span.hpp

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/*
    Copyright (C) 2025 - 2025
    Part of the Battle for Wesnoth Project https://www.wesnoth.org/
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY.
 
    See the COPYING file for more details.
*/
 
#pragma once
 
// We need to include *something* so __cpp_lib_span is defined properly here
#if __has_include(<version>)
#include <version>
#endif
 
#include <boost/version.hpp>
 
// C++20 compatible
#ifdef __cpp_lib_span
 
#include <span>
namespace utils { using std::span; }
 
// Boost 1.78 or later
#elif BOOST_VERSION >= 107800
 
#include <boost/core/span.hpp>
namespace utils { using boost::span; }
 
// Full manual fallback, copied from Boost with some tweaks
#else
 
/*
Copyright 2019-2023 Glen Joseph Fernandes
(glenjofe@gmail.com)
 
Distributed under the Boost Software License, Version 1.0.
(http://www.boost.org/LICENSE_1_0.txt)
*/
 
#include <array>
#include <cstddef>
#include <initializer_list>
#include <iterator>
#include <type_traits>
 
namespace boost {
 
template<class C>
inline constexpr auto
data(C& c) noexcept(noexcept(c.data())) -> decltype(c.data())
{
    return c.data();
}
 
template<class C>
inline constexpr auto
data(const C& c) noexcept(noexcept(c.data())) -> decltype(c.data())
{
    return c.data();
}
 
template<class T, std::size_t N>
inline constexpr T*
data(T(&a)[N]) noexcept
{
    return a;
}
 
template<class T>
inline constexpr const T*
data(std::initializer_list<T> l) noexcept
{
    return l.begin();
}
 
constexpr std::size_t dynamic_extent = static_cast<std::size_t>(-1);
 
template<class T, std::size_t E = dynamic_extent>
class span;
 
namespace detail {
 
template<class U, class T>
struct span_convertible {
    static constexpr bool value = std::is_convertible<U(*)[], T(*)[]>::value;
};
 
template<std::size_t E, std::size_t N>
struct span_capacity {
    static constexpr bool value = E == boost::dynamic_extent || E == N;
};
 
template<class T, std::size_t E, class U, std::size_t N>
struct span_compatible {
    static constexpr bool value = span_capacity<E, N>::value &&
        span_convertible<U, T>::value;
};
 
template<class T>
using span_uncvref = typename std::remove_cv<typename
    std::remove_reference<T>::type>::type;
 
template<class>
struct span_is_span {
    static constexpr bool value = false;
};
 
template<class T, std::size_t E>
struct span_is_span<boost::span<T, E> > {
    static constexpr bool value = true;
};
 
template<class T>
struct span_is_array {
    static constexpr bool value = false;
};
 
template<class T, std::size_t N>
struct span_is_array<std::array<T, N> > {
    static constexpr bool value = true;
};
 
template<class T>
using span_ptr = decltype(boost::data(std::declval<T&>()));
 
template<class, class = void>
struct span_data { };
 
template<class T>
struct span_data<T,
    typename std::enable_if<std::is_pointer<span_ptr<T> >::value>::type> {
    typedef typename std::remove_pointer<span_ptr<T> >::type type;
};
 
template<class, class, class = void>
struct span_has_data {
    static constexpr bool value = false;
};
 
template<class R, class T>
struct span_has_data<R, T, typename std::enable_if<span_convertible<typename
    span_data<R>::type, T>::value>::type> {
    static constexpr bool value = true;
};
 
template<class, class = void>
struct span_has_size {
    static constexpr bool value = false;
};
 
template<class R>
struct span_has_size<R, typename
    std::enable_if<std::is_convertible<decltype(std::declval<R&>().size()),
        std::size_t>::value>::type> {
    static constexpr bool value = true;
};
 
template<class R, class T>
struct span_is_range {
    static constexpr bool value = (std::is_const<T>::value ||
        std::is_lvalue_reference<R>::value) &&
        !span_is_span<span_uncvref<R> >::value &&
        !span_is_array<span_uncvref<R> >::value &&
        !std::is_array<span_uncvref<R> >::value &&
        span_has_data<R, T>::value &&
        span_has_size<R>::value;
};
 
template<std::size_t E, std::size_t N>
struct span_implicit {
    static constexpr bool value = E == boost::dynamic_extent ||
        N != boost::dynamic_extent;
};
 
template<class T, std::size_t E, class U, std::size_t N>
struct span_copyable {
    static constexpr bool value = (N == boost::dynamic_extent ||
        span_capacity<E, N>::value) && span_convertible<U, T>::value;
};
 
