90 lines
2.8 KiB
C
90 lines
2.8 KiB
C
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// Copyright (c) 2002
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// Utrecht University (The Netherlands),
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// ETH Zurich (Switzerland),
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// INRIA Sophia-Antipolis (France),
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// Max-Planck-Institute Saarbruecken (Germany),
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// and Tel-Aviv University (Israel). All rights reserved.
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//
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// This file is part of CGAL (www.cgal.org)
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//
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// $URL: https://github.com/CGAL/cgal/blob/v5.1/Number_types/include/CGAL/to_rational.h $
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// $Id: to_rational.h 0779373 2020-03-26T13:31:46+01:00 Sébastien Loriot
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// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-Commercial
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//
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//
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// Author(s) : Andreas Fabri, Susan Hert, Sylvain Pion
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#ifndef CGAL_TO_RATIONAL_H
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#define CGAL_TO_RATIONAL_H
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#include <CGAL/number_type_basic.h>
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#include <CGAL/Fraction_traits.h>
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#include <cmath>
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namespace CGAL {
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template <class Rational>
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Rational
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to_rational(double x)
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{
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typedef Fraction_traits<Rational> FT;
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typedef typename FT::Is_fraction Is_fraction;
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typedef typename FT::Numerator_type Numerator_type;
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typedef typename FT::Denominator_type Denominator_type;
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typename FT::Compose compose;
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CGAL_static_assertion((::boost::is_same<Is_fraction,Tag_true>::value));
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CGAL_USE_TYPE(Is_fraction);
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CGAL_static_assertion((::boost::is_same<Numerator_type,Denominator_type>::value));
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CGAL_USE_TYPE(Denominator_type);
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Numerator_type num(0),den(1);
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if (x != 0.0)
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{ bool neg = (x < 0);
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if (neg) x = -x;
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const unsigned shift = 15; // a safe shift per step
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const int shift_pow = 32768; // = 2^shift
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const double width = 32768; // = 2^shift
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const int maxiter = 20; // ought not be necessary, but just in
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// case, max 300 bits of precision
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int expt;
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double mantissa = std::frexp(x, &expt);
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long exponent = expt;
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double intpart;
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int k = 0;
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while (mantissa != 0.0 && k++ < maxiter)
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{
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mantissa *= width; // shift double mantissa
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mantissa = std::modf(mantissa, &intpart);
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num *= shift_pow;
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num += (int)intpart;
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exponent -= shift;
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}
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int expsign = (exponent>0 ? +1 : (exponent<0 ? -1 : 0));
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exponent *= expsign;
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Numerator_type twopot(2);
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Numerator_type exppot(1);
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while (exponent!=0) {
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if (exponent & 1)
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exppot *= twopot;
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exponent >>= 1;
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twopot *= twopot;
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}
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if (expsign > 0)
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num *= exppot;
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else if (expsign < 0)
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den *= exppot;
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if (neg)
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num = -num;
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}
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return compose(num,den);
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}
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} //namespace CGAL
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#endif // CGAL_TO_RATIONAL_H
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