return 62;
        } else if (c == '/') {
                return 63;
        } else {
                return uint8_t(-1);
        }
}
 
std::string encode(std::span<const uint8_t> input)
{
        constexpr int CHUNKS = 19;
        constexpr int IN_CHUNKS  = 3 * CHUNKS;
        constexpr int OUT_CHUNKS = 4 * CHUNKS; // 76 chars per line
 
        auto outSize = ((input.size() + (IN_CHUNKS - 1)) / IN_CHUNKS) * (OUT_CHUNKS + 1); // overestimation
        std::string ret(outSize, 0); // too big
 
        size_t out = 0;
        while (!input.empty()) {
                if (out) ret[out++] = '\n';
                auto n = std::min<size_t>(IN_CHUNKS, input.size());
                for (; n >= 3; n -= 3) {
                        ret[out++] = encode(uint8_t( (input[0] & 0xfc) >> 2));
                        ret[out++] = encode(uint8_t(((input[0] & 0x03) << 4) +
                                                    ((input[1] & 0xf0) >> 4)));
                        ret[out++] = encode(uint8_t(((input[1] & 0x0f) << 2) +
                                                    ((input[2] & 0xc0) >> 6)));
                        ret[out++] = encode(uint8_t( (input[2] & 0x3f) >> 0));
                        input = input.subspan(3);
                }
                if (n) {
                        std::array<uint8_t, 3> buf3 = {0, 0, 0};
                        ranges::copy(input.subspan(0, n), buf3);
                        input = input.subspan(n);
 
                        std::array<uint8_t, 4> buf4;
                        buf4[0] = uint8_t( (buf3[0] & 0xfc) >> 2);
                        buf4[1] = uint8_t(((buf3[0] & 0x03) << 4) +
                                          ((buf3[1] & 0xf0) >> 4));
                        buf4[2] = uint8_t(((buf3[1] & 0x0f) << 2) +
                                          ((buf3[2] & 0xc0) >> 6));
                        buf4[3] = uint8_t( (buf3[2] & 0x3f) >> 0);
                        for (auto j : xrange(n + 1)) {
                                ret[out++] = encode(buf4[j]);
                        }
                        for (; n < 3; ++n) {
                                ret[out++] = '=';
                        }
                }
        }
 
        assert(outSize >= out);
        ret.resize(out); // shrink to correct size
        return ret;
}
 
std::pair<MemBuffer<uint8_t>, size_t> decode(std::string_view input)
{
        auto outSize = (input.size() * 3 + 3) / 4; // overestimation
        MemBuffer<uint8_t> ret(outSize); // too big
 
        unsigned i = 0;
        size_t out = 0;
        std::array<uint8_t, 4> buf4;
        for (auto c : input) {
                uint8_t d = decode(c);
                if (d == uint8_t(-1)) continue;
                buf4[i++] = d;
                if (i == 4) {
                        i = 0;
                        ret[out++] = char(((buf4[0] & 0xff) << 2) + ((buf4[1] & 0x30) >> 4));
                        ret[out++] = char(((buf4[1] & 0x0f) << 4) + ((buf4[2] & 0x3c) >> 2));
                        ret[out++] = char(((buf4[2] & 0x03) << 6) + ((buf4[3] & 0xff) >> 0));
                }
        }
        if (i) {
                for (auto j : xrange(i, 4u)) {
                        buf4[j] = 0;
                }
                std::array<uint8_t, 3> buf3;
                buf3[0] = narrow_cast<uint8_t>(((buf4[0] & 0xff) << 2) + ((buf4[1] & 0x30) >> 4));
                buf3[1] = narrow_cast<uint8_t>(((buf4[1] & 0x0f) << 4) + ((buf4[2] & 0x3c) >> 2));
                buf3[2] = narrow_cast<uint8_t>(((buf4[2] & 0x03) << 6) + ((buf4[3] & 0xff) >> 0));
                for (auto j : xrange(i - 1)) {
                        ret[out++] = buf3[j];
                }
        }
 
        assert(outSize >= out);
        ret.resize(out); // shrink to correct size
        return {std::move(ret), out};
}
 
bool decode_inplace(std::string_view input, std::span<uint8_t> output)
{
        auto outSize = output.size();
        unsigned i = 0;
        size_t out = 0;
        std::array<uint8_t, 4> buf4;
        for (auto c : input) {
                uint8_t d = decode(c);
                if (d == uint8_t(-1)) continue;
                buf4[i++] = d;
                if (i == 4) {
                        i = 0;
                        if ((out + 3) > outSize) [[unlikely]] return false;
                        output[out++] = char(((buf4[0] & 0xff) << 2) + ((buf4[1] & 0x30) >> 4));
                        output[out++] = char(((buf4[1] & 0x0f) << 4) + ((buf4[2] & 0x3c) >> 2));
                        output[out++] = char(((buf4[2] & 0x03) << 6) + ((buf4[3] & 0xff) >> 0));
                }
        }
        if (i) {
                for (auto j : xrange(i, 4u)) {
                        buf4[j] = 0;
                }
                std::array<uint8_t, 3> buf3;
                buf3[0] = narrow_cast<uint8_t>(((buf4[0] & 0xff) << 2) + ((buf4[1] & 0x30) >> 4));
                buf3[1] = narrow_cast<uint8_t>(((buf4[1] & 0x0f) << 4) + ((buf4[2] & 0x3c) >> 2));
                buf3[2] = narrow_cast<uint8_t>(((buf4[2] & 0x03) << 6) + ((buf4[3] & 0xff) >> 0));
                for (auto j : xrange(i - 1)) {
                        if (out == outSize) [[unlikely]] return false;
                        output[out++] = buf3[j];
                }
        }
 
        return out == outSize;
}
 
} // namespace Base64