TrinityCore/src/common/Encoding/BaseEncoding.h

/*
 * This file is part of the TrinityCore Project. See AUTHORS file for Copyright information
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation; either version 2 of the License, or (at your
 * option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef TRINITY_BASE_ENCODING_HPP
#define TRINITY_BASE_ENCODING_HPP

#include "Define.h"
#include "Optional.h"
#include <numeric>
#include <string>
#include <vector>

namespace Trinity
{
namespace Impl
{
template <typename Encoding>
struct GenericBaseEncoding
{
    static constexpr std::size_t BITS_PER_CHAR = Encoding::BITS_PER_CHAR;
    static constexpr std::size_t PAD_TO = std::lcm(8u, BITS_PER_CHAR);

    static_assert(BITS_PER_CHAR < 8, "Encoding parameters are invalid");

    static constexpr uint8 DECODE_ERROR = Encoding::DECODE_ERROR;
    static constexpr char PADDING = Encoding::PADDING;

    static constexpr std::size_t EncodedSize(std::size_t size)
    {
        size *= 8; // bits in input
        if (size % PAD_TO) // pad to boundary
            size += (PAD_TO - (size % PAD_TO));
        return (size / BITS_PER_CHAR);
    }

    static constexpr std::size_t DecodedSize(std::size_t size)
    {
        size *= BITS_PER_CHAR; // bits in input
        if (size % PAD_TO) // pad to boundary
            size += (PAD_TO - (size % PAD_TO));
        return (size / 8);
    }

    static std::string Encode(std::vector<uint8> const& data)
    {
        auto it = data.begin(), end = data.end();
        if (it == end)
            return "";

        std::string s;
        s.reserve(EncodedSize(data.size()));

        uint8 bitsLeft = 8; // in current byte
        do
        {
            uint8 thisC = 0;
            if (bitsLeft >= BITS_PER_CHAR)
            {
                bitsLeft -= BITS_PER_CHAR;
                thisC = ((*it >> bitsLeft) & ((1 << BITS_PER_CHAR)-1));
                if (!bitsLeft)
                {
                    ++it;
                    bitsLeft = 8;
                }
            }
            else
            {
                thisC = (*it & ((1 << bitsLeft) - 1)) << (BITS_PER_CHAR - bitsLeft);
                bitsLeft += (8 - BITS_PER_CHAR);
                if ((++it) != end)
                    thisC |= (*it >> bitsLeft);
            }
            s.append(1, Encoding::Encode(thisC));
        } while (it != end);

        while (bitsLeft != 8)
        {
            if (bitsLeft > BITS_PER_CHAR)
                bitsLeft -= BITS_PER_CHAR;
            else
                bitsLeft += (8 - BITS_PER_CHAR);
            s.append(1, PADDING);
        }

        return s;
    }

    static Optional<std::vector<uint8>> Decode(std::string const& data)
    {
        auto it = data.begin(), end = data.end();
        if (it == end)
            return std::vector<uint8>();

        std::vector<uint8> v;
        v.reserve(DecodedSize(data.size()));

        uint8 currentByte = 0;
        uint8 bitsLeft = 8; // in current byte
        while ((it != end) && (*it != PADDING))
        {
            uint8 cur = Encoding::Decode(*(it++));
            if (cur == DECODE_ERROR)
                return {};

            if (bitsLeft > BITS_PER_CHAR)
            {
                bitsLeft -= BITS_PER_CHAR;
                currentByte |= (cur << bitsLeft);
            }
            else
            {
                bitsLeft = BITS_PER_CHAR - bitsLeft; // in encoded char
                currentByte |= (cur >> bitsLeft);
                v.push_back(currentByte);
                currentByte = (cur & ((1 << bitsLeft) - 1));
                bitsLeft = 8 - bitsLeft; // in byte again
                currentByte <<= bitsLeft;
            }
        }

        if (currentByte)
            return {}; // decode error, trailing non-zero bits

        // process padding
        while ((it != end) && (*it == PADDING) && (bitsLeft != 8))
        {
            if (bitsLeft > BITS_PER_CHAR)
                bitsLeft -= BITS_PER_CHAR;
            else
                bitsLeft += (8 - BITS_PER_CHAR);
            ++it;
        }

        // ok, all padding should be consumed, and we should be at end of string
        if (it == end)
            return v;

        // anything else is an error
        return {};
    }
};
}
}

#endif