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1 | /* |
2 | @file | |
3 | Defines experimental views. | |
4 | ||
b32b8144 | 5 | @copyright Louis Dionne 2013-2017 |
7c673cae FG |
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_EXPERIMENTAL_VIEW_HPP | |
11 | #define BOOST_HANA_EXPERIMENTAL_VIEW_HPP | |
12 | ||
13 | #include <boost/hana/and.hpp> | |
14 | #include <boost/hana/at.hpp> | |
15 | #include <boost/hana/bool.hpp> | |
16 | #include <boost/hana/detail/decay.hpp> | |
17 | #include <boost/hana/fold_left.hpp> | |
18 | #include <boost/hana/functional/compose.hpp> | |
b32b8144 | 19 | #include <boost/hana/functional/on.hpp> |
7c673cae FG |
20 | #include <boost/hana/fwd/ap.hpp> |
21 | #include <boost/hana/fwd/concat.hpp> | |
22 | #include <boost/hana/fwd/drop_front.hpp> | |
23 | #include <boost/hana/fwd/empty.hpp> | |
24 | #include <boost/hana/fwd/equal.hpp> | |
25 | #include <boost/hana/fwd/flatten.hpp> | |
26 | #include <boost/hana/fwd/is_empty.hpp> | |
27 | #include <boost/hana/fwd/less.hpp> | |
28 | #include <boost/hana/fwd/lift.hpp> | |
29 | #include <boost/hana/fwd/transform.hpp> | |
30 | #include <boost/hana/integral_constant.hpp> | |
31 | #include <boost/hana/length.hpp> | |
32 | #include <boost/hana/lexicographical_compare.hpp> | |
33 | #include <boost/hana/range.hpp> | |
34 | #include <boost/hana/tuple.hpp> | |
35 | #include <boost/hana/unpack.hpp> | |
36 | ||
37 | #include <cstddef> | |
38 | #include <type_traits> | |
39 | #include <utility> | |
40 | ||
41 | ||
42 | // Pros of views | |
43 | // - No temporary container created between algorithms | |
44 | // - Lazy, so only the minimum is required | |
45 | // | |
46 | // Cons of views | |
47 | // - Reference semantics mean possibility for dangling references | |
48 | // - Lose the ability to move from temporary containers | |
49 | // - When fetching the members of a view multiple times, no caching is done. | |
50 | // So for example, `t = transform(xs, f); at_c<0>(t); at_c<0>(t)` will | |
51 | // compute `f(at_c<0>(xs))` twice. | |
52 | // - push_back creates a joint_view and a single_view. The single_view holds | |
53 | // the value as a member. When doing multiple push_backs, we end up with a | |
54 | // joint_view<xxx, joint_view<single_view<T>, joint_view<single_view<T>, ....>>> | |
55 | // which contains a reference to `xxx` and all the `T`s by value. Such a | |
56 | // "view" is not cheap to copy, which is inconsistent with the usual | |
57 | // expectations about views. | |
58 | ||
1e59de90 | 59 | namespace boost { namespace hana { |
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60 | |
61 | namespace experimental { | |
62 | struct view_tag; | |
63 | ||
64 | namespace detail { | |
65 | template <typename Sequence> | |
66 | struct is_view { | |
67 | static constexpr bool value = false; | |
68 | }; | |
69 | ||
70 | template <typename Sequence> | |
71 | using view_storage = typename std::conditional< | |
72 | detail::is_view<Sequence>::value, Sequence, Sequence& | |
73 | >::type; | |
74 | } | |
75 | ||
76 | ////////////////////////////////////////////////////////////////////////// | |
77 | // sliced_view | |
78 | ////////////////////////////////////////////////////////////////////////// | |
79 | template <typename Sequence, std::size_t ...indices> | |
80 | struct sliced_view_t { | |
81 | detail::view_storage<Sequence> sequence_; | |
82 | using hana_tag = view_tag; | |
83 | }; | |
84 | ||
85 | template <typename Sequence, typename Indices> | |
86 | constexpr auto sliced(Sequence& sequence, Indices const& indices) { | |
87 | return hana::unpack(indices, [&](auto ...i) { | |
88 | return sliced_view_t<Sequence, decltype(i)::value...>{sequence}; | |
89 | }); | |
90 | } | |
91 | ||
92 | namespace detail { | |
93 | template <typename Sequence, std::size_t ...i> | |
94 | struct is_view<sliced_view_t<Sequence, i...>> { | |
95 | static constexpr bool value = true; | |
96 | }; | |
97 | } | |
98 | ||
99 | ////////////////////////////////////////////////////////////////////////// | |
100 | // transformed_view | |
101 | ////////////////////////////////////////////////////////////////////////// | |
102 | template <typename Sequence, typename F> | |
