#include <iostream>
#include <algorithm>
#include <bitset>
 
std::uint64_t key_reg(std::uint8_t* bytes, std::size_t pos, std::size_t n)
{
   std::uint64_t reg = 0;
   auto start = bytes + pos / 8;
   auto end = bytes + n / 8 + ( n % 8 != 0 );
   std::reverse_copy( start,  end, reinterpret_cast<char *>( &reg ) );
   reg >>= ( n + pos ) % 8;
   reg &= ~( -1UL << n );
   return reg;
}
 
int main()
{
    std::uint8_t bt1[] = {255, 0, 255};
    std::cout << std::bitset<64>( key_reg( bt1, 2, 15 ) ).to_string() << std::endl;
    std::uint8_t bt2[] = {7, 128, 7};
    std::cout << std::bitset<64>( key_reg( bt2, 2, 9 ) ).to_string() << std::endl;
}

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