sum_of_bits.hpp

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#pragma once    // Source encoding: UTF-8 with BOM (π is a lowercase Greek "pi").

#include <cppx-core/language/syntax/macro-use.hpp>          // CPPX_USE_CPPX
#include <cppx-core/language/bit-level/bits_per_.hpp>       // cppx::bits_per_

#include <bitset>           // std::bitset
#include <type_traits>      // std::is_unsigned_v

namespace cppx
{
    using std::bitset;

    template< int n >
    inline auto sum_of_bits( const bitset<n>& bits ) noexcept
        -> int
    { return bits.count(); }

    template< class Unsigned >
    inline auto sum_of_bits( const uint16_t x ) noexcept
        -> int
    {
        static_assert( std::is_unsigned_v<Unsigned> );
        return bitset<bits_per_<Unsigned>>( x ).count();
    }

#if 0  // Possible optimizations, probably not needed
#if defined( _MSC_VER )

    inline auto sum_of_bits( const uint16_t x ) noexcept
        -> int
    { return __popcnt16( x ); }

    inline auto sum_of_bits( const uint32_t x ) noexcept
        -> int
    { return __popcnt( x ); }

    inline auto sum_of_bits( const uint64_t x ) noexcept
        -> int
    { return __popcnt64( x ); }

#elif defined( __GNUC__ )

    namespace impl
    {
        template< class Unsigned >
        inline auto gcc_sum_of_bits(  const Unsigned x ) noexcept -> int;

        template<> inline auto gcc_sum_of_bits<unsigned>( const unsigned x ) noexcept
            -> int
        { return  __builtin_popcount( x ); }

        template<> inline auto gcc_sum_of_bits<unsigned long>( const unsigned long x ) noexcept
            -> int
        { return  __builtin_popcountl( x ); }

        template<> inline auto gcc_sum_of_bits<unsigned long long>( const unsigned long long x ) noexcept
            -> int
        { return  __builtin_popcountll( x ); }
    }  // namespace impl

    inline auto sum_of_bits( const uint16_t x ) noexcept
        -> int
    { return impl::gcc_sum_of_bits<uint32_t>( x ); }

    inline auto sum_of_bits( const uint32_t x ) noexcept
        -> int
    { return impl::gcc_sum_of_bits( x ); }

    inline auto sum_of_bits( const uint64_t x ) noexcept
        -> int
    { return impl::gcc_sum_of_bits( x ); }
#endif
#endif // 0

    namespace bitlevel
    {
        CPPX_USE_CPPX( sum_of_bits );
    }  // namespace bitlevel
}  // namespace cppx