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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::permutations`.
Copyright Louis Dionne 2013-2022
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>
namespace boost { namespace hana {
//! 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;
};
BOOST_HANA_INLINE_VARIABLE constexpr permutations_t permutations{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_PERMUTATIONS_HPP
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