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// Copyright 2017 The Chromium Authors
// 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 <stdint.h>
#include <array>
#include <iterator>
#include <limits>
#include <type_traits>
#include <utility>
#include "polyfills/base/check.h"
#include "base/compiler_specific.h"
#include "base/containers/checked_iterators.h"
#include "base/containers/contiguous_iterator.h"
#include "base/cxx20_to_address.h"
#include "polyfills/base/memory/raw_ptr_exclusion.h"
#include "base/numerics/safe_math.h"
namespace gurl_base {
// [views.constants]
constexpr size_t dynamic_extent = std::numeric_limits<size_t>::max();
template <typename T,
size_t Extent = dynamic_extent,
typename InternalPtrType = T*>
class span;
namespace internal {
template <size_t I>
using size_constant = std::integral_constant<size_t, I>;
template <typename T>
struct ExtentImpl : size_constant<dynamic_extent> {};
template <typename T, size_t N>
struct ExtentImpl<T[N]> : size_constant<N> {};
template <typename T, size_t N>
struct ExtentImpl<std::array<T, N>> : size_constant<N> {};
template <typename T, size_t N>
struct ExtentImpl<gurl_base::span<T, N>> : size_constant<N> {};
template <typename T>
using Extent = ExtentImpl<remove_cvref_t<T>>;
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 IsNotSpan = std::negation<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 IsNotStdArray = std::negation<IsStdArrayImpl<std::decay_t<T>>>;
template <typename T>
using IsNotCArray = std::negation<std::is_array<std::remove_reference_t<T>>>;
template <typename From, typename To>
using IsLegalDataConversion = std::is_convertible<From (*)[], To (*)[]>;
template <typename Iter, typename T>
using IteratorHasConvertibleReferenceType =
IsLegalDataConversion<std::remove_reference_t<iter_reference_t<Iter>>, T>;
template <typename Iter, typename T>
using EnableIfCompatibleContiguousIterator = std::enable_if_t<
std::conjunction<IsContiguousIterator<Iter>,
IteratorHasConvertibleReferenceType<Iter, T>>::value>;
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, typename T, size_t Extent>
using EnableIfSpanCompatibleArray =
std::enable_if_t<(Extent == dynamic_extent ||
Extent == internal::Extent<Array>::value) &&
ContainerHasConvertibleData<Array, T>::value>;
// SFINAE check if Container can be converted to a span<T>.
template <typename Container, typename T>
using IsSpanCompatibleContainer =
std::conjunction<IsNotSpan<Container>,
IsNotStdArray<Container>,
IsNotCArray<Container>,
ContainerHasConvertibleData<Container, T>,
ContainerHasIntegralSize<Container>>;
template <typename Container, typename T>
using EnableIfSpanCompatibleContainer =
std::enable_if_t<IsSpanCompatibleContainer<Container, T>::value>;
template <typename Container, typename T, size_t Extent>
using EnableIfSpanCompatibleContainerAndSpanIsDynamic =
std::enable_if_t<IsSpanCompatibleContainer<Container, T>::value &&
Extent == dynamic_extent>;
// A helper template for storing the size of a span. Spans with static extents
// don't require additional storage, since the extent itself is specified in the
// template parameter.
template <size_t Extent>
class ExtentStorage {
public:
constexpr explicit ExtentStorage(size_t size) noexcept {}
constexpr size_t size() const noexcept { return Extent; }
};
// Specialization of ExtentStorage for dynamic extents, which do require
// explicit storage for the size.
template <>
struct ExtentStorage<dynamic_extent> {
constexpr explicit ExtentStorage(size_t size) noexcept : size_(size) {}
constexpr size_t size() const noexcept { return size_; }
private:
size_t size_;
};
// must_not_be_dynamic_extent prevents |dynamic_extent| from being returned in a
// constexpr context.
template <size_t kExtent>
constexpr size_t must_not_be_dynamic_extent() {
static_assert(
kExtent != dynamic_extent,
"EXTENT should only be used for containers with a static extent.");
return kExtent;
}
} // 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 StringPiece, 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(gurl_base::span<const uint8_t> data);
// std::vector<uint8_t> data_buffer = GenerateData();
// std::string r = HexEncode(data_buffer);
//
// Mutable:
// // ssize_t SafeSNPrintf(gurl_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*> => gurl_base::span<int* const>
// std::vector<const int*> => gurl_base::span<const int*>
// const std::vector<const int*> => gurl_base::span<const int* const>
//
// Differences from the C++20 draft
// --------------------------------
//
// http://eel.is/c++draft/views contains the latest C++20 draft of std::span.
