Best practices for circular shift (rotate) operations in C++

Question

Left and right shift operators (<< and >>) are already available in C++. However, I couldn\'t find out how I could perform circular shift or rotate operations.

Answer 1

C++20 std::rotl and std::rotr

It has arrived! http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0553r4.html and should add it to the header.

cppreference says that the usage will be like:

#include 
#include 
#include 
#include 

int main()
{
    std::uint8_t i = 0b00011101;
    std::cout << "i          = " << std::bitset<8>(i) << '\n';
    std::cout << "rotl(i,0)  = " << std::bitset<8>(std::rotl(i,0)) << '\n';
    std::cout << "rotl(i,1)  = " << std::bitset<8>(std::rotl(i,1)) << '\n';
    std::cout << "rotl(i,4)  = " << std::bitset<8>(std::rotl(i,4)) << '\n';
    std::cout << "rotl(i,9)  = " << std::bitset<8>(std::rotl(i,9)) << '\n';
    std::cout << "rotl(i,-1) = " << std::bitset<8>(std::rotl(i,-1)) << '\n';
}

giving output:

i          = 00011101
rotl(i,0)  = 00011101
rotl(i,1)  = 00111010
rotl(i,4)  = 11010001
rotl(i,9)  = 00111010
rotl(i,-1) = 10001110

I'll give it a try when support arrives to GCC, GCC 9.1.0 with g++-9 -std=c++2a still doesn't support it.

The proposal says:

Header:

namespace std {
  // 25.5.5, rotating   
  template
    [[nodiscard]] constexpr T rotl(T x, int s) noexcept;
  template
    [[nodiscard]] constexpr T rotr(T x, int s) noexcept;

and:

25.5.5 Rotating [bitops.rot]

In the following descriptions, let N denote std::numeric_limits::digits.

template
  [[nodiscard]] constexpr T rotl(T x, int s) noexcept;

Constraints: T is an unsigned integer type (3.9.1 [basic.fundamental]).

Let r be s % N.

Returns: If r is 0, x; if r is positive, (x << r) | (x >> (N - r)); if r is negative, rotr(x, -r).

template
  [[nodiscard]] constexpr T rotr(T x, int s) noexcept;

Constraints: T is an unsigned integer type (3.9.1 [basic.fundamental]). Let r be s % N.

Returns: If r is 0, x; if r is positive, (x >> r) | (x << (N - r)); if r is negative, rotl(x, -r).

A std::popcount was also added to count the number of 1 bits: How to count the number of set bits in a 32-bit integer?