103 | struct transformed_view_t { | |
104 | detail::view_storage<Sequence> sequence_; | |
105 | F f_; | |
106 | using hana_tag = view_tag; | |
107 | }; | |
108 | ||
109 | template <typename Sequence, typename F> | |
110 | constexpr transformed_view_t<Sequence, typename hana::detail::decay<F>::type> | |
111 | transformed(Sequence& sequence, F&& f) { | |
112 | return {sequence, static_cast<F&&>(f)}; | |
113 | } | |
114 | ||
115 | namespace detail { | |
116 | template <typename Sequence, typename F> | |
117 | struct is_view<transformed_view_t<Sequence, F>> { | |
118 | static constexpr bool value = true; | |
119 | }; | |
120 | } | |
121 | ||
122 | ////////////////////////////////////////////////////////////////////////// | |
123 | // filtered_view | |
124 | ////////////////////////////////////////////////////////////////////////// | |
125 | #if 0 | |
126 | template <typename Sequence, typename Pred> | |
127 | using filtered_view_t = sliced_view_t<Sequence, detail::filtered_indices<...>>; | |
128 | ||
129 | template <typename Sequence, typename Pred> | |
130 | constexpr filtered_view_t<Sequence, Pred> filtered(Sequence& sequence, Pred&& pred) { | |
131 | return {sequence}; | |
132 | } | |
133 | #endif | |
134 | ||
135 | ////////////////////////////////////////////////////////////////////////// | |
136 | // joined_view | |
137 | ////////////////////////////////////////////////////////////////////////// | |
138 | template <typename Sequence1, typename Sequence2> | |
139 | struct joined_view_t { | |
140 | detail::view_storage<Sequence1> sequence1_; | |
141 | detail::view_storage<Sequence2> sequence2_; | |
142 | using hana_tag = view_tag; | |
143 | }; | |
144 | ||
145 | struct make_joined_view_t { | |
146 | template <typename Sequence1, typename Sequence2> | |
147 | constexpr joined_view_t<Sequence1, Sequence2> operator()(Sequence1& s1, Sequence2& s2) const { | |
148 | return {s1, s2}; | |
149 | } | |
150 | }; | |
1e59de90 | 151 | BOOST_HANA_INLINE_VARIABLE constexpr make_joined_view_t joined{}; |
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152 | |
153 | namespace detail { | |
154 | template <typename Sequence1, typename Sequence2> | |
155 | struct is_view<joined_view_t<Sequence1, Sequence2>> { | |
156 | static constexpr bool value = true; | |
157 | }; | |
158 | } | |
159 | ||
160 | ////////////////////////////////////////////////////////////////////////// | |
161 | // single_view | |
162 | ////////////////////////////////////////////////////////////////////////// | |
163 | template <typename T> | |
164 | struct single_view_t { | |
165 | T value_; | |
166 | using hana_tag = view_tag; | |
167 | }; | |
168 | ||
169 | template <typename T> | |
170 | constexpr single_view_t<typename hana::detail::decay<T>::type> single_view(T&& t) { | |
171 | return {static_cast<T&&>(t)}; | |
172 | } | |
173 | ||
174 | namespace detail { | |
175 | template <typename T> | |
176 | struct is_view<single_view_t<T>> { | |
177 | static constexpr bool value = true; | |
178 | }; | |
179 | } | |
180 | ||
181 | ////////////////////////////////////////////////////////////////////////// | |
182 | // empty_view | |
183 | ////////////////////////////////////////////////////////////////////////// | |
184 | struct empty_view_t { | |
185 | using hana_tag = view_tag; | |
186 | }; | |
187 | ||
188 | constexpr empty_view_t empty_view() { | |
189 | return {}; | |
190 | } | |
191 | ||
192 | namespace detail { | |
193 | template <> | |
194 | struct is_view<empty_view_t> { | |
195 | static constexpr bool value = true; | |
196 | }; | |
197 | } | |
198 | } // end namespace experimental | |
199 | ||
200 | ////////////////////////////////////////////////////////////////////////// | |
201 | // Foldable | |
202 | ////////////////////////////////////////////////////////////////////////// | |
203 | template <> | |
204 | struct unpack_impl<experimental::view_tag> { | |
205 | // sliced_view | |
206 | template <typename Sequence, std::size_t ...i, typename F> | |
207 | static constexpr decltype(auto) | |
208 | apply(experimental::sliced_view_t<Sequence, i...> view, F&& f) { | |
209 | (void)view; // Remove spurious unused variable warning with GCC | |
