LCOV - code coverage report
Current view: top level - src - span.h (source / functions) Coverage Total Hit
Test: total_coverage.info Lines: 100.0 % 45 45
Test Date: 2024-11-04 05:10:19 Functions: 95.3 % 107 102
Branches: 45.6 % 2536 1157

             Branch data     Line data    Source code
       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                 :             :     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 [ +  - ][ +  -  :     1312172 :     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   [ +  -  +  + ]:  7834112779 :     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         [ +  - ]:      100958 :     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                 :             :     }
     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   [ +  -  +  - ]:     4093176 :     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   [ +  -  +  - ]:    12048895 :     constexpr Span(C (&a)[N]) noexcept : m_data(a), m_size(N) {}
         [ +  - ][ +  -  
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     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   [ +  +  +  -  :    90513766 :     constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
           +  - ][ +  -  
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                      - ]
     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   [ +  +  +  +  :    97131328 :         : m_data(other.data()), m_size(other.size()){}
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     166                 :             : 
     167                 :             :     template <typename V>
     168   [ +  -  +  -  :    59314387 :     constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
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     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   [ +  +  +  +  :    90514240 :         : m_data(other.data()), m_size(other.size()){}
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     173                 :             : 
     174         [ +  + ]:   267362922 :     constexpr C* data() const noexcept { return m_data; }
           [ +  +  +  + ]
     175   [ +  +  +  + ]:    51723085 :     constexpr C* begin() const noexcept { return m_data; }
           [ +  +  #  # ]
     176 [ +  + ][ +  +  :    79899724 :     constexpr C* end() const noexcept { return m_data + m_size; }
          +  +  +  -  +  
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     177                 :  7542213559 :     CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
     178                 :             :     {
     179                 :             :         ASSERT_IF_DEBUG(size() > 0);
     180         [ +  + ]:  7542213559 :         return m_data[0];
     181                 :             :     }
     182                 :      111408 :     CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
     183                 :             :     {
     184                 :             :         ASSERT_IF_DEBUG(size() > 0);
     185         [ +  + ]:      111408 :         return m_data[m_size - 1];
     186                 :             :     }
     187   [ -  +  -  +  : 20863329282 :     constexpr std::size_t size() const noexcept { return m_size; }
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     188                 :   322265006 :     constexpr std::size_t size_bytes() const noexcept { return sizeof(C) * m_size; }
     189   [ +  +  +  +  :    73265443 :     constexpr bool empty() const noexcept { return size() == 0; }
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     190                 : 12939947121 :     CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
     191                 :             :     {
     192                 :             :         ASSERT_IF_DEBUG(size() > pos);
     193   [ +  +  #  #  : 12939921689 :         return m_data[pos];
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     194                 :             :     }
     195                 :  7597174121 :     CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
     196                 :             :     {
     197                 :             :         ASSERT_IF_DEBUG(size() >= offset);
     198   [ +  +  +  +  :  7597141972 :         return Span<C>(m_data + offset, m_size - offset);
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     199                 :             :     }
     200                 :     6680154 :     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   [ #  #  #  #  :     6680154 :         return Span<C>(m_data + offset, count);
           #  # ][ +  - ]
     204                 :             :     }
     205                 :    24765316 :     CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
     206                 :             :     {
     207                 :             :         ASSERT_IF_DEBUG(size() >= count);
     208 [ +  + ][ +  -  :    23323424 :         return Span<C>(m_data, count);
             -  -  -  - ]
     209                 :             :     }
     210                 :    15974613 :     CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
     211                 :             :     {
     212                 :             :          ASSERT_IF_DEBUG(size() >= count);
     213   [ +  -  +  -  :    15974613 :          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                 :      239486 : T& SpanPopBack(Span<T>& span)
     249                 :             : {
     250                 :      239486 :     size_t size = span.size();
     251                 :             :     ASSERT_IF_DEBUG(size > 0);
     252                 :      239486 :     T& back = span[size - 1];
     253                 :      239486 :     span = Span<T>(span.data(), size - 1);
     254                 :      239486 :     return back;
     255                 :             : }
     256                 :             : 
     257                 :             : // From C++20 as_bytes and as_writeable_bytes
     258                 :             : template <typename T>
     259                 :   282629204 : Span<const std::byte> AsBytes(Span<T> s) noexcept
     260                 :             : {
     261                 :   282629204 :     return {reinterpret_cast<const std::byte*>(s.data()), s.size_bytes()};
     262                 :             : }
     263                 :             : template <typename T>
     264                 :    39635802 : Span<std::byte> AsWritableBytes(Span<T> s) noexcept
     265                 :             : {
     266                 :    39635802 :     return {reinterpret_cast<std::byte*>(s.data()), s.size_bytes()};
     267                 :             : }
     268                 :             : 
     269                 :             : template <typename V>
     270         [ +  + ]:    34849560 : Span<const std::byte> MakeByteSpan(V&& v) noexcept
     271                 :             : {
     272                 :    34849560 :     return AsBytes(Span{std::forward<V>(v)});
     273                 :             : }
     274                 :             : template <typename V>
     275                 :     7038167 : Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
     276                 :             : {
     277                 :     7038167 :     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                 :             : inline unsigned char* UCharCast(unsigned char* c) { return c; }
     283                 :             : inline unsigned char* UCharCast(signed char* c) { return reinterpret_cast<unsigned char*>(c); }
     284                 :             : inline unsigned char* UCharCast(std::byte* c) { return reinterpret_cast<unsigned char*>(c); }
     285                 :             : inline const unsigned char* UCharCast(const char* c) { return reinterpret_cast<const unsigned char*>(c); }
     286                 :             : 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         [ +  - ]:     1429518 : 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                 :     8947587 : 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         [ +  + ]:     8947587 : 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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