summaryrefslogtreecommitdiffstats
path: root/third_party/libwebrtc/api/array_view.h
blob: 7e01959b01a4c2ee27b8e20c98912b378355f57f (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
/*
 *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#ifndef API_ARRAY_VIEW_H_
#define API_ARRAY_VIEW_H_

#include <algorithm>
#include <array>
#include <iterator>
#include <type_traits>

#include "rtc_base/checks.h"
#include "rtc_base/type_traits.h"

namespace rtc {

// tl;dr: rtc::ArrayView is the same thing as gsl::span from the Guideline
//        Support Library.
//
// Many functions read from or write to arrays. The obvious way to do this is
// to use two arguments, a pointer to the first element and an element count:
//
//   bool Contains17(const int* arr, size_t size) {
//     for (size_t i = 0; i < size; ++i) {
//       if (arr[i] == 17)
//         return true;
//     }
//     return false;
//   }
//
// This is flexible, since it doesn't matter how the array is stored (C array,
// std::vector, rtc::Buffer, ...), but it's error-prone because the caller has
// to correctly specify the array length:
//
//   Contains17(arr, arraysize(arr));     // C array
//   Contains17(arr.data(), arr.size());  // std::vector
//   Contains17(arr, size);               // pointer + size
//   ...
//
// It's also kind of messy to have two separate arguments for what is
// conceptually a single thing.
//
// Enter rtc::ArrayView<T>. It contains a T pointer (to an array it doesn't
// own) and a count, and supports the basic things you'd expect, such as
// indexing and iteration. It allows us to write our function like this:
//
//   bool Contains17(rtc::ArrayView<const int> arr) {
//     for (auto e : arr) {
//       if (e == 17)
//         return true;
//     }
//     return false;
//   }
//
// And even better, because a bunch of things will implicitly convert to
// ArrayView, we can call it like this:
//
//   Contains17(arr);                             // C array
//   Contains17(arr);                             // std::vector
//   Contains17(rtc::ArrayView<int>(arr, size));  // pointer + size
//   Contains17(nullptr);                         // nullptr -> empty ArrayView
//   ...
//
// ArrayView<T> stores both a pointer and a size, but you may also use
// ArrayView<T, N>, which has a size that's fixed at compile time (which means
// it only has to store the pointer).
//
// One important point is that ArrayView<T> and ArrayView<const T> are
// different types, which allow and don't allow mutation of the array elements,
// respectively. The implicit conversions work just like you'd hope, so that
// e.g. vector<int> will convert to either ArrayView<int> or ArrayView<const
// int>, but const vector<int> will convert only to ArrayView<const int>.
// (ArrayView itself can be the source type in such conversions, so
// ArrayView<int> will convert to ArrayView<const int>.)
//
// Note: ArrayView is tiny (just a pointer and a count if variable-sized, just
// a pointer if fix-sized) and trivially copyable, so it's probably cheaper to
// pass it by value than by const reference.

namespace array_view_internal {

// Magic constant for indicating that the size of an ArrayView is variable
// instead of fixed.
enum : std::ptrdiff_t { kArrayViewVarSize = -4711 };

// Base class for ArrayViews of fixed nonzero size.
template <typename T, std::ptrdiff_t Size>
class ArrayViewBase {
  static_assert(Size > 0, "ArrayView size must be variable or non-negative");

 public:
  ArrayViewBase(T* data, size_t size) : data_(data) {}

  static constexpr size_t size() { return Size; }
  static constexpr bool empty() { return false; }
  T* data() const { return data_; }

 protected:
  static constexpr bool fixed_size() { return true; }

 private:
  T* data_;
};

// Specialized base class for ArrayViews of fixed zero size.
template <typename T>
class ArrayViewBase<T, 0> {
 public:
  explicit ArrayViewBase(T* data, size_t size) {}

  static constexpr size_t size() { return 0; }
  static constexpr bool empty() { return true; }
  T* data() const { return nullptr; }

 protected:
  static constexpr bool fixed_size() { return true; }
};