210 | return static_cast<F&&>(f)(hana::at_c<i>(view.sequence_)...); | |
211 | } | |
212 | ||
213 | // transformed_view | |
214 | template <typename Sequence, typename F, typename G> | |
215 | static constexpr decltype(auto) | |
216 | apply(experimental::transformed_view_t<Sequence, F> view, G&& g) { | |
217 | return hana::unpack(view.sequence_, hana::on(static_cast<G&&>(g), view.f_)); | |
218 | } | |
219 | ||
220 | // joined_view | |
221 | template <typename View, typename F, std::size_t ...i1, std::size_t ...i2> | |
222 | static constexpr decltype(auto) | |
223 | unpack_joined(View view, F&& f, std::index_sequence<i1...>, | |
224 | std::index_sequence<i2...>) | |
225 | { | |
226 | (void)view; // Remove spurious unused variable warning with GCC | |
227 | return static_cast<F&&>(f)(hana::at_c<i1>(view.sequence1_)..., | |
228 | hana::at_c<i2>(view.sequence2_)...); | |
229 | } | |
230 | ||
231 | template <typename S1, typename S2, typename F> | |
232 | static constexpr decltype(auto) | |
233 | apply(experimental::joined_view_t<S1, S2> view, F&& f) { | |
234 | constexpr auto N1 = decltype(hana::length(view.sequence1_))::value; | |
235 | constexpr auto N2 = decltype(hana::length(view.sequence2_))::value; | |
236 | return unpack_joined(view, static_cast<F&&>(f), | |
237 | std::make_index_sequence<N1>{}, | |
238 | std::make_index_sequence<N2>{}); | |
239 | } | |
240 | ||
241 | // single_view | |
242 | template <typename T, typename F> | |
243 | static constexpr decltype(auto) apply(experimental::single_view_t<T> view, F&& f) { | |
244 | return static_cast<F&&>(f)(view.value_); | |
245 | } | |
246 | ||
247 | // empty_view | |
248 | template <typename F> | |
249 | static constexpr decltype(auto) apply(experimental::empty_view_t, F&& f) { | |
250 | return static_cast<F&&>(f)(); | |
251 | } | |
252 | }; | |
253 | ||
254 | ////////////////////////////////////////////////////////////////////////// | |
255 | // Iterable | |
256 | ////////////////////////////////////////////////////////////////////////// | |
257 | template <> | |
258 | struct at_impl<experimental::view_tag> { | |
259 | // sliced_view | |
260 | template <typename Sequence, std::size_t ...i, typename N> | |
261 | static constexpr decltype(auto) | |
262 | apply(experimental::sliced_view_t<Sequence, i...> view, N const&) { | |
263 | constexpr std::size_t indices[] = {i...}; | |
264 | constexpr std::size_t n = indices[N::value]; | |
265 | return hana::at_c<n>(view.sequence_); | |
266 | } | |
267 | ||
268 | // transformed_view | |
269 | template <typename Sequence, typename F, typename N> | |
270 | static constexpr decltype(auto) | |
271 | apply(experimental::transformed_view_t<Sequence, F> view, N const& n) { | |
272 | return view.f_(hana::at(view.sequence_, n)); | |
273 | } | |
274 | ||
275 | // joined_view | |
276 | template <std::size_t Left, typename View, typename N> | |
277 | static constexpr decltype(auto) at_joined_view(View view, N const&, hana::true_) { | |
278 | return hana::at_c<N::value>(view.sequence1_); | |
279 | } | |
280 | ||
281 | template <std::size_t Left, typename View, typename N> | |
282 | static constexpr decltype(auto) at_joined_view(View view, N const&, hana::false_) { | |
283 | return hana::at_c<N::value - Left>(view.sequence2_); | |
284 | } | |
285 | ||
286 | template <typename S1, typename S2, typename N> | |
287 | static constexpr decltype(auto) | |
288 | apply(experimental::joined_view_t<S1, S2> view, N const& n) { | |
289 | constexpr auto Left = decltype(hana::length(view.sequence1_))::value; | |
290 | return at_joined_view<Left>(view, n, hana::bool_c<(N::value < Left)>); | |
291 | } | |
292 | ||
293 | // single_view | |
294 | template <typename T, typename N> | |
295 | static constexpr decltype(auto) apply(experimental::single_view_t<T> view, N const&) { | |
296 | static_assert(N::value == 0, | |
297 | "trying to fetch an out-of-bounds element in a hana::single_view"); | |
298 | return view.value_; | |
299 | } | |
300 | ||
301 | // empty_view | |
302 | template <typename N> | |
303 | static constexpr decltype(auto) apply(experimental::empty_view_t, N const&) = delete; | |
304 | }; | |
305 | ||
306 | template <> | |
