Insane_DNS/include/insane_dns/util.h

/*
 *  BLT Memory Util for parsing DNS packets.
 *  This software is unlikely to become part of BLT main but is provided under the BLT license (GPL 3).
 *  Copyright (C) 2023  Brett Terpstra
 *
 *  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 3 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 <https://www.gnu.org/licenses/>.
 */

#ifndef INSANE_DNS_UTIL_H
#define INSANE_DNS_UTIL_H

#include <insane_dns/constants.h>

namespace blt
{
    
    /**
     * Basic parser for processing strings of bytes received from UDP sockets. This class provides a simple interface for efficiently copying the
     * big endian bytes from the network into the little endian bytes required by x86 processors. The class also provides basic safety guarantees
     * in that the program will halt execution if reading the bytes would overflow the internal buffer.
     */
    class byte_reader
    {
        private:
            unsigned char* _data;
            const size_t _size;
            mutable size_t _current_byte = DNS_HEADER_END;
        public:
            explicit byte_reader(unsigned char* data, size_t size, bool tcp = false): _data(data), _size(size)
            {
                if (tcp)
                    _current_byte += 2;
            }
            
            /**
             * Read the next byte in the data stream then increment the internal counter by 1
             * @return the next byte in the array
             */
            inline unsigned char& next() const
            {
                BLT_ASSERT(_current_byte < _size);
                return _data[_current_byte++];
            }
            
            /**
             * Reads data into the provided template parameter, reordering the byte into the correct little endian format
             * @tparam T type to read (deduction is automatic)
             * @param t variable reference to read into
             */
            template<typename T>
            inline void to(T& t) const
            {
                BLT_ASSERT(_current_byte + sizeof(T) <= _size);
                // I hate little endian. I hate dealing with converting between endianness
                // it is very easy to do it very slowly
                // So I made BLT provide a simple interface to convert between bytes dependent on the platform
                // for small integral types (16, 32, and 64) it will compile to 3 instructions using compiler intrinsics.
                // Larger POD types are supported but will use std::reverse and therefore be slow.
                blt::mem::fromBytes(&_data[_current_byte], t);
                skip(sizeof(T));
            }
            
            /**
             * Copy from the internal data buffer, advancing the internal stream, into the provided unsigned char array. *No reordering is done*
             * @param out character array to write to
             * @param size number of bytes to copy
             */
            inline void copy(unsigned char*& out, size_t size) const
            {
                BLT_ASSERT(_current_byte + size < _size);
                std::memcpy(out, &_data[_current_byte], size);
                skip(size);
            }
            
            /**
             * Skip bytes in the data stream, useful for skipping sections of the DNS packet we may not care about.
             * @param s number of bytes to skip.
             */
            inline void skip(size_t s = 1) const
            {
                _current_byte += s;
            }
            
            /**
             * @return 1 past the last byte we wrote to, otherwise known as the next byte we will be writing to.
             */
            inline size_t last() const
            {
                return _current_byte;
            }
            
            /**
             * @return a pointer to a location in the internal buffer starting at last()
             */
            inline unsigned char* from()
            {
                BLT_ASSERT(_current_byte < _size);
                return &_data[_current_byte];
            }
    };

}

#endif //INSANE_DNS_UTIL_H