92 lines
3.3 KiB
C++
92 lines
3.3 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::unique`.
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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_FWD_UNIQUE_HPP
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#define BOOST_HANA_FWD_UNIQUE_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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#include <boost/hana/detail/nested_by_fwd.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Removes all consecutive duplicate elements from a Sequence.
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//! @ingroup group-Sequence
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//!
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//! Given a `Sequence` and an optional binary predicate, `unique` returns
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//! a new sequence containing only the first element of every subrange
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//! of the original sequence whose elements are all equal. In other words,
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//! it turns a sequence of the form `[a, a, b, c, c, c, d, d, d, a]` into
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//! a sequence `[a, b, c, d, a]`. The equality of two elements is
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//! determined by the provided `predicate`, or by `equal` if no
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//! `predicate` is provided.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a binary predicate
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//! \f$ T \times T \to Bool \f$, `unique` has the following signature:
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//! \f[
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//! \mathtt{unique} : S(T) \times (T \times T \to Bool) \to S(T)
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//! \f]
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//!
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//! @param xs
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//! The sequence from which to remove consecutive duplicates.
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//!
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//! @param predicate
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//! A function called as `predicate(x, y)`, where `x` and `y` are adjacent
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//! elements of the sequence, and returning a `Logical` representing
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//! whether `x` and `y` should be considered equal. `predicate` should
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//! define an [equivalence relation][1] over the elements of the sequence.
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//! In the current implementation of the library, `predicate` has to
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//! return a compile-time `Logical`. This parameter is optional; it
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//! defaults to `equal` if it is not provided, which then requires the
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//! elements of the sequence to be compile-time `Comparable`.
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//!
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//!
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//! Syntactic sugar (`unique.by`)
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//! -----------------------------
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//! `unique` can be called in an alternate way, which provides a nice
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//! syntax, especially in conjunction with the `comparing` combinator:
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//! @code
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//! unique.by(predicate, xs) == unique(xs, predicate)
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//! unique.by(predicate) == unique(-, predicate)
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//! @endcode
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//!
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//! where `unique(-, predicate)` denotes the partial application of
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//! `unique` to `predicate`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/unique.cpp
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//!
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//! [1]: http://en.wikipedia.org/wiki/Equivalence_relation#Definition
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#if defined(BOOST_HANA_DOXYGEN_INVOKED)
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constexpr auto unique = [](auto&& xs[, auto&& predicate]) {
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return tag-dispatched;
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};
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#else
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template <typename S, typename = void>
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struct unique_impl : unique_impl<S, when<true>> { };
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struct unique_t : detail::nested_by<unique_t> {
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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const;
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template <typename Xs, typename Predicate>
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constexpr auto operator()(Xs&& xs, Predicate&& predicate) const;
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};
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constexpr unique_t unique{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_UNIQUE_HPP
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