307 | struct length_impl<experimental::view_tag> { | |
308 | // sliced_view | |
309 | template <typename Sequence, std::size_t ...i> | |
310 | static constexpr auto | |
311 | apply(experimental::sliced_view_t<Sequence, i...>) { | |
312 | return hana::size_c<sizeof...(i)>; | |
313 | } | |
314 | ||
315 | // transformed_view | |
316 | template <typename Sequence, typename F> | |
317 | static constexpr auto apply(experimental::transformed_view_t<Sequence, F> view) { | |
318 | return hana::length(view.sequence_); | |
319 | } | |
320 | ||
321 | // joined_view | |
322 | template <typename S1, typename S2> | |
323 | static constexpr auto apply(experimental::joined_view_t<S1, S2> view) { | |
324 | return hana::size_c< | |
325 | decltype(hana::length(view.sequence1_))::value + | |
326 | decltype(hana::length(view.sequence2_))::value | |
327 | >; | |
328 | } | |
329 | ||
330 | // single_view | |
331 | template <typename T> | |
332 | static constexpr auto apply(experimental::single_view_t<T>) { | |
333 | return hana::size_c<1>; | |
334 | } | |
335 | ||
336 | // empty_view | |
337 | static constexpr auto apply(experimental::empty_view_t) { | |
338 | return hana::size_c<0>; | |
339 | } | |
340 | }; | |
341 | ||
342 | template <> | |
343 | struct is_empty_impl<experimental::view_tag> { | |
344 | // sliced_view | |
345 | template <typename Sequence, std::size_t ...i> | |
346 | static constexpr auto | |
347 | apply(experimental::sliced_view_t<Sequence, i...>) { | |
348 | return hana::bool_c<sizeof...(i) == 0>; | |
349 | } | |
350 | ||
351 | // transformed_view | |
352 | template <typename Sequence, typename F> | |
353 | static constexpr auto apply(experimental::transformed_view_t<Sequence, F> view) { | |
354 | return hana::is_empty(view.sequence_); | |
355 | } | |
356 | ||
357 | // joined_view | |
358 | template <typename S1, typename S2> | |
359 | static constexpr auto apply(experimental::joined_view_t<S1, S2> view) { | |
360 | return hana::and_(hana::is_empty(view.sequence1_), | |
361 | hana::is_empty(view.sequence2_)); | |
362 | } | |
363 | ||
364 | // single_view | |
365 | template <typename T> | |
366 | static constexpr auto apply(experimental::single_view_t<T>) { | |
367 | return hana::false_c; | |
368 | } | |
369 | ||
370 | // empty_view | |
371 | static constexpr auto apply(experimental::empty_view_t) { | |
372 | return hana::true_c; | |
373 | } | |
374 | }; | |
375 | ||
376 | template <> | |
377 | struct drop_front_impl<experimental::view_tag> { | |
378 | template <typename View, typename N> | |
379 | static constexpr auto apply(View view, N const&) { | |
380 | constexpr auto n = N::value; | |
381 | constexpr auto Length = decltype(hana::length(view))::value; | |
382 | return experimental::sliced(view, hana::range_c<std::size_t, n, Length>); | |
383 | } | |
384 | }; | |
385 | ||
386 | ////////////////////////////////////////////////////////////////////////// | |
387 | // Functor | |
388 | ////////////////////////////////////////////////////////////////////////// | |
389 | template <> | |
390 | struct transform_impl<experimental::view_tag> { | |
391 | template <typename Sequence, typename F, typename G> | |
392 | static constexpr auto | |
393 | apply(experimental::transformed_view_t<Sequence, F> view, G&& g) { | |
394 | return experimental::transformed(view.sequence_, | |
395 | hana::compose(static_cast<G&&>(g), view.f_)); | |
396 | } | |
397 | ||
398 | template <typename View, typename F> | |
399 | static constexpr auto apply(View view, F&& f) { | |
400 | return experimental::transformed(view, static_cast<F&&>(f)); | |
401 | } | |
402 | }; | |
403 | ||
404 | ////////////////////////////////////////////////////////////////////////// | |
405 | // Applicative | |
406 | ////////////////////////////////////////////////////////////////////////// | |
407 | template <> | |
408 | struct lift_impl<experimental::view_tag> { | |
409 | template <typename T> | |
410 | static constexpr auto apply(T&& t) { | |
411 | return experimental::single_view(static_cast<T&&>(t)); | |
412 | } | |
413 | }; | |
414 | ||
415 | template <> | |
416 | struct ap_impl<experimental::view_tag> { | |
417 | template <typename F, typename X> | |