// Specialized base class for ArrayViews of variable size.
template <typename T>
class ArrayViewBase<T, array_view_internal::kArrayViewVarSize> {
 public:
  ArrayViewBase(T* data, size_t size)
      : data_(size == 0 ? nullptr : data), size_(size) {}

  size_t size() const { return size_; }
  bool empty() const { return size_ == 0; }
  T* data() const { return data_; }

 protected:
  static constexpr bool fixed_size() { return false; }

 private:
  T* data_;
  size_t size_;
};

}  // namespace array_view_internal

template <typename T,
          std::ptrdiff_t Size = array_view_internal::kArrayViewVarSize>
class ArrayView final : public array_view_internal::ArrayViewBase<T, Size> {
 public:
  using value_type = T;
  using reference = value_type&;
  using const_reference = const value_type&;
  using pointer = value_type*;
  using const_pointer = const value_type*;
  using const_iterator = const T*;

  // Construct an ArrayView from a pointer and a length.
  template <typename U>
  ArrayView(U* data, size_t size)
      : array_view_internal::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
    RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
    RTC_DCHECK_EQ(size, this->size());
    RTC_DCHECK_EQ(!this->data(),
                  this->size() == 0);  // data is null iff size == 0.
  }

  // Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
  // cannot be empty.
  ArrayView() : ArrayView(nullptr, 0) {}
  ArrayView(std::nullptr_t)  // NOLINT
      : ArrayView() {}
  ArrayView(std::nullptr_t, size_t size)
      : ArrayView(static_cast<T*>(nullptr), size) {
    static_assert(Size == 0 || Size == array_view_internal::kArrayViewVarSize,
                  "");
    RTC_DCHECK_EQ(0, size);
  }

  // Construct an ArrayView from a C-style array.
  template <typename U, size_t N>
  ArrayView(U (&array)[N])  // NOLINT
      : ArrayView(array, N) {
    static_assert(Size == N || Size == array_view_internal::kArrayViewVarSize,
                  "Array size must match ArrayView size");
  }

  // (Only if size is fixed.) Construct a fixed size ArrayView<T, N> from a
  // non-const std::array instance. For an ArrayView with variable size, the
  // used ctor is ArrayView(U& u) instead.
  template <typename U,
            size_t N,
            typename std::enable_if<
                Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
  ArrayView(std::array<U, N>& u)  // NOLINT
      : ArrayView(u.data(), u.size()) {}

  // (Only if size is fixed.) Construct a fixed size ArrayView<T, N> where T is
  // const from a const(expr) std::array instance. For an ArrayView with
  // variable size, the used ctor is ArrayView(U& u) instead.
  template <typename U,
            size_t N,
            typename std::enable_if<
                Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
  ArrayView(const std::array<U, N>& u)  // NOLINT
      : ArrayView(u.data(), u.size()) {}

  // (Only if size is fixed.) Construct an ArrayView from any type U that has a
  // static constexpr size() method whose return value is equal to Size, and a
  // data() method whose return value converts implicitly to T*. In particular,
  // this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
  // N>, but not the other way around. We also don't allow conversion from
  // ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
  // N> when M != N.
  template <
      typename U,
      typename std::enable_if<Size != array_view_internal::kArrayViewVarSize &&
                              HasDataAndSize<U, T>::value>::type* = nullptr>
  ArrayView(U& u)  // NOLINT
      : ArrayView(u.data(), u.size()) {
    static_assert(U::size() == Size, "Sizes must match exactly");
  }
  template <
      typename U,
      typename std::enable_if<Size != array_view_internal::kArrayViewVarSize &&
                              HasDataAndSize<U, T>::value>::type* = nullptr>
  ArrayView(const U& u)  // NOLINT(runtime/explicit)
      : ArrayView(u.data(), u.size()) {
    static_assert(U::size() == Size, "Sizes must match exactly");
  }

