boost/math/tools/polynomial_gcd.hpp
// (C) Copyright Jeremy William Murphy 2016. // (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_TOOLS_POLYNOMIAL_GCD_HPP #define BOOST_MATH_TOOLS_POLYNOMIAL_GCD_HPP #ifdef _MSC_VER #pragma once #endif #include <algorithm> #include <type_traits> #include <boost/math/tools/is_standalone.hpp> #include <boost/math/tools/polynomial.hpp> #ifndef BOOST_MATH_STANDALONE #include <boost/integer/common_factor_rt.hpp> // std::gcd was introduced in C++17 #elif (__cplusplus > 201700L || _MSVC_LANG > 201700L) #define BOOST_MATH_CXX17_NUMERIC #include <numeric> #include <utility> #include <iterator> #include <type_traits> #include <boost/math/tools/assert.hpp> namespace boost::integer { template <typename Iter, typename T = typename std::iterator_traits<Iter>::value_type> std::pair<T, Iter> gcd_range(Iter first, Iter last) noexcept(std::is_arithmetic_v<T>) { using std::gcd; BOOST_MATH_ASSERT(first != last); T d = *first; ++first; while (d != T(1) && first != last) { d = gcd(d, *first); ++first; } return std::make_pair(d, first); } namespace gcd_detail { template <typename EuclideanDomain> inline EuclideanDomain Euclid_gcd(EuclideanDomain a, EuclideanDomain b) noexcept(std::is_arithmetic_v<EuclideanDomain>) { using std::swap; while (b != EuclideanDomain(0)) { a %= b; swap(a, b); } return a; } enum method_type { method_euclid = 0, method_binary = 1, method_mixed = 2 }; } // namespace gcd_detail } // namespace boost::integer #endif namespace boost{ namespace integer { namespace gcd_detail { template <class T> struct gcd_traits; template <class T> struct gcd_traits<boost::math::tools::polynomial<T> > { inline static const boost::math::tools::polynomial<T>& abs(const boost::math::tools::polynomial<T>& val) { return val; } static const method_type method = method_euclid; }; } } namespace math{ namespace tools{ /* From Knuth, 4.6.1: * * We may write any nonzero polynomial u(x) from R[x] where R is a UFD as * * u(x) = cont(u) . pp(u(x)) * * where cont(u), the content of u, is an element of S, and pp(u(x)), the primitive * part of u(x), is a primitive polynomial over S. * When u(x) = 0, it is convenient to define cont(u) = pp(u(x)) = O. */ template <class T> T content(polynomial<T> const &x) { #if defined(BOOST_MATH_STANDALONE) && !defined(BOOST_MATH_CXX17_NUMERIC) static_assert(sizeof(T) == 0, "polynomial gcd can only be used in standalone mode with C++17 or higher"); #endif return x ? boost::integer::gcd_range(x.data().begin(), x.data().end()).first : T(0); } // Knuth, 4.6.1 template <class T> polynomial<T> primitive_part(polynomial<T> const &x, T const &cont) { return x ? x / cont : polynomial<T>(); } template <class T> polynomial<T> primitive_part(polynomial<T> const &x) { return primitive_part(x, content(x)); } // Trivial but useful convenience function referred to simply as l() in Knuth. template <class T> T leading_coefficient(polynomial<T> const &x) { return x ? x.data().back() : T(0); } namespace detail { /* Reduce u and v to their primitive parts and return the gcd of their * contents. Used in a couple of gcd algorithms. */ template <class T> T reduce_to_primitive(polynomial<T> &u, polynomial<T> &v) { #ifndef BOOST_MATH_STANDALONE using boost::integer::gcd; #elif defined(BOOST_MATH_CXX17_NUMERIC) using std::gcd; #else static_assert(sizeof(T) == 0, "polynomial gcd can only be used in standalone mode with C++17 or higher"); #endif T const u_cont = content(u), v_cont = content(v); u /= u_cont; v /= v_cont; return gcd(u_cont, v_cont); } } /** * Knuth, The Art of Computer