418 | static constexpr auto apply(F&& f, X&& x) { | |
419 | // TODO: Implement cleverly; we most likely need a cartesian_product | |
420 | // view or something like that. | |
421 | return hana::ap(hana::to_tuple(f), hana::to_tuple(x)); | |
422 | } | |
423 | }; | |
424 | ||
425 | ////////////////////////////////////////////////////////////////////////// | |
426 | // Monad | |
427 | ////////////////////////////////////////////////////////////////////////// | |
428 | template <> | |
429 | struct flatten_impl<experimental::view_tag> { | |
430 | template <typename View> | |
431 | static constexpr auto apply(View view) { | |
432 | // TODO: Implement a flattened_view instead | |
433 | return hana::fold_left(view, experimental::empty_view(), | |
434 | experimental::joined); | |
435 | } | |
436 | }; | |
437 | ||
438 | ////////////////////////////////////////////////////////////////////////// | |
439 | // MonadPlus | |
440 | ////////////////////////////////////////////////////////////////////////// | |
441 | template <> | |
442 | struct concat_impl<experimental::view_tag> { | |
443 | template <typename View1, typename View2> | |
444 | static constexpr auto apply(View1 view1, View2 view2) { | |
445 | return experimental::joined(view1, view2); | |
446 | } | |
447 | }; | |
448 | ||
449 | template <> | |
450 | struct empty_impl<experimental::view_tag> { | |
451 | static constexpr auto apply() { | |
452 | return experimental::empty_view(); | |
453 | } | |
454 | }; | |
455 | ||
456 | ////////////////////////////////////////////////////////////////////////// | |
457 | // Comparable | |
458 | ////////////////////////////////////////////////////////////////////////// | |
459 | template <> | |
460 | struct equal_impl<experimental::view_tag, experimental::view_tag> { | |
461 | template <typename View1, typename View2> | |
462 | static constexpr auto apply(View1 v1, View2 v2) { | |
463 | // TODO: Use a lexicographical comparison algorithm. | |
464 | return hana::equal(hana::to_tuple(v1), hana::to_tuple(v2)); | |
465 | } | |
466 | }; | |
467 | ||
468 | template <typename S> | |
469 | struct equal_impl<experimental::view_tag, S, hana::when<hana::Sequence<S>::value>> { | |
470 | template <typename View1, typename Seq> | |
471 | static constexpr auto apply(View1 v1, Seq const& s) { | |
472 | // TODO: Use a lexicographical comparison algorithm. | |
473 | return hana::equal(hana::to_tuple(v1), hana::to_tuple(s)); | |
474 | } | |
475 | }; | |
476 | ||
477 | template <typename S> | |
478 | struct equal_impl<S, experimental::view_tag, hana::when<hana::Sequence<S>::value>> { | |
479 | template <typename Seq, typename View2> | |
480 | static constexpr auto apply(Seq const& s, View2 v2) { | |
481 | // TODO: Use a lexicographical comparison algorithm. | |
482 | return hana::equal(hana::to_tuple(s), hana::to_tuple(v2)); | |
483 | } | |
484 | }; | |
485 | ||
486 | ////////////////////////////////////////////////////////////////////////// | |
487 | // Orderable | |
488 | ////////////////////////////////////////////////////////////////////////// | |
489 | template <> | |
490 | struct less_impl<experimental::view_tag, experimental::view_tag> { | |
491 | template <typename View1, typename View2> | |
492 | static constexpr auto apply(View1 v1, View2 v2) { | |
493 | return hana::lexicographical_compare(v1, v2); | |
494 | } | |
495 | }; | |
496 | ||
497 | template <typename S> | |
498 | struct less_impl<experimental::view_tag, S, hana::when<hana::Sequence<S>::value>> { | |
499 | template <typename View1, typename Seq> | |
500 | static constexpr auto apply(View1 v1, Seq const& s) { | |
501 | return hana::lexicographical_compare(v1, s); | |
502 | } | |
503 | }; | |
504 | ||
505 | template <typename S> | |
506 | struct less_impl<S, experimental::view_tag, hana::when<hana::Sequence<S>::value>> { | |
507 | template <typename Seq, typename View2> | |
508 | static constexpr auto apply(Seq const& s, View2 v2) { | |
509 | return hana::lexicographical_compare(s, v2); | |
510 | } | |
511 | }; | |
512 | ||
1e59de90 | 513 | }} // end namespace boost::hana |
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514 | |
515 | #endif // !BOOST_HANA_EXPERIMENTAL_VIEW_HPP |