#define UNIT r4_bitptr
#include <cstddef>
#include <cassert>

class bitptr
{
    std::byte *_ptr;
    int _offset;
    bool _valid = true;
public:
    bitptr() : _valid( false ) {}
    bitptr( std::byte *p, int o ) : _ptr( p ), _offset( o )
    {
        assert( o >= 0 && o <= 7 );
    }

    static auto one() { return std::byte( 1 ); }

    bool get() const { return bool( *_ptr & one() << _offset ); }
    void set( bool b ) const
    {
        *_ptr = ( *_ptr & ~( one() << _offset ) ) |
                std::byte( b ) << _offset;
    }

    void advance( int n = 1 ) { _offset += n; _ptr += _offset / 8; _offset %= 8; }
    bool valid() const { return _valid; }
};

class const_bitptr
{
    bitptr _ptr;
public:
    const_bitptr() = default;
    const_bitptr( const std::byte *p, int o )
        : _ptr( const_cast< std::byte * >( p ), o )
    {}

    bool get() const { return _ptr.get(); }
    void set( bool b ) const { return _ptr.set( b ); }
    void advance( int n = 1 ) { _ptr.advance( n ); }
    bool valid() const { return _ptr.valid(); }
};

#include "test_main.cpp"