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// Copyright 2021 the V8 project 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 INCLUDE_V8_MEMORY_SPAN_H_
#define INCLUDE_V8_MEMORY_SPAN_H_
#include <stddef.h>
#include <array>
#include <iterator>
#include <type_traits>
#include "v8config.h" // NOLINT(build/include_directory)
namespace v8 {
/**
* Points to an unowned contiguous buffer holding a known number of elements.
*
* This is similar to std::span (under consideration for C++20), but does not
* require advanced C++ support. In the (far) future, this may be replaced with
* or aliased to std::span.
*
* To facilitate future migration, this class exposes a subset of the interface
* implemented by std::span.
*/
template <typename T>
class V8_EXPORT MemorySpan {
private:
/** Some C++ machinery, brought from the future. */
template <typename From, typename To>
using is_array_convertible = std::is_convertible<From (*)[], To (*)[]>;
template <typename From, typename To>
static constexpr bool is_array_convertible_v =
is_array_convertible<From, To>::value;
template <typename It>
using iter_reference_t = decltype(*std::declval<It&>());
template <typename It, typename = void>
struct is_compatible_iterator : std::false_type {};
template <typename It>
struct is_compatible_iterator<
It,
std::void_t<
std::is_base_of<std::random_access_iterator_tag,
typename std::iterator_traits<It>::iterator_category>,
is_array_convertible<std::remove_reference_t<iter_reference_t<It>>,
T>>> : std::true_type {};
template <typename It>
static constexpr bool is_compatible_iterator_v =
is_compatible_iterator<It>::value;
template <typename U>
static constexpr U* to_address(U* p) noexcept {
return p;
}
template <typename It,
typename = std::void_t<decltype(std::declval<It&>().operator->())>>
static constexpr auto to_address(It it) noexcept {
return it.operator->();
}
public:
/** The default constructor creates an empty span. */
constexpr MemorySpan() = default;
/** Constructor from nullptr and count, for backwards compatibility.
* This is not compatible with C++20 std::span.
*/
constexpr MemorySpan(std::nullptr_t, size_t) {}
/** Constructor from "iterator" and count. */
template <typename Iterator,
std::enable_if_t<is_compatible_iterator_v<Iterator>, bool> = true>
constexpr MemorySpan(Iterator first,
size_t count) // NOLINT(runtime/explicit)
: data_(to_address(first)), size_(count) {}
/** Constructor from two "iterators". */
template <typename Iterator,
std::enable_if_t<is_compatible_iterator_v<Iterator> &&
!std::is_convertible_v<Iterator, size_t>,
bool> = true>
constexpr MemorySpan(Iterator first,
Iterator last) // NOLINT(runtime/explicit)
: data_(to_address(first)), size_(last - first) {}
/** Implicit conversion from C-style array. */
template <size_t N>
constexpr MemorySpan(T (&a)[N]) noexcept // NOLINT(runtime/explicit)
: data_(a), size_(N) {}
/** Implicit conversion from std::array. */
template <typename U, size_t N,
std::enable_if_t<is_array_convertible_v<U, T>, bool> = true>
constexpr MemorySpan(
std::array<U, N>& a) noexcept // NOLINT(runtime/explicit)
: data_(a.data()), size_{N} {}
/** Implicit conversion from const std::array. */
template <typename U, size_t N,
std::enable_if_t<is_array_convertible_v<const U, T>, bool> = true>
constexpr MemorySpan(
const std::array<U, N>& a) noexcept // NOLINT(runtime/explicit)
: data_(a.data()), size_{N} {}
/** Returns a pointer to the beginning of the buffer. */
constexpr T* data() const { return data_; }
/** Returns the number of elements that the buffer holds. */
constexpr size_t size() const { return size_; }
constexpr T& operator[](size_t i) const { return data_[i]; }
/** Returns true if the buffer is empty. */
constexpr bool empty() const { return size() == 0; }
class Iterator {
public:
using iterator_category = std::forward_iterator_tag;
using value_type = T;
using difference_type = std::ptrdiff_t;
using pointer = value_type*;
using reference = value_type&;
T& operator*() const { return *ptr_; }
T* operator->() const { return ptr_; }
bool operator==(Iterator other) const { return ptr_ == other.ptr_; }
bool operator!=(Iterator other) const { return !(*this == other); }
Iterator& operator++() {
++ptr_;
return *this;
}
Iterator operator++(int) {
Iterator temp(*this);
++(*this);
return temp;
}
private:
friend class MemorySpan<T>;
explicit Iterator(T* ptr) : ptr_(ptr) {}
T* ptr_ = nullptr;
};
Iterator begin() const { return Iterator(data_); }
Iterator end() const { return Iterator(data_ + size_); }
private:
T* data_ = nullptr;
size_t size_ = 0;
};
/**
* Helper function template to create an array of fixed length, initialized by
* the provided initializer list, without explicitly specifying the array size,
* e.g.
*
* auto arr = v8::to_array<Local<String>>({v8_str("one"), v8_str("two")});
*
* In the future, this may be replaced with or aliased to std::to_array (under
* consideration for C++20).
*/
namespace detail {
template <class T, std::size_t N, std::size_t... I>
constexpr std::array<std::remove_cv_t<T>, N> to_array_lvalue_impl(
T (&a)[N], std::index_sequence<I...>) {
return {{a[I]...}};
}
template <class T, std::size_t N, std::size_t... I>
constexpr std::array<std::remove_cv_t<T>, N> to_array_rvalue_impl(
T (&&a)[N], std::index_sequence<I...>) {
return {{std::move(a[I])...}};
}
} // namespace detail
template <class T, std::size_t N>
constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&a)[N]) {
return detail::to_array_lvalue_impl(a, std::make_index_sequence<N>{});
}
template <class T, std::size_t N>
constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&&a)[N]) {
return detail::to_array_rvalue_impl(std::move(a),
std::make_index_sequence<N>{});
}
} // namespace v8
#endif // INCLUDE_V8_MEMORY_SPAN_H_
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