#include <type_traits>

/// Integral part of log2(n), n - integral (interpreted as unsigned)
template <class _Int, class = std::enable_if_t<std::is_integral<_Int>::value>>
  constexpr int ilog2(_Int n) noexcept
    {
      struct Bits {
        constexpr auto bias() const noexcept { return 1023; }
        uint64_t m: 52;
        uint64_t e: 11;
        uint64_t s:  1;
      };

      union FRep {
        constexpr FRep(double f) noexcept : v(f) {}
        constexpr int ilog2() const noexcept
          { return b.e >= b.bias() ? int(b.e) - b.bias() : -1; }
        double v;
        Bits b;
      };

      return FRep(double(uint64_t(n))).ilog2();
    }

/// Most significant bit // Старший значащий бит
/// @return 2^i or 0, i = index of the MSB;
template <class _Int, class = std::enable_if_t<std::is_integral<_Int>::value>>
  constexpr auto msb(_Int n) noexcept
    {
      using Unsigned = std::make_unsigned_t<_Int>;
      return !n ? 0 : n == _Int(-1) ? Unsigned(-1) ^ (Unsigned(-1) >> 1)
                                    : Unsigned(1) << ilog2(n);
    }