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// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_CONTAINERS_SPAN_H_
#define BASE_CONTAINERS_SPAN_H_
#include <stddef.h>
#include <algorithm>
#include <array>
#include <iterator>
#include <string_view>
#include <type_traits>
#include <utility>
#include "base/logging.h"
#include "base/stl_util.h"
namespace base {
// [views.constants]
constexpr size_t dynamic_extent = static_cast<size_t>(-1);
template <typename T, size_t Extent = dynamic_extent>
class span;
namespace internal {
template <typename T>
struct IsSpanImpl : std::false_type {};
template <typename T, size_t Extent>
struct IsSpanImpl<span<T, Extent>> : std::true_type {};
template <typename T>
using IsSpan = IsSpanImpl<std::decay_t<T>>;
template <typename T>
struct IsStdArrayImpl : std::false_type {};
template <typename T, size_t N>
struct IsStdArrayImpl<std::array<T, N>> : std::true_type {};
template <typename T>
using IsStdArray = IsStdArrayImpl<std::decay_t<T>>;
template <typename T>
using IsCArray = std::is_array<std::remove_reference_t<T>>;
template <typename From, typename To>
using IsLegalDataConversion = std::is_convertible<From (*)[], To (*)[]>;
template <typename Container, typename T>
using ContainerHasConvertibleData = IsLegalDataConversion<
std::remove_pointer_t<decltype(std::data(std::declval<Container>()))>,
T>;
template <typename Container>
using ContainerHasIntegralSize =
std::is_integral<decltype(std::size(std::declval<Container>()))>;
template <typename From, size_t FromExtent, typename To, size_t ToExtent>
using EnableIfLegalSpanConversion =
std::enable_if_t<(ToExtent == dynamic_extent || ToExtent == FromExtent) &&
IsLegalDataConversion<From, To>::value>;
// SFINAE check if Array can be converted to a span<T>.
template <typename Array, size_t N, typename T, size_t Extent>
using EnableIfSpanCompatibleArray =
std::enable_if_t<(Extent == dynamic_extent || Extent == N) &&
ContainerHasConvertibleData<Array, T>::value>;
// SFINAE check if Container can be converted to a span<T>.
template <typename Container, typename T>
using EnableIfSpanCompatibleContainer =
std::enable_if_t<!internal::IsSpan<Container>::value &&
!internal::IsStdArray<Container>::value &&
!internal::IsCArray<Container>::value &&
ContainerHasConvertibleData<Container, T>::value &&
ContainerHasIntegralSize<Container>::value>;
} // namespace internal
// A span is a value type that represents an array of elements of type T. Since
// it only consists of a pointer to memory with an associated size, it is very
// light-weight. It is cheap to construct, copy, move and use spans, so that
// users are encouraged to use it as a pass-by-value parameter. A span does not
// own the underlying memory, so care must be taken to ensure that a span does
// not outlive the backing store.
//
// span is somewhat analogous to std::string_view, but with arbitrary element
// types, allowing mutation if T is non-const.
//
// span is implicitly convertible from C++ arrays, as well as most [1]
// container-like types that provide a data() and size() method (such as
// std::vector<T>). A mutable span<T> can also be implicitly converted to an
// immutable span<const T>.
//
// Consider using a span for functions that take a data pointer and size
// parameter: it allows the function to still act on an array-like type, while
// allowing the caller code to be a bit more concise.
//
// For read-only data access pass a span<const T>: the caller can supply either
// a span<const T> or a span<T>, while the callee will have a read-only view.
// For read-write access a mutable span<T> is required.
//
// Without span:
// Read-Only:
// // std::string HexEncode(const uint8_t* data, size_t size);
// std::vector<uint8_t> data_buffer = GenerateData();
// std::string r = HexEncode(data_buffer.data(), data_buffer.size());
//
// Mutable:
// // ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt, Args...);
// char str_buffer[100];
// SafeSNPrintf(str_buffer, sizeof(str_buffer), "Pi ~= %lf", 3.14);
//
// With span:
// Read-Only:
// // std::string HexEncode(base::span<const uint8_t> data);
// std::vector<uint8_t> data_buffer = GenerateData();
// std::string r = HexEncode(data_buffer);
//
// Mutable:
// // ssize_t SafeSNPrintf(base::span<char>, const char* fmt, Args...);
// char str_buffer[100];
// SafeSNPrintf(str_buffer, "Pi ~= %lf", 3.14);
//
// Spans with "const" and pointers
// -------------------------------
//
// Const and pointers can get confusing. Here are vectors of pointers and their
// corresponding spans:
//
// const std::vector<int*> => base::span<int* const>
// std::vector<const int*> => base::span<const int*>
// const std::vector<const int*> => base::span<const int* const>
//
// Differences from the working group proposal
// -------------------------------------------
//
// https://wg21.link/P0122 is the latest working group proposal, Chromium
// currently implements R7. Differences between the proposal and the
// implementation are documented in subsections below.
