45 lines
1.3 KiB
C++
45 lines
1.3 KiB
C++
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// (C) Copyright Nick Thompson 2020.
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// Use, modification and distribution are subject to the
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// Boost Software License, Version 1.0. (See accompanying file
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// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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#ifndef BOOST_MATH_SPECIAL_FUNCTIONS_RSQRT_HPP
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#define BOOST_MATH_SPECIAL_FUNCTIONS_RSQRT_HPP
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#include <cmath>
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#include <type_traits>
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#include <limits>
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namespace boost::math {
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template<typename Real>
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inline Real rsqrt(Real const & x)
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{
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using std::sqrt;
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if constexpr (std::is_same_v<Real, float> || std::is_same_v<Real, double> || std::is_same_v<Real, long double>)
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{
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return 1/sqrt(x);
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}
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else
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{
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// if it's so tiny it rounds to 0 as long double,
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// no performance gains are possible:
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if (x < std::numeric_limits<long double>::denorm_min() || x > (std::numeric_limits<long double>::max)()) {
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return 1/sqrt(x);
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}
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Real x0 = 1/sqrt(static_cast<long double>(x));
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// Divide by 512 for leeway:
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Real s = sqrt(std::numeric_limits<Real>::epsilon())*x0/512;
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Real x1 = x0 + x0*(1-x*x0*x0)/2;
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while(abs(x1 - x0) > s) {
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x0 = x1;
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x1 = x0 + x0*(1-x*x0*x0)/2;
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}
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// Final iteration get ~2ULPs:
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return x1 + x1*(1-x*x1*x1)/2;;
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}
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}
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}
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#endif
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