third-party/boost-math/include/boost/math/ccmath/logb.hpp

//  (C) Copyright Matt Borland 2021.
//  Use, modification and distribution are subject to the
//  Boost Software License, Version 1.0. (See accompanying file
//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#ifndef BOOST_MATH_CCMATH_LOGB_HPP
#define BOOST_MATH_CCMATH_LOGB_HPP

#include <boost/math/ccmath/detail/config.hpp>

#ifdef BOOST_MATH_NO_CCMATH
#error "The header <boost/math/logb.hpp> can only be used in C++17 and later."
#endif

#include <boost/math/ccmath/frexp.hpp>
#include <boost/math/ccmath/isinf.hpp>
#include <boost/math/ccmath/isnan.hpp>
#include <boost/math/ccmath/abs.hpp>

namespace boost::math::ccmath {

namespace detail {

// The value of the exponent returned by std::logb is always 1 less than the exponent returned by 
// std::frexp because of the different normalization requirements: for the exponent e returned by std::logb, 
// |arg*r^-e| is between 1 and r (typically between 1 and 2), but for the exponent e returned by std::frexp, 
// |arg*2^-e| is between 0.5 and 1. 
template <typename T>
constexpr T logb_impl(T arg) noexcept
{
    int exp = 0;
    boost::math::ccmath::frexp(arg, &exp);

    return static_cast<T>(exp - 1);
}

} // Namespace detail

template <typename Real, std::enable_if_t<!std::is_integral_v<Real>, bool> = true>
constexpr Real logb(Real arg) noexcept
{
    if(BOOST_MATH_IS_CONSTANT_EVALUATED(arg))
    {
        if (boost::math::ccmath::abs(arg) == Real(0))
        {
            return -std::numeric_limits<Real>::infinity();
        }
        else if (boost::math::ccmath::isinf(arg))
        {
            return std::numeric_limits<Real>::infinity();
        }
        else if (boost::math::ccmath::isnan(arg))
        {
            return arg;
        }
        
        return boost::math::ccmath::detail::logb_impl(arg);
    }
    else
    {
        using std::logb;
        return logb(arg);
    }
}

template <typename Z, std::enable_if_t<std::is_integral_v<Z>, bool> = true>
constexpr double logb(Z arg) noexcept
{
    return boost::math::ccmath::logb(static_cast<double>(arg));
}

constexpr float logbf(float arg) noexcept
{
    return boost::math::ccmath::logb(arg);
}

#ifndef BOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS
constexpr long double logbl(long double arg) noexcept
{
    return boost::math::ccmath::logb(arg);
}
#endif

} // Namespaces

#endif // BOOST_MATH_CCMATH_LOGB_HPP