//
// Differences from [span.objectrep]:
// - as_bytes() and as_writable_bytes() return spans of uint8_t instead of
// std::byte
//
// Differences in constants and types:
// - index_type is aliased to size_t
//
// Differences from [span.sub]:
// - using size_t instead of ptrdiff_t for indexing
//
// Differences from [span.obs]:
// - using size_t instead of ptrdiff_t to represent size()
//
// Differences from [span.elem]:
// - using size_t instead of ptrdiff_t for indexing
//
// Furthermore, all constructors and methods are marked noexcept due to the lack
// of exceptions in Chromium.
//
// Due to the lack of class template argument deduction guides in C++14
// appropriate make_span() utility functions are provided.
// [span], class template span
template <typename T, size_t Extent>
class span {
public:
using element_type = T;
using value_type = std::remove_cv_t<T>;
using index_type = size_t;
using difference_type = ptrdiff_t;
using pointer = T*;
using reference = T&;
using iterator = T*;
using const_iterator = const T*;
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
static constexpr index_type extent = Extent;
// [span.cons], span constructors, copy, assignment, and destructor
constexpr span() noexcept : data_(nullptr), size_(0) {
static_assert(Extent == dynamic_extent || Extent == 0, "Invalid Extent");
}
constexpr span(T* data, size_t size) noexcept : data_(data), size_(size) {
CHECK(Extent == dynamic_extent || Extent == size);
}
// Artificially templatized to break ambiguity for span(ptr, 0).
template <typename = void>
constexpr span(T* begin, T* end) noexcept : span(begin, end - begin) {
// Note: CHECK_LE is not constexpr, hence regular CHECK must be used.
CHECK(begin <= end);
}
template <
size_t N,
typename = internal::EnableIfSpanCompatibleArray<T (&)[N], N, T, Extent>>
constexpr span(T (&array)[N]) noexcept : span(std::data(array), N) {}
template <
size_t N,
typename = internal::
EnableIfSpanCompatibleArray<std::array<value_type, N>&, N, T, Extent>>
constexpr span(std::array<value_type, N>& array) noexcept
: span(std::data(array), N) {}
template <size_t N,
typename = internal::EnableIfSpanCompatibleArray<
const std::array<value_type, N>&,
N,
T,
Extent>>
constexpr span(const std::array<value_type, N>& array) noexcept
: span(std::data(array), N) {}
// Conversion from a container that has compatible std::data() and integral
// std::size().
template <typename Container,
typename = internal::EnableIfSpanCompatibleContainer<Container&, T>>
constexpr span(Container& container) noexcept
: span(std::data(container), std::size(container)) {}
template <
typename Container,
typename = internal::EnableIfSpanCompatibleContainer<const Container&, T>>
span(const Container& container) noexcept
: span(std::data(container), std::size(container)) {}
constexpr span(const span& other) noexcept = default;
// Conversions from spans of compatible types and extents: this allows a
// span<T> to be seamlessly used as a span<const T>, but not the other way
// around. If extent is not dynamic, OtherExtent has to be equal to Extent.
template <
typename U,
size_t OtherExtent,
typename =
internal::EnableIfLegalSpanConversion<U, OtherExtent, T, Extent>>
constexpr span(const span<U, OtherExtent>& other)
: span(other.data(), other.size()) {}
constexpr span& operator=(const span& other) noexcept = default;
~span() noexcept = default;
// [span.sub], span subviews
template <size_t Count>
constexpr span<T, Count> first() const noexcept {
static_assert(Extent == dynamic_extent || Count <= Extent,
"Count must not exceed Extent");
CHECK(Extent != dynamic_extent || Count <= size());
return {data(), Count};
}
template <size_t Count>
constexpr span<T, Count> last() const noexcept {
static_assert(Extent == dynamic_extent || Count <= Extent,
"Count must not exceed Extent");
CHECK(Extent != dynamic_extent || Count <= size());
return {data() + (size() - Count), Count};
}
template <size_t Offset, size_t Count = dynamic_extent>
constexpr span<T,
(Count != dynamic_extent
? Count
: (Extent != dynamic_extent ? Extent - Offset
: dynamic_extent))>
subspan() const noexcept {
static_assert(Extent == dynamic_extent || Offset <= Extent,
"Offset must not exceed Extent");
static_assert(Extent == dynamic_extent || Count == dynamic_extent ||
Count <= Extent - Offset,
"Count must not exceed Extent - Offset");
CHECK(Extent != dynamic_extent || Offset <= size());
CHECK(Extent != dynamic_extent || Count == dynamic_extent ||
Count <= size() - Offset);
return {data() + Offset, Count != dynamic_extent ? Count : size() - Offset};
}
constexpr span<T, dynamic_extent> first(size_t count) const noexcept {
// Note: CHECK_LE is not constexpr, hence regular CHECK must be used.
CHECK(count <= size());
return {data(), count};
}
constexpr span<T, dynamic_extent> last(size_t count) const noexcept {
// Note: CHECK_LE is not constexpr, hence regular CHECK must be used.
CHECK(count <= size());
return {data() + (size() - count), count};
}
constexpr span<T, dynamic_extent> subspan(size_t offset,
size_t count = dynamic_extent) const
noexcept {
// Note: CHECK_LE is not constexpr, hence regular CHECK must be used.
