Added thirdparty: boost library

thirdparty/include/boost/math/special_functions/rsqrt.hpp

@@ -0,0 +1,52 @@
+//  (C) Copyright Nick Thompson 2020.
+//  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_SPECIAL_FUNCTIONS_RSQRT_HPP
+#define BOOST_MATH_SPECIAL_FUNCTIONS_RSQRT_HPP
+#include <cmath>
+#include <type_traits>
+#include <limits>
+
+#include <boost/math/tools/is_standalone.hpp>
+#ifndef BOOST_MATH_STANDALONE
+#  include <boost/config.hpp>
+#  ifdef BOOST_NO_CXX17_IF_CONSTEXPR
+#    error "The header <boost/math/rqrt.hpp> can only be used in C++17 and later."
+#  endif
+#endif
+
+namespace boost::math {
+
+template<typename Real>
+inline Real rsqrt(Real const & x)
+{
+    using std::sqrt;
+    if constexpr (std::is_arithmetic_v<Real> && !std::is_integral_v<Real>)
+    {
+        return 1/sqrt(x);
+    }
+    else
+    {
+        // if it's so tiny it rounds to 0 as long double,
+        // no performance gains are possible:
+        if (x < std::numeric_limits<long double>::denorm_min() || x > (std::numeric_limits<long double>::max)()) {
+            return 1/sqrt(x);
+        }
+        Real x0 = 1/sqrt(static_cast<long double>(x));
+        // Divide by 512 for leeway:
+        Real s = sqrt(std::numeric_limits<Real>::epsilon())*x0/512;
+        Real x1 = x0 + x0*(1-x*x0*x0)/2;
+        while(abs(x1 - x0) > s) {
+            x0 = x1;
+            x1 = x0 + x0*(1-x*x0*x0)/2;
+        }
+        // Final iteration get ~2ULPs:
+        return  x1 + x1*(1-x*x1*x1)/2;;
+    }
+}
+
+
+}
+#endif