isqrt
Implementing integer isqrt ( integer ), the fastest way:
#include <cassert>
#include <cmath>
// Integer square root.
template<typename SizeType>
[[nodiscard]] inline constexpr SizeType isqrt ( SizeType val_ ) noexcept {
assert ( val_ > 0 );
return static_cast<SizeType> ( std::sqrt ( static_cast<double> ( val_ ) ) );
}