CHECK(offset <= size());
CHECK(count == dynamic_extent || count <= size() - offset);
return {data() + offset, count != dynamic_extent ? count : size() - offset};
}
// [span.obs], span observers
constexpr size_t size() const noexcept { return size_; }
constexpr size_t size_bytes() const noexcept { return size() * sizeof(T); }
constexpr bool empty() const noexcept { return size() == 0; }
// [span.elem], span element access
constexpr T& operator[](size_t idx) const noexcept {
// Note: CHECK_LT is not constexpr, hence regular CHECK must be used.
CHECK(idx < size());
return *(data() + idx);
}
constexpr T& operator()(size_t idx) const noexcept {
// Note: CHECK_LT is not constexpr, hence regular CHECK must be used.
CHECK(idx < size());
return *(data() + idx);
}
constexpr T* data() const noexcept { return data_; }
// [span.iter], span iterator support
constexpr iterator begin() const noexcept { return data(); }
constexpr iterator end() const noexcept { return data() + size(); }
constexpr const_iterator cbegin() const noexcept { return begin(); }
constexpr const_iterator cend() const noexcept { return end(); }
constexpr reverse_iterator rbegin() const noexcept {
return reverse_iterator(end());
}
constexpr reverse_iterator rend() const noexcept {
return reverse_iterator(begin());
}
constexpr const_reverse_iterator crbegin() const noexcept {
return const_reverse_iterator(cend());
}
constexpr const_reverse_iterator crend() const noexcept {
return const_reverse_iterator(cbegin());
}
private:
T* data_;
size_t size_;
};
// span<T, Extent>::extent can not be declared inline prior to C++17, hence this
// definition is required.
template <class T, size_t Extent>
constexpr size_t span<T, Extent>::extent;
// [span.comparison], span comparison operators
// Relational operators. Equality is a element-wise comparison.
template <typename T, size_t X, typename U, size_t Y>
constexpr bool operator==(span<T, X> lhs, span<U, Y> rhs) noexcept {
return std::equal(lhs.cbegin(), lhs.cend(), rhs.cbegin(), rhs.cend());
}
template <typename T, size_t X, typename U, size_t Y>
constexpr bool operator!=(span<T, X> lhs, span<U, Y> rhs) noexcept {
return !(lhs == rhs);
}
template <typename T, size_t X, typename U, size_t Y>
constexpr bool operator<(span<T, X> lhs, span<U, Y> rhs) noexcept {
return std::lexicographical_compare(lhs.cbegin(), lhs.cend(), rhs.cbegin(),
rhs.cend());
}
template <typename T, size_t X, typename U, size_t Y>
constexpr bool operator<=(span<T, X> lhs, span<U, Y> rhs) noexcept {
return !(rhs < lhs);
}
template <typename T, size_t X, typename U, size_t Y>
constexpr bool operator>(span<T, X> lhs, span<U, Y> rhs) noexcept {
return rhs < lhs;
}
template <typename T, size_t X, typename U, size_t Y>
constexpr bool operator>=(span<T, X> lhs, span<U, Y> rhs) noexcept {
return !(lhs < rhs);
}
// [span.objectrep], views of object representation
template <typename T, size_t X>
span<const uint8_t, (X == dynamic_extent ? dynamic_extent : sizeof(T) * X)>
as_bytes(span<T, X> s) noexcept {
return {reinterpret_cast<const uint8_t*>(s.data()), s.size_bytes()};
}
template <typename T,
size_t X,
typename = std::enable_if_t<!std::is_const<T>::value>>
span<uint8_t, (X == dynamic_extent ? dynamic_extent : sizeof(T) * X)>
as_writable_bytes(span<T, X> s) noexcept {
return {reinterpret_cast<uint8_t*>(s.data()), s.size_bytes()};
}
// Type-deducing helpers for constructing a span.
template <typename T>
constexpr span<T> make_span(T* data, size_t size) noexcept {
return {data, size};
}
template <typename T>
constexpr span<T> make_span(T* begin, T* end) noexcept {
return {begin, end};
}
template <typename T, size_t N>
constexpr span<T, N> make_span(T (&array)[N]) noexcept {
return array;
}
template <typename T, size_t N>
constexpr span<T, N> make_span(std::array<T, N>& array) noexcept {
return array;
}
template <typename T, size_t N>
constexpr span<const T, N> make_span(const std::array<T, N>& array) noexcept {
return array;
}
template <typename Container,
typename T = typename Container::value_type,
typename = internal::EnableIfSpanCompatibleContainer<Container&, T>>
constexpr span<T> make_span(Container& container) noexcept {
return container;
}
template <
typename Container,
typename T = const typename Container::value_type,
typename = internal::EnableIfSpanCompatibleContainer<const Container&, T>>
constexpr span<T> make_span(const Container& container) noexcept {
return container;
}
template <typename T, size_t X>
constexpr span<T, X> make_span(const span<T, X>& span) noexcept {
return span;
}
} // namespace base
#endif // BASE_CONTAINERS_SPAN_H_