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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::sort`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
6 | Distributed under the Boost Software License, Version 1.0. | |
7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
8 | */ | |
9 | ||
10 | #ifndef BOOST_HANA_FWD_SORT_HPP | |
11 | #define BOOST_HANA_FWD_SORT_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | #include <boost/hana/detail/nested_by_fwd.hpp> | |
16 | ||
17 | ||
18 | BOOST_HANA_NAMESPACE_BEGIN | |
19 | //! Sort a sequence, optionally based on a custom `predicate`. | |
20 | //! @ingroup group-Sequence | |
21 | //! | |
22 | //! Given a Sequence and an optional predicate (by default `less`), `sort` | |
23 | //! returns a new sequence containing the same elements as the original, | |
24 | //! except they are ordered in such a way that if `x` comes before `y` in | |
25 | //! the sequence, then either `predicate(x, y)` is true, or both | |
26 | //! `predicate(x, y)` and `predicate(y, x)` are false. | |
27 | //! | |
28 | //! Also note that the sort is guaranteed to be stable. Hence, if `x` | |
29 | //! comes before `y` in the original sequence and both `predicate(x, y)` | |
30 | //! and `predicate(y, x)` are false, then `x` will come before `y` in the | |
31 | //! resulting sequence. | |
32 | //! | |
33 | //! If no predicate is provided, the elements in the sequence must all be | |
34 | //! compile-time `Orderable`. | |
35 | //! | |
36 | //! Signature | |
37 | //! --------- | |
38 | //! Given a `Sequence` `S(T)`, a boolean `IntegralConstant` `Bool` and a | |
39 | //! binary predicate \f$ T \times T \to Bool \f$, `sort` has the following | |
40 | //! signatures. For the variant with a provided predicate, | |
41 | //! \f[ | |
42 | //! \mathtt{sort} : S(T) \times (T \times T \to Bool) \to S(T) | |
43 | //! \f] | |
44 | //! | |
45 | //! for the variant without a custom predicate, `T` is required to be | |
46 | //! `Orderable`. The signature is then | |
47 | //! \f[ | |
48 | //! \mathtt{sort} : S(T) \to S(T) | |
49 | //! \f] | |
50 | //! | |
51 | //! @param xs | |
52 | //! The sequence to sort. | |
53 | //! | |
54 | //! @param predicate | |
55 | //! A function called as `predicate(x, y)` for two elements `x` and `y` of | |
56 | //! the sequence, and returning a boolean `IntegralConstant` representing | |
57 | //! whether `x` is to be considered _less_ than `y`, i.e. whether `x` should | |
58 | //! appear _before_ `y` in the resulting sequence. More specifically, | |
59 | //! `predicate` must define a [strict weak ordering][1] on the elements | |
60 | //! of the sequence. When the predicate is not specified, this defaults | |
61 | //! to `less`. In the current version of the library, the predicate has | |
62 | //! to return an `IntegralConstant` holding a value convertible to a `bool`. | |
63 | //! | |
64 | //! | |
65 | //! Syntactic sugar (`sort.by`) | |
66 | //! --------------------------- | |
67 | //! `sort` can be called in a third way, which provides a nice syntax | |
68 | //! especially when working with the `ordering` combinator: | |
69 | //! @code | |
70 | //! sort.by(predicate, xs) == sort(xs, predicate) | |
71 | //! sort.by(predicate) == sort(-, predicate) | |
72 | //! @endcode | |
73 | //! | |
74 | //! where `sort(-, predicate)` denotes the partial application of | |
75 | //! `sort` to `predicate`. | |
76 | //! | |
77 | //! | |
78 | //! Example | |
79 | //! ------- | |
80 | //! @include example/sort.cpp | |
81 | //! | |
82 | //! [1]: http://en.wikipedia.org/wiki/Strict_weak_ordering | |
83 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
84 | constexpr auto sort = [](auto&& xs[, auto&& predicate]) { | |
85 | return tag-dispatched; | |
86 | }; | |
87 | #else | |
88 | template <typename S, typename = void> | |
89 | struct sort_impl : sort_impl<S, when<true>> { }; | |
90 | ||
91 | struct sort_t : detail::nested_by<sort_t> { | |
92 | template <typename Xs> | |
93 | constexpr auto operator()(Xs&& xs) const; | |
94 | ||
95 | template <typename Xs, typename Predicate> | |
96 | constexpr auto operator()(Xs&& xs, Predicate&& pred) const; | |
97 | }; | |
98 | ||
99 | constexpr sort_t sort{}; | |
100 | #endif | |
101 | BOOST_HANA_NAMESPACE_END | |
102 | ||
103 | #endif // !BOOST_HANA_FWD_SORT_HPP |