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# 1 : : // Copyright (c) 2018-2020 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 <type_traits>
# 9 : : #include <cstddef>
# 10 : : #include <algorithm>
# 11 : : #include <assert.h>
# 12 : :
# 13 : : #ifdef DEBUG
# 14 : : #define CONSTEXPR_IF_NOT_DEBUG
# 15 : 31054476 : #define ASSERT_IF_DEBUG(x) assert((x))
# 16 : : #else
# 17 : : #define CONSTEXPR_IF_NOT_DEBUG constexpr
# 18 : : #define ASSERT_IF_DEBUG(x)
# 19 : : #endif
# 20 : :
# 21 : : #if defined(__clang__)
# 22 : : #if __has_attribute(lifetimebound)
# 23 : : #define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
# 24 : : #else
# 25 : : #define SPAN_ATTR_LIFETIMEBOUND
# 26 : : #endif
# 27 : : #else
# 28 : : #define SPAN_ATTR_LIFETIMEBOUND
# 29 : : #endif
# 30 : :
# 31 : : /** A Span is an object that can refer to a contiguous sequence of objects.
# 32 : : *
# 33 : : * It implements a subset of C++20's std::span.
# 34 : : *
# 35 : : * Things to be aware of when writing code that deals with Spans:
# 36 : : *
# 37 : : * - Similar to references themselves, Spans are subject to reference lifetime
# 38 : : * issues. The user is responsible for making sure the objects pointed to by
# 39 : : * a Span live as long as the Span is used. For example:
# 40 : : *
# 41 : : * std::vector<int> vec{1,2,3,4};
# 42 : : * Span<int> sp(vec);
# 43 : : * vec.push_back(5);
# 44 : : * printf("%i\n", sp.front()); // UB!
# 45 : : *
# 46 : : * may exhibit undefined behavior, as increasing the size of a vector may
# 47 : : * invalidate references.
# 48 : : *
# 49 : : * - One particular pitfall is that Spans can be constructed from temporaries,
# 50 : : * but this is unsafe when the Span is stored in a variable, outliving the
# 51 : : * temporary. For example, this will compile, but exhibits undefined behavior:
# 52 : : *
# 53 : : * Span<const int> sp(std::vector<int>{1, 2, 3});
# 54 : : * printf("%i\n", sp.front()); // UB!
# 55 : : *
# 56 : : * The lifetime of the vector ends when the statement it is created in ends.
# 57 : : * Thus the Span is left with a dangling reference, and using it is undefined.
# 58 : : *
# 59 : : * - Due to Span's automatic creation from range-like objects (arrays, and data
# 60 : : * types that expose a data() and size() member function), functions that
# 61 : : * accept a Span as input parameter can be called with any compatible
# 62 : : * range-like object. For example, this works:
# 63 : : *
# 64 : : * void Foo(Span<const int> arg);
# 65 : : *
# 66 : : * Foo(std::vector<int>{1, 2, 3}); // Works
# 67 : : *
# 68 : : * This is very useful in cases where a function truly does not care about the
# 69 : : * container, and only about having exactly a range of elements. However it
# 70 : : * may also be surprising to see automatic conversions in this case.
# 71 : : *
# 72 : : * When a function accepts a Span with a mutable element type, it will not
# 73 : : * accept temporaries; only variables or other references. For example:
# 74 : : *
# 75 : : * void FooMut(Span<int> arg);
# 76 : : *
# 77 : : * FooMut(std::vector<int>{1, 2, 3}); // Does not compile
# 78 : : * std::vector<int> baz{1, 2, 3};
# 79 : : * FooMut(baz); // Works
# 80 : : *
# 81 : : * This is similar to how functions that take (non-const) lvalue references
# 82 : : * as input cannot accept temporaries. This does not work either:
# 83 : : *
# 84 : : * void FooVec(std::vector<int>& arg);
# 85 : : * FooVec(std::vector<int>{1, 2, 3}); // Does not compile
# 86 : : *
# 87 : : * The idea is that if a function accepts a mutable reference, a meaningful
# 88 : : * result will be present in that variable after the call. Passing a temporary
# 89 : : * is useless in that context.
# 90 : : */
# 91 : : template<typename C>
# 92 : : class Span
# 93 : : {
# 94 : : C* m_data;
# 95 : : std::size_t m_size;
# 96 : :
# 97 : : template <class T>
# 98 : : struct is_Span_int : public std::false_type {};
# 99 : : template <class T>
# 100 : : struct is_Span_int<Span<T>> : public std::true_type {};
# 101 : : template <class T>
# 102 : : struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
# 103 : :
# 104 : :
# 105 : : public:
# 106 : 2 : constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
# 107 : :
# 108 : : /** Construct a span from a begin pointer and a size.
# 109 : : *
# 110 : : * This implements a subset of the iterator-based std::span constructor in C++20,
# 111 : : * which is hard to implement without std::address_of.
# 112 : : */
# 113 : : template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
# 114 : 24581255 : constexpr Span(T* begin, std::size_t size) noexcept : m_data(begin), m_size(size) {}
# 115 : :
# 116 : : /** Construct a span from a begin and end pointer.
# 117 : : *
# 118 : : * This implements a subset of the iterator-based std::span constructor in C++20,
# 119 : : * which is hard to implement without std::address_of.
# 120 : : */
# 121 : : template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
# 122 : : CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
# 123 : 23178 : {
# 124 : 23178 : ASSERT_IF_DEBUG(end >= begin);
# 125 : 23178 : }
# 126 : :
# 127 : : /** Implicit conversion of spans between compatible types.
# 128 : : *
# 129 : : * Specifically, if a pointer to an array of type O can be implicitly converted to a pointer to an array of type
