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Showing content from https://en.cppreference.com/w/cpp/language/../keywords/../numeric/bit_floor.html below:

std::bit_floor - cppreference.com

template< class T > constexpr T bit_floor( T x ) noexcept;

(since C++20)

If x is not zero, calculates the largest integral power of two that is not greater than x. If x is zero, returns zero.

This overload participates in overload resolution only if T is an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type).

[edit] Parameters x - unsigned integer value [edit] Return value

Zero if x is zero; otherwise, the largest integral power of two that is not greater than x.

[edit] Notes

Prior to P1956R1, the proposed name for this function template was floor2.

[edit] Possible implementation

template<typename T, typename ... U>
concept neither = (!std::same_as<T, U> && ...);
 
template<std::unsigned_integral T>
    requires neither<T, bool, char, char8_t, char16_t, char32_t, wchar_t>
constexpr T bit_floor(T x) noexcept
{
    if (x != 0)
        return T{1} << (std::bit_width(x) - 1);
    return 0;
}

[edit] Example

#include <bit>
#include <bitset>
#include <iostream>
 
int main()
{
    using bin = std::bitset<8>;
    for (unsigned x{}; x != 012; ++x)
        std::cout << "bit_floor( " << bin(x) << " ) = "
                  << bin(std::bit_floor(x)) << '\n';
}

Output:

bit_floor( 00000000 ) = 00000000
bit_floor( 00000001 ) = 00000001
bit_floor( 00000010 ) = 00000010
bit_floor( 00000011 ) = 00000010
bit_floor( 00000100 ) = 00000100
bit_floor( 00000101 ) = 00000100
bit_floor( 00000110 ) = 00000100
bit_floor( 00000111 ) = 00000100
bit_floor( 00001000 ) = 00001000
bit_floor( 00001001 ) = 00001000

[edit] See also finds the smallest integral power of 2 not less than the given value
(function template) [edit] computes the result of bitwise right-rotation
(function template) [edit] finds the smallest number of bits needed to represent the given value
(function template) [edit] checks if a number is an integral power of 2
(function template) [edit]

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