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1 : : // Copyright (c) 2018-2022 The Bitcoin Core developers
2 : : // Distributed under the MIT software license, see the accompanying
3 : : // file COPYING or http://www.opensource.org/licenses/mit-license.php.
4 : :
5 : : #ifndef BITCOIN_SPAN_H
6 : : #define BITCOIN_SPAN_H
7 : :
8 : : #include <cassert>
9 : : #include <cstddef>
10 : : #include <span>
11 : : #include <type_traits>
12 : : #include <utility>
13 : :
14 : : #ifdef DEBUG
15 : : #define CONSTEXPR_IF_NOT_DEBUG
16 : : #define ASSERT_IF_DEBUG(x) assert((x))
17 : : #else
18 : : #define CONSTEXPR_IF_NOT_DEBUG constexpr
19 : : #define ASSERT_IF_DEBUG(x)
20 : : #endif
21 : :
22 : : #if defined(__clang__)
23 : : #if __has_attribute(lifetimebound)
24 : : #define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
25 : : #else
26 : : #define SPAN_ATTR_LIFETIMEBOUND
27 : : #endif
28 : : #else
29 : : #define SPAN_ATTR_LIFETIMEBOUND
30 : : #endif
31 : :
32 : : /** A Span is an object that can refer to a contiguous sequence of objects.
33 : : *
34 : : * This file implements a subset of C++20's std::span. It can be considered
35 : : * temporary compatibility code until C++20 and is designed to be a
36 : : * self-contained abstraction without depending on other project files. For this
37 : : * reason, Clang lifetimebound is defined here instead of including
38 : : * <attributes.h>, which also defines it.
39 : : *
40 : : * Things to be aware of when writing code that deals with Spans:
41 : : *
42 : : * - Similar to references themselves, Spans are subject to reference lifetime
43 : : * issues. The user is responsible for making sure the objects pointed to by
44 : : * a Span live as long as the Span is used. For example:
45 : : *
46 : : * std::vector<int> vec{1,2,3,4};
47 : : * Span<int> sp(vec);
48 : : * vec.push_back(5);
49 : : * printf("%i\n", sp.front()); // UB!
50 : : *
51 : : * may exhibit undefined behavior, as increasing the size of a vector may
52 : : * invalidate references.
53 : : *
54 : : * - One particular pitfall is that Spans can be constructed from temporaries,
55 : : * but this is unsafe when the Span is stored in a variable, outliving the
56 : : * temporary. For example, this will compile, but exhibits undefined behavior:
57 : : *
58 : : * Span<const int> sp(std::vector<int>{1, 2, 3});
59 : : * printf("%i\n", sp.front()); // UB!
60 : : *
61 : : * The lifetime of the vector ends when the statement it is created in ends.
62 : : * Thus the Span is left with a dangling reference, and using it is undefined.
63 : : *
64 : : * - Due to Span's automatic creation from range-like objects (arrays, and data
65 : : * types that expose a data() and size() member function), functions that
66 : : * accept a Span as input parameter can be called with any compatible
67 : : * range-like object. For example, this works:
68 : : *
69 : : * void Foo(Span<const int> arg);
70 : : *
71 : : * Foo(std::vector<int>{1, 2, 3}); // Works
72 : : *
73 : : * This is very useful in cases where a function truly does not care about the
74 : : * container, and only about having exactly a range of elements. However it
75 : : * may also be surprising to see automatic conversions in this case.
76 : : *
77 : : * When a function accepts a Span with a mutable element type, it will not
78 : : * accept temporaries; only variables or other references. For example:
79 : : *
80 : : * void FooMut(Span<int> arg);
81 : : *
82 : : * FooMut(std::vector<int>{1, 2, 3}); // Does not compile
83 : : * std::vector<int> baz{1, 2, 3};
84 : : * FooMut(baz); // Works
85 : : *
86 : : * This is similar to how functions that take (non-const) lvalue references
87 : : * as input cannot accept temporaries. This does not work either:
88 : : *
89 : : * void FooVec(std::vector<int>& arg);
90 : : * FooVec(std::vector<int>{1, 2, 3}); // Does not compile
91 : : *
92 : : * The idea is that if a function accepts a mutable reference, a meaningful
93 : : * result will be present in that variable after the call. Passing a temporary
94 : : * is useless in that context.
95 : : */
96 : : template<typename C>
97 : : class Span
98 : : {
99 : : C* m_data;
100 : 2472235 : std::size_t m_size{0};
101 : :
102 : : template <class T>
103 : : struct is_Span_int : public std::false_type {};
104 : : template <class T>
105 : : struct is_Span_int<Span<T>> : public std::true_type {};
106 : : template <class T>
107 : : struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
108 : :
109 : :
110 : : public:
111 : 4944470 : constexpr Span() noexcept : m_data(nullptr) {}
112 : :
113 : : /** Construct a span from a begin pointer and a size.
114 : : *
115 : : * This implements a subset of the iterator-based std::span constructor in C++20,
116 : : * which is hard to implement without std::address_of.