  // (Only if size is variable.) Construct an ArrayView from any type U that
  // has a size() method whose return value converts implicitly to size_t, and
  // a data() method whose return value converts implicitly to T*. In
  // particular, this means we allow conversion from ArrayView<T> to
  // ArrayView<const T>, but not the other way around. Other allowed
  // conversions include
  // ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
  // std::vector<T> to ArrayView<T> or ArrayView<const T>,
  // const std::vector<T> to ArrayView<const T>,
  // rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
  // const rtc::Buffer to ArrayView<const uint8_t>.
  template <
      typename U,
      typename std::enable_if<Size == array_view_internal::kArrayViewVarSize &&
                              HasDataAndSize<U, T>::value>::type* = nullptr>
  ArrayView(U& u)  // NOLINT
      : ArrayView(u.data(), u.size()) {}
  template <
      typename U,
      typename std::enable_if<Size == array_view_internal::kArrayViewVarSize &&
                              HasDataAndSize<U, T>::value>::type* = nullptr>
  ArrayView(const U& u)  // NOLINT(runtime/explicit)
      : ArrayView(u.data(), u.size()) {}

  // Indexing and iteration. These allow mutation even if the ArrayView is
  // const, because the ArrayView doesn't own the array. (To prevent mutation,
  // use a const element type.)
  T& operator[](size_t idx) const {
    RTC_DCHECK_LT(idx, this->size());
    RTC_DCHECK(this->data());
    return this->data()[idx];
  }
  T* begin() const { return this->data(); }
  T* end() const { return this->data() + this->size(); }
  const T* cbegin() const { return this->data(); }
  const T* cend() const { return this->data() + this->size(); }
  std::reverse_iterator<T*> rbegin() const {
    return std::make_reverse_iterator(end());
  }
  std::reverse_iterator<T*> rend() const {
    return std::make_reverse_iterator(begin());
  }
  std::reverse_iterator<const T*> crbegin() const {
    return std::make_reverse_iterator(cend());
  }
  std::reverse_iterator<const T*> crend() const {
    return std::make_reverse_iterator(cbegin());
  }

  ArrayView<T> subview(size_t offset, size_t size) const {
    return offset < this->size()
               ? ArrayView<T>(this->data() + offset,
                              std::min(size, this->size() - offset))
               : ArrayView<T>();
  }
  ArrayView<T> subview(size_t offset) const {
    return subview(offset, this->size());
  }
};

// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
// dereference the pointers.
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator==(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
  return a.data() == b.data() && a.size() == b.size();
}
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator!=(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
  return !(a == b);
}

// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
// are the size of one pointer. (And as a special case, fixed-size ArrayViews
// of size 0 require no storage.)
static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int*), "");
static_assert(sizeof(ArrayView<int, 17>) == sizeof(int*), "");
static_assert(std::is_empty<ArrayView<int, 0>>::value, "");

template <typename T>
inline ArrayView<T> MakeArrayView(T* data, size_t size) {
  return ArrayView<T>(data, size);
}

// Only for primitive types that have the same size and aligment.
// Allow reinterpret cast of the array view to another primitive type of the
// same size.
// Template arguments order is (U, T, Size) to allow deduction of the template
// arguments in client calls: reinterpret_array_view<target_type>(array_view).
template <typename U, typename T, std::ptrdiff_t Size>
inline ArrayView<U, Size> reinterpret_array_view(ArrayView<T, Size> view) {
  static_assert(sizeof(U) == sizeof(T) && alignof(U) == alignof(T),
                "ArrayView reinterpret_cast is only supported for casting "
                "between views that represent the same chunk of memory.");
  static_assert(
      std::is_fundamental<T>::value && std::is_fundamental<U>::value,
      "ArrayView reinterpret_cast is only supported for casting between "
      "fundamental types.");
  return ArrayView<U, Size>(reinterpret_cast<U*>(view.data()), view.size());
}

}  // namespace rtc

#endif  // API_ARRAY_VIEW_H_