Programming: Volume 2, Third edition, 1998 * Algorithm 4.6.1C: Greatest common divisor over a unique factorization domain. * * The subresultant algorithm by George E. Collins [JACM 14 (1967), 128-142], * later improved by W. S. Brown and J. F. Traub [JACM 18 (1971), 505-514]. * * Although step C3 keeps the coefficients to a "reasonable" size, they are * still potentially several binary orders of magnitude larger than the inputs. * Thus, this algorithm should only be used where T is a multi-precision type. * * @tparam T Polynomial coefficient type. * @param u First polynomial. * @param v Second polynomial. * @return Greatest common divisor of polynomials u and v. */ template <class T> typename std::enable_if< std::numeric_limits<T>::is_integer, polynomial<T> >::type subresultant_gcd(polynomial<T> u, polynomial<T> v) { using std::swap; BOOST_MATH_ASSERT(u || v); if (!u) return v; if (!v) return u; typedef typename polynomial<T>::size_type N; if (u.degree() < v.degree()) swap(u, v); T const d = detail::reduce_to_primitive(u, v); T g = 1, h = 1; polynomial<T> r; while (true) { BOOST_MATH_ASSERT(u.degree() >= v.degree()); // Pseudo-division. r = u % v; if (!r) return d * primitive_part(v); // Attach the content. if (r.degree() == 0) return d * polynomial<T>(T(1)); // The content is the result. N const delta = u.degree() - v.degree(); // Adjust remainder. u = v; v = r / (g * detail::integer_power(h, delta)); g = leading_coefficient(u); T const tmp = detail::integer_power(g, delta); if (delta <= N(1)) h = tmp * detail::integer_power(h, N(1) - delta); else h = tmp / detail::integer_power(h, delta - N(1)); } } /** * @brief GCD for polynomials with unbounded multi-precision integral coefficients. * * The multi-precision constraint is enforced via numeric_limits. * * Note that intermediate terms in the evaluation can grow arbitrarily large, hence the need for * unbounded integers, otherwise numeric loverflow would break the algorithm. * * @tparam T A multi-precision integral type. */ template <typename T> typename std::enable_if<std::numeric_limits<T>::is_integer && !std::numeric_limits<T>::is_bounded, polynomial<T> >::type gcd(polynomial<T> const &u, polynomial<T> const &v) { return subresultant_gcd(u, v); } // GCD over bounded integers is not currently allowed: template <typename T> typename std::enable_if<std::numeric_limits<T>::is_integer && std::numeric_limits<T>::is_bounded, polynomial<T> >::type gcd(polynomial<T> const &u, polynomial<T> const &v) { static_assert(sizeof(v) == 0, "GCD on polynomials of bounded integers is disallowed due to the excessive growth in the size of intermediate terms."); return subresultant_gcd(u, v); } // GCD over polynomials of floats can go via the Euclid algorithm: template <typename T> typename std::enable_if<!std::numeric_limits<T>::is_integer && (std::numeric_limits<T>::min_exponent != std::numeric_limits<T>::max_exponent) && !std::numeric_limits<T>::is_exact, polynomial<T> >::type gcd(polynomial<T> const &u, polynomial<T> const &v) { #if defined(BOOST_MATH_STANDALONE) && !defined(BOOST_MATH_CXX17_NUMERIC) static_assert(sizeof(T) == 0, "polynomial gcd can only be used in standalone mode with C++17 or higher"); #endif return boost::integer::gcd_detail::Euclid_gcd(u, v); } } // // Using declaration so we overload the default implementation in this namespace: // using boost::math::tools::gcd; } namespace integer { // // Using declaration so we overload the default implementation in this namespace: // using boost::math::tools::gcd; } } // namespace boost::math::tools #endif