117 : : */
118 : : template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
119 : 12637006283 : constexpr Span(T* begin, std::size_t size) noexcept : m_data(begin), m_size(size) {}
120 : :
121 : : /** Construct a span from a begin and end pointer.
122 : : *
123 : : * This implements a subset of the iterator-based std::span constructor in C++20,
124 : : * which is hard to implement without std::address_of.
125 : : */
126 : : template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
127 : 1400869 : CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
128 : : {
129 : : ASSERT_IF_DEBUG(end >= begin);
130 : 1400869 : }
131 : :
132 : : /** Implicit conversion of spans between compatible types.
133 : : *
134 : : * Specifically, if a pointer to an array of type O can be implicitly converted to a pointer to an array of type
135 : : * C, then permit implicit conversion of Span<O> to Span<C>. This matches the behavior of the corresponding
136 : : * C++20 std::span constructor.
137 : : *
138 : : * For example this means that a Span<T> can be converted into a Span<const T>.
139 : : */
140 : : template <typename O, typename std::enable_if<std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
141 : 14204957 : constexpr Span(const Span<O>& other) noexcept : m_data(other.m_data), m_size(other.m_size) {}
142 : :
143 : : /** Default copy constructor. */
144 : : constexpr Span(const Span&) noexcept = default;
145 : :
146 : : /** Default assignment operator. */
147 : : Span& operator=(const Span& other) noexcept = default;
148 : :
149 : : /** Construct a Span from an array. This matches the corresponding C++20 std::span constructor. */
150 : : template <int N>
151 : 55243268 : constexpr Span(C (&a)[N]) noexcept : m_data(a), m_size(N) {}
152 : :
153 : : /** Construct a Span for objects with .data() and .size() (std::string, std::array, std::vector, ...).
154 : : *
155 : : * This implements a subset of the functionality provided by the C++20 std::span range-based constructor.
156 : : *
157 : : * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
158 : : * Note that this restriction does not exist when converting arrays or other Spans (see above).
159 : : */
160 : : template <typename V>
161 : 406868530 : constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
162 : : typename std::enable_if<!is_Span<V>::value &&
163 : : std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
164 : : std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
165 : 406868530 : : m_data(other.data()), m_size(other.size()){}
166 : :
167 : : template <typename V>
168 : 1118260717 : constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
169 : : typename std::enable_if<!is_Span<V>::value &&
170 : : std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
171 : : std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
172 : 1118260717 : : m_data(other.data()), m_size(other.size()){}
173 : :
174 : 6646250100 : constexpr C* data() const noexcept { return m_data; }
175 : 595903939 : constexpr C* begin() const noexcept { return m_data; }
176 : 815269768 : constexpr C* end() const noexcept { return m_data + m_size; }
177 : 5353335841 : CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
178 : : {
179 : : ASSERT_IF_DEBUG(size() > 0);
180 : 5353335841 : return m_data[0];
181 : : }
182 : 11838 : CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
183 : : {
184 : : ASSERT_IF_DEBUG(size() > 0);
185 : 11838 : return m_data[m_size - 1];
186 : : }
187 : 12442196423 : constexpr std::size_t size() const noexcept { return m_size; }
188 : 3441301068 : constexpr std::size_t size_bytes() const noexcept { return sizeof(C) * m_size; }
189 : 249171480 : constexpr bool empty() const noexcept { return size() == 0; }
190 : 834477608 : CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
191 : : {
192 : : ASSERT_IF_DEBUG(size() > pos);
193 : 834477608 : return m_data[pos];
194 : : }
195 : 6626541340 : CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
196 : : {
197 : : ASSERT_IF_DEBUG(size() >= offset);
198 : 6626541340 : return Span<C>(m_data + offset, m_size - offset);
199 : : }
200 : 2015093 : CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset, std::size_t count) const noexcept
201 : : {
202 : : ASSERT_IF_DEBUG(size() >= offset + count);
203 : 2015093 : return Span<C>(m_data + offset, count);
204 : : }
205 : 48261629 : CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
206 : : {
207 : : ASSERT_IF_DEBUG(size() >= count);
208 : 48261629 : return Span<C>(m_data, count);
209 : : }
210 : 53047307 : CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
211 : : {
212 : : ASSERT_IF_DEBUG(size() >= count);
213 : 53047307 : return Span<C>(m_data + m_size - count, count);
214 : : }
215 : :
216 : : template <typename O> friend class Span;
217 : : };
218 : :
219 : : // Return result of calling .data() method on type T. This is used to be able to
220 : : // write template deduction guides for the single-parameter Span constructor
221 : : // below that will work if the value that is passed has a .data() method, and if
222 : : // the data method does not return a void pointer.
223 : : //
224 : : // It is important to check for the void type specifically below, so the
225 : : // deduction guides can be used in SFINAE contexts to check whether objects can
226 : : // be converted to spans. If the deduction guides did not explicitly check for
227 : : // void, and an object was passed that returned void* from data (like
228 : : // std::vector<bool>), the template deduction would succeed, but the Span<void>
229 : : // object instantiation would fail, resulting in a hard error, rather than a
230 : : // SFINAE error.