// Chromium tries to follow the draft as close as possible. Differences between
// the draft 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 (std::byte is a C++17 feature)
//
// Differences from [span.cons]:
// - Constructing a static span (i.e. Extent != dynamic_extent) from a dynamic
// sized container (e.g. std::vector) requires an explicit conversion (in the
// C++20 draft this is simply UB)
//
// 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, typename InternalPtrType>
class GSL_POINTER span : public internal::ExtentStorage<Extent> {
private:
using ExtentStorage = internal::ExtentStorage<Extent>;
public:
using element_type = T;
using value_type = std::remove_cv_t<T>;
using size_type = size_t;
using difference_type = ptrdiff_t;
using pointer = T*;
using const_pointer = const T*;
using reference = T&;
using const_reference = const T&;
using iterator = CheckedContiguousIterator<T>;
using reverse_iterator = std::reverse_iterator<iterator>;
static constexpr size_t extent = Extent;
// [span.cons], span constructors, copy, assignment, and destructor
constexpr span() noexcept : ExtentStorage(0), data_(nullptr) {
static_assert(Extent == dynamic_extent || Extent == 0, "Invalid Extent");
}
template <typename It,
typename = internal::EnableIfCompatibleContiguousIterator<It, T>>
constexpr span(It first, StrictNumeric<size_t> count) noexcept
: ExtentStorage(count),
// The use of to_address() here is to handle the case where the iterator
// `first` is pointing to the container's `end()`. In that case we can
// not use the address returned from the iterator, or dereference it
// through the iterator's `operator*`, but we can store it. We must
// assume in this case that `count` is 0, since the iterator does not
// point to valid data. Future hardening of iterators may disallow
// pulling the address from `end()`, as demonstrated by asserts() in
// libstdc++: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93960.
//
// The span API dictates that the `data()` is accessible when size is 0,
// since the pointer may be valid, so we cannot prevent storing and
// giving out an invalid pointer here without breaking API compatibility
// and our unit tests. Thus protecting against this can likely only be
// successful from inside iterators themselves, where the context about
// the pointer is known.
//
// We can not protect here generally against an invalid iterator/count
// being passed in, since we have no context to determine if the
// iterator or count are valid.
data_(gurl_base::to_address(first)) {
GURL_CHECK(Extent == dynamic_extent || Extent == count);
}
template <
typename It,
typename End,
typename = internal::EnableIfCompatibleContiguousIterator<It, T>,
typename = std::enable_if_t<!std::is_convertible<End, size_t>::value>>
constexpr span(It begin, End end) noexcept
// Subtracting two iterators gives a ptrdiff_t, but the result should be
// non-negative: see GURL_CHECK below.
: span(begin, static_cast<size_t>(end - begin)) {
// Note: GURL_CHECK_LE is not constexpr, hence regular GURL_CHECK must be used.
GURL_CHECK(begin <= end);
}
template <
size_t N,
typename = internal::EnableIfSpanCompatibleArray<T (&)[N], T, Extent>>
constexpr span(T (&array)[N]) noexcept : span(std::data(array), N) {}
template <
typename U,
size_t N,
typename =
internal::EnableIfSpanCompatibleArray<std::array<U, N>&, T, Extent>>
constexpr span(std::array<U, N>& array) noexcept
: span(std::data(array), N) {}
template <typename U,
size_t N,
typename = internal::
EnableIfSpanCompatibleArray<const std::array<U, N>&, T, Extent>>
constexpr span(const std::array<U, 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::EnableIfSpanCompatibleContainerAndSpanIsDynamic<Container&,
T,
Extent>>
constexpr span(Container& container) noexcept
: span(std::data(container), std::size(container)) {}
template <
typename Container,
typename = internal::EnableIfSpanCompatibleContainerAndSpanIsDynamic<
const Container&,
T,
Extent>>
constexpr 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(Count <= Extent, "Count must not exceed Extent");
GURL_CHECK(Extent != dynamic_extent || Count <= size());
return {data(), Count};
}
template <size_t Count>
constexpr span<T, Count> last() const noexcept {
static_assert(Count <= Extent, "Count must not exceed Extent");
GURL_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(Offset <= Extent, "Offset must not exceed Extent");
static_assert(Count == dynamic_extent || Count <= Extent - Offset,
"Count must not exceed Extent - Offset");
GURL_CHECK(Extent != dynamic_extent || Offset <= size());
GURL_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: GURL_CHECK_LE is not constexpr, hence regular GURL_CHECK must be used.
GURL_CHECK(count <= size());
return {data(), count};
}
constexpr span<T, dynamic_extent> last(size_t count) const noexcept {
// Note: GURL_CHECK_LE is not constexpr, hence regular GURL_CHECK must be used.
GURL_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: GURL_CHECK_LE is not constexpr, hence regular GURL_CHECK must be used.
GURL_CHECK(offset <= size());
GURL_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 ExtentStorage::size(); }
constexpr size_t size_bytes() const noexcept { return size() * sizeof(T); }
[[nodiscard]] constexpr bool empty() const noexcept { return size() == 0; }
// [span.elem], span element access
constexpr T& operator[](size_t idx) const noexcept {
// Note: GURL_CHECK_LT is not constexpr, hence regular GURL_CHECK must be used.
