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+/*!
+@file
+Defines `boost::hana::one`.
+
+@copyright Louis Dionne 2013-2016
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_ONE_HPP
+#define BOOST_HANA_ONE_HPP
+
+#include <boost/hana/fwd/one.hpp>
+
+#include <boost/hana/concept/constant.hpp>
+#include <boost/hana/concept/ring.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/to.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/detail/canonical_constant.hpp>
+
+#include <type_traits>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ template <typename R>
+ struct one_t {
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Ring<R>::value,
+ "hana::one<R>() requires 'R' to be a Ring");
+ #endif
+
+ constexpr decltype(auto) operator()() const {
+ using One = BOOST_HANA_DISPATCH_IF(one_impl<R>,
+ hana::Ring<R>::value
+ );
+
+ return One::apply();
+ }
+ };
+
+ template <typename R, bool condition>
+ struct one_impl<R, when<condition>> : default_ {
+ template <typename ...Args>
+ static constexpr auto apply(Args&& ...) = delete;
+ };
+
+ //////////////////////////////////////////////////////////////////////////
+ // Model for non-boolean arithmetic data types
+ //////////////////////////////////////////////////////////////////////////
+ template <typename T>
+ struct one_impl<T, when<std::is_arithmetic<T>::value &&
+ !std::is_same<bool, T>::value>> {
+ static constexpr T apply()
+ { return static_cast<T>(1); }
+ };
+
+ //////////////////////////////////////////////////////////////////////////
+ // Model for Constants over a Ring
+ //////////////////////////////////////////////////////////////////////////
+ namespace detail {
+ template <typename C>
+ struct constant_from_one {
+ static constexpr auto value = hana::one<typename C::value_type>();
+ using hana_tag = detail::CanonicalConstant<typename C::value_type>;
+ };
+ }
+
+ template <typename C>
+ struct one_impl<C, when<
+ hana::Constant<C>::value &&
+ Ring<typename C::value_type>::value
+ >> {
+ static constexpr decltype(auto) apply()
+ { return hana::to<C>(detail::constant_from_one<C>{}); }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_ONE_HPP