# 130 : : * C, then permit implicit conversion of Span<O> to Span<C>. This matches the behavior of the corresponding
# 131 : : * C++20 std::span constructor.
# 132 : : *
# 133 : : * For example this means that a Span<T> can be converted into a Span<const T>.
# 134 : : */
# 135 : : template <typename O, typename std::enable_if<std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
# 136 : 821082 : constexpr Span(const Span<O>& other) noexcept : m_data(other.m_data), m_size(other.m_size) {}
# 137 : :
# 138 : : /** Default copy constructor. */
# 139 : : constexpr Span(const Span&) noexcept = default;
# 140 : :
# 141 : : /** Default assignment operator. */
# 142 : : Span& operator=(const Span& other) noexcept = default;
# 143 : :
# 144 : : /** Construct a Span from an array. This matches the corresponding C++20 std::span constructor. */
# 145 : : template <int N>
# 146 : 5875268 : constexpr Span(C (&a)[N]) noexcept : m_data(a), m_size(N) {}
# 147 : :
# 148 : : /** Construct a Span for objects with .data() and .size() (std::string, std::array, std::vector, ...).
# 149 : : *
# 150 : : * This implements a subset of the functionality provided by the C++20 std::span range-based constructor.
# 151 : : *
# 152 : : * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
# 153 : : * Note that this restriction does not exist when converting arrays or other Spans (see above).
# 154 : : */
# 155 : : template <typename V>
# 156 : : constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
# 157 : : typename std::enable_if<!is_Span<V>::value &&
# 158 : : std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
# 159 : : std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
# 160 : 14444660 : : m_data(other.data()), m_size(other.size()){}
# 161 : :
# 162 : : template <typename V>
# 163 : : constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
# 164 : : typename std::enable_if<!is_Span<V>::value &&
# 165 : : std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
# 166 : : std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
# 167 : 12724042 : : m_data(other.data()), m_size(other.size()){}
# 168 : :
# 169 : 39586358 : constexpr C* data() const noexcept { return m_data; }
# 170 : 8020830 : constexpr C* begin() const noexcept { return m_data; }
# 171 : 9870905 : constexpr C* end() const noexcept { return m_data + m_size; }
# 172 : : CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
# 173 : 1021 : {
# 174 : 1021 : ASSERT_IF_DEBUG(size() > 0);
# 175 : 1021 : return m_data[0];
# 176 : 1021 : }
# 177 : : CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
# 178 : 22617 : {
# 179 : 22617 : ASSERT_IF_DEBUG(size() > 0);
# 180 : 22617 : return m_data[m_size - 1];
# 181 : 22617 : }
# 182 : 94570655 : constexpr std::size_t size() const noexcept { return m_size; }
# 183 : 705 : constexpr bool empty() const noexcept { return size() == 0; }
# 184 : : CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
# 185 : 16788373 : {
# 186 : 16788373 : ASSERT_IF_DEBUG(size() > pos);
# 187 : 16788373 : return m_data[pos];
# 188 : 16788373 : }
# 189 : : CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
# 190 : 10625281 : {
# 191 : 10625281 : ASSERT_IF_DEBUG(size() >= offset);
# 192 : 10625281 : return Span<C>(m_data + offset, m_size - offset);
# 193 : 10625281 : }
# 194 : : CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset, std::size_t count) const noexcept
# 195 : 3926 : {
# 196 : 3926 : ASSERT_IF_DEBUG(size() >= offset + count);
# 197 : 3926 : return Span<C>(m_data + offset, count);
# 198 : 3926 : }
# 199 : : CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
# 200 : 3542132 : {
# 201 : 3542132 : ASSERT_IF_DEBUG(size() >= count);
# 202 : 3542132 : return Span<C>(m_data, count);
# 203 : 3542132 : }
# 204 : : CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
# 205 : 116 : {
# 206 : 116 : ASSERT_IF_DEBUG(size() >= count);
# 207 : 116 : return Span<C>(m_data + m_size - count, count);
# 208 : 116 : }
# 209 : :
# 210 [ + - ][ + + ]: 2413 : friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
# [ + + ][ + + ]
# 211 : 34 : friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
# 212 : : friend constexpr bool operator<(const Span& a, const Span& b) noexcept { return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end()); }
# 213 : : friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
# 214 : : friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
# 215 : : friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
# 216 : :
# 217 : : template <typename O> friend class Span;
# 218 : : };
# 219 : :
# 220 : : // MakeSpan helps constructing a Span of the right type automatically.
# 221 : : /** MakeSpan for arrays: */
# 222 : 3387391 : template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
# 223 : : /** MakeSpan for temporaries / rvalue references, only supporting const output. */
# 224 : 1484 : template <typename V> constexpr auto MakeSpan(V&& v SPAN_ATTR_LIFETIMEBOUND) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
# 225 : : /** MakeSpan for (lvalue) references, supporting mutable output. */
# 226 : 8177540 : template <typename V> constexpr auto MakeSpan(V& v SPAN_ATTR_LIFETIMEBOUND) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
# 227 : :
# 228 : : /** Pop the last element off a span, and return a reference to that element. */
# 229 : : template <typename T>
# 230 : : T& SpanPopBack(Span<T>& span)
# 231 : 47832 : {
# 232 : 47832 : size_t size = span.size();
# 233 : 47832 : ASSERT_IF_DEBUG(size > 0);
# 234 : 47832 : T& back = span[size - 1];
# 235 : 47832 : span = Span<T>(span.data(), size - 1);
# 236 : 47832 : return back;
# 237 : 47832 : }
# 238 : :
# 239 : : // Helper functions to safely cast to unsigned char pointers.
# 240 : 252595 : inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; }
# 241 : 12909 : inline unsigned char* UCharCast(unsigned char* c) { return c; }
# 242 : 161424 : inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; }
# 243 : 6379961 : inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
# 244 : :
# 245 : : // Helper function to safely convert a Span to a Span<[const] unsigned char>.
# 246 : 6806890 : 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()}; }
# 247 : :
# 248 : : /** Like MakeSpan, but for (const) unsigned char member types only. Only works for (un)signed char containers. */
# 249 : 6806892 : template <typename V> constexpr auto MakeUCharSpan(V&& v) -> decltype(UCharSpanCast(MakeSpan(std::forward<V>(v)))) { return UCharSpanCast(MakeSpan(std::forward<V>(v))); }
# 250 : :
# 251 : : #endif
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