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- /*!
- @file
- Forward declares `boost::hana::permutations`.
- @copyright Louis Dionne 2013-2017
- 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_FWD_PERMUTATIONS_HPP
- #define BOOST_HANA_FWD_PERMUTATIONS_HPP
- #include <boost/hana/config.hpp>
- #include <boost/hana/core/when.hpp>
- BOOST_HANA_NAMESPACE_BEGIN
- //! Return a sequence of all the permutations of the given sequence.
- //! @ingroup group-Sequence
- //!
- //! Specifically, `permutations(xs)` is a sequence whose elements are
- //! permutations of the original sequence `xs`. The permutations are not
- //! guaranteed to be in any specific order. Also note that the number
- //! of permutations grows very rapidly as the length of the original
- //! sequence increases. The growth rate is `O(length(xs)!)`; with a
- //! sequence `xs` of length only 8, `permutations(xs)` contains over
- //! 40 000 elements!
- //!
- //!
- //! Example
- //! -------
- //! @include example/permutations.cpp
- #ifdef BOOST_HANA_DOXYGEN_INVOKED
- constexpr auto permutations = [](auto&& xs) {
- return tag-dispatched;
- };
- #else
- template <typename S, typename = void>
- struct permutations_impl : permutations_impl<S, when<true>> { };
- struct permutations_t {
- template <typename Xs>
- constexpr auto operator()(Xs&& xs) const;
- };
- constexpr permutations_t permutations{};
- #endif
- BOOST_HANA_NAMESPACE_END
- #endif // !BOOST_HANA_FWD_PERMUTATIONS_HPP
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