Span.hpp

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#pragma once
 
#include "AstGlobal.h"
#include <cstddef>
#include <iterator>
#include <type_traits>
#include <initializer_list>
 
AST_NAMESPACE_BEGIN
 
constexpr size_t dynamic_extent = static_cast<size_t>(-1);
 
template<typename T, size_t Extent>
class SpanStorage{
public:
    A_CONSTEXPR_CXX14 SpanStorage(T* data, size_t size) noexcept 
        : data_(data) 
    {
        static_assert(size == Extent, "Size must match the extent for fixed-size spans");
    }
    T* data() const noexcept { return data_; }
    A_CONSTEXPR_CXX14 size_t size() const noexcept { return Extent; }
public:
    T* data_;
};
 
 
template<typename T>
class SpanStorage<T, dynamic_extent>{
public:
    SpanStorage(T* data, size_t size) noexcept : data_(data), size_(size) {}
    T* data() const noexcept { return data_; }
    size_t size() const noexcept { return size_; }
public:
    T* data_;
    size_t size_;
};
 
template <typename T, size_t Extent = dynamic_extent>
class Span
{
public:
    using element_type = T;
    using value_type = typename std::remove_cv<T>::type;
    using index_type = std::size_t;
    using difference_type = std::ptrdiff_t;
    using pointer = T*;
    using reference = T&;
    using const_pointer = const T*;
    using const_reference = const T&;
    using iterator = pointer;
    using const_iterator = const_pointer;
    using reverse_iterator = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
    static constexpr std::ptrdiff_t extent = Extent;
 
    // 构造函数
    constexpr Span() noexcept 
        : storage_(nullptr, 0) 
        {}
 
    constexpr Span(std::initializer_list<T> init) noexcept
        : storage_(init.begin(), init.size()) {}
 
    constexpr Span(pointer ptr, index_type count) noexcept
        : storage_(ptr, count) {}
 
    constexpr Span(pointer first, pointer last) noexcept
        : storage_(first, last - first) {}
 
    template <std::size_t N>
    constexpr Span(element_type (&arr)[N]) noexcept
        : storage_(arr, N) {}
 
    template <typename U, std::size_t N>
    constexpr Span(std::array<U, N>& arr) noexcept
        : storage_(arr.data(), N) {}
 
    template <typename U, std::size_t N>
    constexpr Span(const std::array<U, N>& arr) noexcept
        : storage_(arr.data(), N) {}
 
#if 0 // 暂时不支持其他容器类型，因为无法确定其内存是否连续
    template <typename Container>
    constexpr Span(Container& cont) noexcept
        : storage_(cont.data(), cont.size()) {}
 
    template <typename Container>
    constexpr Span(const Container& cont) noexcept
        : storage_(cont.data(), cont.size()) {}
#endif
 
    // 元素访问
    constexpr reference operator[](index_type idx) const noexcept {
        return data()[idx];
    }
 
    constexpr reference front() const noexcept {
        return data()[0];
    }
 
    constexpr reference back() const noexcept {
        return data()[size() - 1];
    }
 
    constexpr pointer data() const noexcept {
        return storage_.data_;
    }
 
    // 迭代器
    constexpr iterator begin() const noexcept {
        return data();
    }
 
    constexpr iterator end() const noexcept {
        return data() + size();
    }
 
    constexpr const_iterator cbegin() const noexcept {
        return data();
    }
 
    constexpr const_iterator cend() const noexcept {
        return data() + size();
    }
 
    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());
    }
 
    // 容量
    constexpr bool empty() const noexcept {
        return size() == 0;
    }
 
    constexpr index_type size() const noexcept {
        return storage_.size_;
    }
 
    constexpr index_type size_bytes() const noexcept {
        return size() * sizeof(element_type);
    }
 
    // 子视图
    A_CONSTEXPR_CXX14 Span subspan(index_type offset, index_type count = static_cast<index_type>(-1)) const noexcept {
        if (count == static_cast<index_type>(-1)) {
            count = size() - offset;
        }
        return Span(data() + offset, count);
    }
 
    constexpr Span first(index_type count) const noexcept {
        return Span(data(), count);
    }
 
    constexpr Span last(index_type count) const noexcept {
        return Span(data() + size() - count, count);
    }
 
private:
    SpanStorage<T, Extent> storage_;
};
 
#ifdef A_CXX17
// 推导指南
template <typename T, std::size_t N>
Span(T (&)[N]) -> Span<T, N>;
 
template <typename T, std::size_t N>
Span(std::array<T, N>&) -> Span<T, N>;
 
template <typename T, std::size_t N>
Span(const std::array<T, N>&) -> Span<const T, N>;
 
template <typename Container>
Span(Container&) -> Span<typename Container::value_type>;
 
template <typename Container>
Span(const Container&) -> Span<const typename Container::value_type>;
#endif
 
AST_NAMESPACE_END