66 lines
2.3 KiB
C++
66 lines
2.3 KiB
C++
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/*!
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@file
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Defines `boost::hana::detail::has_duplicates`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_DETAIL_HAS_DUPLICATES_HPP
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#define BOOST_HANA_DETAIL_HAS_DUPLICATES_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/detail/fast_and.hpp>
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#include <boost/hana/equal.hpp>
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#include <cstddef>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN namespace detail {
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template <typename T, typename ...U>
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constexpr std::size_t pack_count() {
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std::size_t c = 0;
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std::size_t expand[] = {0, // avoid empty array
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(decltype(hana::equal(std::declval<T>(), std::declval<U>()))::value
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? ++c
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: c)...
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};
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(void)expand;
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return c;
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}
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//! @ingroup group-details
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//! Returns whether any of the `T`s are duplicate w.r.t. `hana::equal`.
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//!
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//! In particular, this does not check whether all of the `T`s are unique
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//! as _types_, but rather whether they are unique when compared as
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//! `hana::equal(std::declval<T>(), std::declval<U>())`. This assumes
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//! the comparison to return an `IntegralConstant` that can be explicitly
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//! converted to `bool`.
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//!
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//! @note
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//! Since this utility is mostly used in assertions to check that there
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//! are no duplicates in a sequence, we expect it to return `false` most
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//! of the time (otherwise we will assert). Hence, this implementation is
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//! biased towards the fact that we __will__ have to compare every pair of
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//! elements in most cases, and it does not try to be lazy.
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//!
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//! @todo
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//! This implementation is O(n^2). We could do it in O(n), but that would
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//! require a more elaborate setup including storage with O(1) lookup
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//! (which could be based on a compile-time hash). If we implement such
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//! storage for associative sequences, we could use it to optimize this.
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template <typename ...T>
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struct has_duplicates {
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static constexpr bool value =
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sizeof...(T) > 0 &&
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!detail::fast_and<(detail::pack_count<T, T...>() == 1)...>::value
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;
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};
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} BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_DETAIL_HAS_DUPLICATES_HPP
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