base64.h

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#ifndef BASE64_H
#define BASE64_H
 
#include <vector>
#include <string>
#include <cctype>
#include <cstring>
#include <limits>
 
class Base64 {
private:
    static unsigned char const PAD = '=';
 
    static unsigned char enc(int c)
    {
        static const unsigned char table[] = {
            0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50,
            0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66,
            0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76,
            0x77, 0x78, 0x79, 0x7a, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x2b, 0x2f,
        };
        return table[c & 63];
    }
 
    static unsigned char dec(int c)
    {
        static const unsigned char table[] = {
            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3e, 0xff, 0xff, 0xff, 0x3f,
            0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
            0xff, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e,
            0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0xff, 0xff, 0xff, 0xff, 0xff,
            0xff, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28,
            0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33, 0xff, 0xff, 0xff, 0xff, 0xff,
        };
        return table[c & 127];
    }
public:
    static bool decode_checked(char const *src, size_t length, std::vector<char> *out)
    {
        if (!out) return false;
        out->clear();
        if (!src && length != 0) return false;
 
        unsigned char quartet[4];
        size_t count = 0;
        bool finished = false;
        for (size_t i = 0; i < length; ++i) {
            unsigned char ch = static_cast<unsigned char>(src[i]);
            if (std::isspace(ch)) continue;
            if (finished) return false;
            if (ch == PAD) {
                quartet[count++] = 0x40;
            } else {
                if (ch >= 0x80) return false;
                unsigned char value = dec(ch);
                if (value >= 0x40) return false;
                quartet[count++] = value;
            }
            if (count != 4) continue;
 
            if (quartet[0] >= 0x40 || quartet[1] >= 0x40) return false;
            if (quartet[2] == 0x40 && quartet[3] != 0x40) return false;
            out->push_back(static_cast<char>((quartet[0] << 2) | (quartet[1] >> 4)));
            if (quartet[2] != 0x40) {
                out->push_back(static_cast<char>((quartet[1] << 4) | (quartet[2] >> 2)));
                if (quartet[3] != 0x40) {
                    out->push_back(static_cast<char>((quartet[2] << 6) | quartet[3]));
                }
            }
            finished = quartet[2] == 0x40 || quartet[3] == 0x40;
            count = 0;
        }
        if (count != 0) {
            out->clear();
            return false;
        }
        return true;
    }
 
    static void encode(char const *src, size_t length, std::vector<char> *out)
    {
        if (!out) return;
        out->clear();
        if (!src && length != 0) return;
        if (length > std::numeric_limits<size_t>::max() / 4 * 3) return;
        size_t srcpos = 0;
        size_t dstlen = length / 3 * 4;
        if (length % 3 != 0) dstlen += 4;
        out->resize(dstlen);
        if (dstlen == 0) {
            return;
        }
        char *dst = &out->at(0);
        size_t dstpos = 0;
        for (srcpos = 0; srcpos < length; srcpos += 3) {
            int v = static_cast<unsigned char>(src[srcpos]) << 16;
            if (srcpos + 1 < length) {
                v |= static_cast<unsigned char>(src[srcpos + 1]) << 8;
                if (srcpos + 2 < length) {
                    v |= static_cast<unsigned char>(src[srcpos + 2]);
                    dst[dstpos + 3] = enc(v);
                } else {
                    dst[dstpos + 3] = PAD;
                }
                dst[dstpos + 2] = enc(v >> 6);
            } else {
                dst[dstpos + 2] = PAD;
                dst[dstpos + 3] = PAD;
            }
            dst[dstpos + 1] = enc(v >> 12);
            dst[dstpos] = enc(v >> 18);
            dstpos += 4;
        }
    }
 
    static void decode(char const *src, size_t length, std::vector<char> *out)
    {
        (void)decode_checked(src, length, out);
    }
 
    static std::string _to_s_(std::vector<char> const *vec)
    {
        if (!vec || vec->empty()) return std::string();
        return std::string((char const *)&(*vec)[0], vec->size());
    }
};
 
static inline void base64_encode(char const *src, size_t length, std::vector<char> *out)
{
    Base64::encode(src, length, out);
}
 
static inline void base64_decode(char const *src, size_t length, std::vector<char> *out)
{
    Base64::decode(src, length, out);
}
 
static inline void base64_encode(std::vector<char> const *src, std::vector<char> *out)
{
    if (!src) {
        out->clear();
        return;
    }
    Base64::encode(src->data(), src->size(), out);
}
 
static inline void base64_decode(std::vector<char> const *src, std::vector<char> *out)
{
    if (!src) {
        out->clear();
        return;
    }
    Base64::decode(src->data(), src->size(), out);
}
 
static inline void base64_encode(char const *src, std::vector<char> *out)
{
    Base64::encode((char const *)src, strlen(src), out);
}
 
static inline void base64_decode(char const *src, std::vector<char> *out)
{
    Base64::decode((char const *)src, strlen(src), out);
}
 
static inline std::string base64_encode(std::string_view const &src)
{
    std::vector<char> vec;
    base64_encode((char const *)src.data(), src.size(), &vec);
    return Base64::_to_s_(&vec);
}
 
static inline std::string base64_decode(std::string_view const &src)
{
    std::vector<char> vec;
    base64_decode((char const *)src.data(), src.size(), &vec);
    return Base64::_to_s_(&vec);
}
 
#endif