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