231 : : // https://stackoverflow.com/questions/68759148/sfinae-to-detect-the-explicitness-of-a-ctad-deduction-guide
232 : : // https://stackoverflow.com/questions/16568986/what-happens-when-you-call-data-on-a-stdvectorbool
233 : : template<typename T>
234 : : using DataResult = std::remove_pointer_t<decltype(std::declval<T&>().data())>;
235 : :
236 : : // Deduction guides for Span
237 : : // For the pointer/size based and iterator based constructor:
238 : : template <typename T, typename EndOrSize> Span(T*, EndOrSize) -> Span<T>;
239 : : // For the array constructor:
240 : : template <typename T, std::size_t N> Span(T (&)[N]) -> Span<T>;
241 : : // For the temporaries/rvalue references constructor, only supporting const output.
242 : : template <typename T> Span(T&&) -> Span<std::enable_if_t<!std::is_lvalue_reference_v<T> && !std::is_void_v<DataResult<T&&>>, const DataResult<T&&>>>;
243 : : // For (lvalue) references, supporting mutable output.
244 : : template <typename T> Span(T&) -> Span<std::enable_if_t<!std::is_void_v<DataResult<T&>>, DataResult<T&>>>;
245 : :
246 : : /** Pop the last element off a span, and return a reference to that element. */
247 : : template <typename T>
248 : 309299 : T& SpanPopBack(Span<T>& span)
249 : : {
250 : 309299 : size_t size = span.size();
251 : : ASSERT_IF_DEBUG(size > 0);
252 : 309299 : T& back = span[size - 1];
253 : 309299 : span = Span<T>(span.data(), size - 1);
254 : 618598 : return back;
255 : 309299 : }
256 : :
257 : : // From C++20 as_bytes and as_writeable_bytes
258 : : template <typename T>
259 : 2782280011 : Span<const std::byte> AsBytes(Span<T> s) noexcept
260 : : {
261 : 2782280011 : return {reinterpret_cast<const std::byte*>(s.data()), s.size_bytes()};
262 : : }
263 : : template <typename T>
264 : 659021056 : Span<std::byte> AsWritableBytes(Span<T> s) noexcept
265 : : {
266 : 659021056 : return {reinterpret_cast<std::byte*>(s.data()), s.size_bytes()};
267 : : }
268 : :
269 : : template <typename V>
270 : 513127909 : Span<const std::byte> MakeByteSpan(V&& v) noexcept
271 : : {
272 [ + - + - : 513127909 : return AsBytes(Span{std::forward<V>(v)});
# # + - ]
[ + - + - ]
[ + - # #
# # ][ + -
+ - + - +
- + - ][ +
- + - + -
# # ][ + -
# # + - +
- # # +
- ]
273 : : }
274 : : template <typename V>
275 : 52641126 : Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
276 : : {
277 [ + - # # ]: 52641126 : return AsWritableBytes(Span{std::forward<V>(v)});
[ + - + - ]
[ + - + -
+ - ][ + -
+ - # # +
- + - ][ +
- + - + -
+ - ]
278 : : }
279 : :
280 : : // Helper functions to safely cast basic byte pointers to unsigned char pointers.
281 : : inline unsigned char* UCharCast(char* c) { return reinterpret_cast<unsigned char*>(c); }
282 : 434731 : inline unsigned char* UCharCast(unsigned char* c) { return c; }
283 : : inline unsigned char* UCharCast(signed char* c) { return reinterpret_cast<unsigned char*>(c); }
284 : 1051843650 : inline unsigned char* UCharCast(std::byte* c) { return reinterpret_cast<unsigned char*>(c); }
285 : 60513 : inline const unsigned char* UCharCast(const char* c) { return reinterpret_cast<const unsigned char*>(c); }
286 : 32332727 : inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
287 : : inline const unsigned char* UCharCast(const signed char* c) { return reinterpret_cast<const unsigned char*>(c); }
288 : 1451197633 : inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }
289 : : // Helper concept for the basic byte types.
290 : : template <typename B>
291 : : concept BasicByte = requires { UCharCast(std::span<B>{}.data()); };
292 : :
293 : : // Helper function to safely convert a Span to a Span<[const] unsigned char>.
294 : 26133678 : template <typename T> constexpr auto UCharSpanCast(Span<T> s) -> Span<typename std::remove_pointer<decltype(UCharCast(s.data()))>::type> { return {UCharCast(s.data()), s.size()}; }
295 : :
296 : : /** Like the Span constructor, but for (const) unsigned char member types only. Only works for (un)signed char containers. */
297 : 26133678 : template <typename V> constexpr auto MakeUCharSpan(V&& v) -> decltype(UCharSpanCast(Span{std::forward<V>(v)})) { return UCharSpanCast(Span{std::forward<V>(v)}); }
298 : :
299 : : #endif // BITCOIN_SPAN_H
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