Line data Source code
1 : // Copyright (c) 2018-2021 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 <algorithm>
9 : #include <cassert>
10 : #include <cstddef>
11 : #include <span>
12 : #include <type_traits>
13 :
14 : #ifdef DEBUG_CORE
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 1388945 : 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 4166835 : 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 431765448 : 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 2662 : CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
128 1331 : {
129 : ASSERT_IF_DEBUG(end >= begin);
130 2662 : }
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 8221476 : 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 25200542 : 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 51603576 : 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 51603576 : : m_data(other.data()), m_size(other.size()){}
166 :
167 : template <typename V>
168 47438340 : 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 47438340 : : m_data(other.data()), m_size(other.size()){}
173 :
174 158927986 : constexpr C* data() const noexcept { return m_data; }
175 88134416 : constexpr C* begin() const noexcept { return m_data; }
176 71577603 : constexpr C* end() const noexcept { return m_data + m_size; }
177 : CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
178 : {
179 : ASSERT_IF_DEBUG(size() > 0);
180 : return m_data[0];
181 : }
182 : CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
183 : {
184 : ASSERT_IF_DEBUG(size() > 0);
185 : return m_data[m_size - 1];
186 : }
187 185716518 : constexpr std::size_t size() const noexcept { return m_size; }
188 102286738 : constexpr std::size_t size_bytes() const noexcept { return sizeof(C) * m_size; }
189 36940197 : constexpr bool empty() const noexcept { return size() == 0; }
190 21443241 : CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
191 : {
192 : ASSERT_IF_DEBUG(size() > pos);
193 21443241 : return m_data[pos];
194 : }
195 19380920 : CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
196 : {
197 : ASSERT_IF_DEBUG(size() >= offset);
198 19380920 : return Span<C>(m_data + offset, m_size - offset);
199 : }
200 152237 : 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 152237 : return Span<C>(m_data + offset, count);
204 : }
205 11567318 : CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
206 : {
207 : ASSERT_IF_DEBUG(size() >= count);
208 11567318 : return Span<C>(m_data, count);
209 : }
210 1525047 : CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
211 : {
212 : ASSERT_IF_DEBUG(size() >= count);
213 1525047 : return Span<C>(m_data + m_size - count, count);
214 : }
215 :
216 595319 : friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
217 53 : friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
218 : friend constexpr bool operator<(const Span& a, const Span& b) noexcept { return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end()); }
219 : friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
220 : friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
221 : friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
222 :
223 : template <typename O> friend class Span;
224 : };
225 :
226 : // Deduction guides for Span
227 : // For the pointer/size based and iterator based constructor:
228 : template <typename T, typename EndOrSize> Span(T*, EndOrSize) -> Span<T>;
229 : // For the array constructor:
230 : template <typename T, std::size_t N> Span(T (&)[N]) -> Span<T>;
231 : // For the temporaries/rvalue references constructor, only supporting const output.
232 : template <typename T> Span(T&&) -> Span<std::enable_if_t<!std::is_lvalue_reference_v<T>, const std::remove_pointer_t<decltype(std::declval<T&&>().data())>>>;
233 : // For (lvalue) references, supporting mutable output.
234 : template <typename T> Span(T&) -> Span<std::remove_pointer_t<decltype(std::declval<T&>().data())>>;
235 :
236 : /** Pop the last element off a span, and return a reference to that element. */
237 : template <typename T>
238 : T& SpanPopBack(Span<T>& span)
239 : {
240 : size_t size = span.size();
241 : ASSERT_IF_DEBUG(size > 0);
242 : T& back = span[size - 1];
243 : span = Span<T>(span.data(), size - 1);
244 : return back;
245 : }
246 :
247 : // From C++20 as_bytes and as_writeable_bytes
248 : template <typename T>
249 96065836 : Span<const std::byte> AsBytes(Span<T> s) noexcept
250 : {
251 96065836 : return {reinterpret_cast<const std::byte*>(s.data()), s.size_bytes()};
252 : }
253 : template <typename T>
254 6220996 : Span<std::byte> AsWritableBytes(Span<T> s) noexcept
255 : {
256 6220996 : return {reinterpret_cast<std::byte*>(s.data()), s.size_bytes()};
257 : }
258 :
259 : template <typename V>
260 21744492 : Span<const std::byte> MakeByteSpan(V&& v) noexcept
261 : {
262 21744492 : return AsBytes(Span{std::forward<V>(v)});
263 : }
264 : template <typename V>
265 1265326 : Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
266 : {
267 1265326 : return AsWritableBytes(Span{std::forward<V>(v)});
268 : }
269 :
270 : // Helper functions to safely cast basic byte pointers to unsigned char pointers.
271 29565 : inline unsigned char* UCharCast(char* c) { return reinterpret_cast<unsigned char*>(c); }
272 7811 : inline unsigned char* UCharCast(unsigned char* c) { return c; }
273 25666002 : inline unsigned char* UCharCast(std::byte* c) { return reinterpret_cast<unsigned char*>(c); }
274 9764 : inline const unsigned char* UCharCast(const char* c) { return reinterpret_cast<const unsigned char*>(c); }
275 2731457 : inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
276 29706668 : inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }
277 : // Helper concept for the basic byte types.
278 : template <typename B>
279 : concept BasicByte = requires { UCharCast(std::span<B>{}.data()); };
280 :
281 : // Helper function to safely convert a Span to a Span<[const] unsigned char>.
282 2336451 : 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()}; }
283 :
284 : /** Like the Span constructor, but for (const) unsigned char member types only. Only works for (un)signed char containers. */
285 2336451 : template <typename V> constexpr auto MakeUCharSpan(V&& v) -> decltype(UCharSpanCast(Span{std::forward<V>(v)})) { return UCharSpanCast(Span{std::forward<V>(v)}); }
286 :
287 : template<typename C>
288 : [[nodiscard]] constexpr auto begin(const Span<C>& span) noexcept -> C* {
289 : return span.begin();
290 : }
291 :
292 : template<typename C>
293 : [[nodiscard]] constexpr auto end(const Span<C>& span) noexcept -> C* {
294 : return span.end();
295 : }
296 :
297 : #endif // BITCOIN_SPAN_H
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