GURL_CHECK(idx < size());
return *(data() + idx);
}
constexpr T& front() const noexcept {
static_assert(Extent == dynamic_extent || Extent > 0,
"Extent must not be 0");
GURL_CHECK(Extent != dynamic_extent || !empty());
return *data();
}
constexpr T& back() const noexcept {
static_assert(Extent == dynamic_extent || Extent > 0,
"Extent must not be 0");
GURL_CHECK(Extent != dynamic_extent || !empty());
return *(data() + size() - 1);
}
constexpr T* data() const noexcept { return data_; }
// [span.iter], span iterator support
constexpr iterator begin() const noexcept {
return iterator(data(), data() + size());
}
constexpr iterator end() const noexcept {
return iterator(data(), data() + size(), data() + size());
}
constexpr reverse_iterator rbegin() const noexcept {
return reverse_iterator(end());
}
constexpr reverse_iterator rend() const noexcept {
return reverse_iterator(begin());
}
private:
// This field is not a raw_ptr<> because it was filtered by the rewriter
// for: #constexpr-ctor-field-initializer, #global-scope, #union
InternalPtrType data_;
};
// span<T, Extent>::extent can not be declared inline prior to C++17, hence this
// definition is required.
template <class T, size_t Extent, typename InternalPtrType>
constexpr size_t span<T, Extent, InternalPtrType>::extent;
template <typename It,
typename T = std::remove_reference_t<iter_reference_t<It>>>
span(It, StrictNumeric<size_t>) -> span<T>;
template <typename It,
typename End,
typename = std::enable_if_t<!std::is_convertible_v<End, size_t>>,
typename T = std::remove_reference_t<iter_reference_t<It>>>
span(It, End) -> span<T>;
template <typename T, size_t N>
span(T (&)[N]) -> span<T, N>;
template <typename T, size_t N>
span(std::array<T, N>&) -> span<T, N>;
template <typename T, size_t N>
span(const std::array<T, N>&) -> span<const T, N>;
template <typename Container,
typename T = std::remove_pointer_t<
decltype(std::data(std::declval<Container>()))>,
size_t X = internal::Extent<Container>::value>
span(Container&&) -> span<T, X>;
// [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 <int&... ExplicitArgumentBarrier, typename It>
constexpr auto make_span(It it, StrictNumeric<size_t> size) noexcept {
using T = std::remove_reference_t<iter_reference_t<It>>;
return span<T>(it, size);
}
template <int&... ExplicitArgumentBarrier,
typename It,
typename End,
typename = std::enable_if_t<!std::is_convertible_v<End, size_t>>>
constexpr auto make_span(It it, End end) noexcept {
using T = std::remove_reference_t<iter_reference_t<It>>;
return span<T>(it, end);
}
// make_span utility function that deduces both the span's value_type and extent
// from the passed in argument.
//
// Usage: auto span = gurl_base::make_span(...);
template <int&... ExplicitArgumentBarrier, typename Container>
constexpr auto make_span(Container&& container) noexcept {
using T =
std::remove_pointer_t<decltype(std::data(std::declval<Container>()))>;
using Extent = internal::Extent<Container>;
return span<T, Extent::value>(std::forward<Container>(container));
}
// make_span utility functions that allow callers to explicit specify the span's
// extent, the value_type is deduced automatically. This is useful when passing
// a dynamically sized container to a method expecting static spans, when the
// container is known to have the correct size.
//
// Note: This will GURL_CHECK that N indeed matches size(container).
//
// Usage: auto static_span = gurl_base::make_span<N>(...);
template <size_t N, int&... ExplicitArgumentBarrier, typename It>
constexpr auto make_span(It it, StrictNumeric<size_t> size) noexcept {
using T = std::remove_reference_t<iter_reference_t<It>>;
return span<T, N>(it, size);
}
template <size_t N,
int&... ExplicitArgumentBarrier,
typename It,
typename End,
typename = std::enable_if_t<!std::is_convertible_v<End, size_t>>>
constexpr auto make_span(It it, End end) noexcept {
using T = std::remove_reference_t<iter_reference_t<It>>;
return span<T, N>(it, end);
}
template <size_t N, int&... ExplicitArgumentBarrier, typename Container>
constexpr auto make_span(Container&& container) noexcept {
using T =
std::remove_pointer_t<decltype(std::data(std::declval<Container>()))>;
return span<T, N>(std::data(container), std::size(container));
}
} // namespace base
// EXTENT returns the size of any type that can be converted to a |gurl_base::span|
// with definite extent, i.e. everything that is a contiguous storage of some
// sort with static size. Specifically, this works for std::array in a constexpr
// context. Note:
// * |std::size| should be preferred for plain arrays.
// * In run-time contexts, functions such as |std::array::size| should be
// preferred.
#define EXTENT(x) \
::gurl_base::internal::must_not_be_dynamic_extent<decltype( \
::gurl_base::make_span(x))::extent>()
#endif // BASE_CONTAINERS_SPAN_H_