template<std::size_t E, std::size_t O>
struct span_sub {
    static constexpr std::size_t value = E == boost::dynamic_extent ?
        boost::dynamic_extent : E - O;
};
 
template<class T, std::size_t E>
struct span_store {
    constexpr span_store(T* p_, std::size_t) noexcept
        : p(p_) { }
    static constexpr std::size_t n = E;
    T* p;
};
 
template<class T>
struct span_store<T, boost::dynamic_extent> {
    constexpr span_store(T* p_, std::size_t n_) noexcept
        : p(p_)
        , n(n_) { }
    T* p;
    std::size_t n;
};
 
template<class T, std::size_t E>
struct span_bytes {
    static constexpr std::size_t value = sizeof(T) * E;
};
 
template<class T>
struct span_bytes<T, boost::dynamic_extent> {
    static constexpr std::size_t value = boost::dynamic_extent;
};
 
} /* detail */
 
template<class T, std::size_t E>
class span {
public:
    typedef T element_type;
    typedef typename std::remove_cv<T>::type value_type;
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;
    typedef T* pointer;
    typedef const T* const_pointer;
    typedef T& reference;
    typedef const T& const_reference;
    typedef T* iterator;
    typedef const T* const_iterator;
    typedef std::reverse_iterator<T*> reverse_iterator;
    typedef std::reverse_iterator<const T*> const_reverse_iterator;
 
    static constexpr std::size_t extent = E;
 
    template<std::size_t N = E,
        typename std::enable_if<N == dynamic_extent || N == 0, int>::type = 0>
    constexpr span() noexcept
        : s_(0, 0) { }
 
    template<class I,
        typename std::enable_if<E == dynamic_extent &&
            detail::span_convertible<I, T>::value, int>::type = 0>
    constexpr span(I* f, size_type c)
        : s_(f, c) { }
 
    template<class I,
        typename std::enable_if<E != dynamic_extent &&
            detail::span_convertible<I, T>::value, int>::type = 0>
    explicit constexpr span(I* f, size_type c)
        : s_(f, c) { }
 
    template<class I, class L,
        typename std::enable_if<E == dynamic_extent &&
            detail::span_convertible<I, T>::value, int>::type = 0>
    constexpr span(I* f, L* l)
        : s_(f, l - f) { }
 
    template<class I, class L,
        typename std::enable_if<E != dynamic_extent &&
            detail::span_convertible<I, T>::value, int>::type = 0>
    explicit constexpr span(I* f, L* l)
        : s_(f, l - f) { }
 
    template<std::size_t N,
        typename std::enable_if<detail::span_capacity<E, N>::value,
            int>::type = 0>
    constexpr span(typename std::enable_if<true, T>::type (&a)[N]) noexcept
        : s_(a, N) { }
 
    template<class U, std::size_t N,
        typename std::enable_if<detail::span_compatible<T, E, U, N>::value,
            int>::type = 0>
    constexpr span(std::array<U, N>& a) noexcept
        : s_(a.data(), N) { }
 
    template<class U, std::size_t N,
        typename std::enable_if<detail::span_compatible<T, E, const U,
            N>::value, int>::type = 0>
    constexpr span(const std::array<U, N>& a) noexcept
        : s_(a.data(), N) { }
 
    template<class R,
        typename std::enable_if<E == dynamic_extent &&
            detail::span_is_range<R, T>::value, int>::type = 0>
    constexpr span(R&& r) noexcept(noexcept(boost::data(r)) &&
        noexcept(r.size()))
        : s_(boost::data(r), r.size()) { }
 
    template<class R,
        typename std::enable_if<E != dynamic_extent &&
            detail::span_is_range<R, T>::value, int>::type = 0>
    explicit constexpr span(R&& r) noexcept(noexcept(boost::data(r)) &&
        noexcept(r.size()))
        : s_(boost::data(r), r.size()) { }
 
    template<class U, std::size_t N,
        typename std::enable_if<detail::span_implicit<E, N>::value &&
            detail::span_copyable<T, E, U, N>::value, int>::type = 0>
    constexpr span(const span<U, N>& s) noexcept
        : s_(s.data(), s.size()) { }
 
    template<class U, std::size_t N,
        typename std::enable_if<!detail::span_implicit<E, N>::value &&
            detail::span_copyable<T, E, U, N>::value, int>::type = 0>
    explicit constexpr span(const span<U, N>& s) noexcept
        : s_(s.data(), s.size()) { }
 
    template<std::size_t C>
    constexpr span<T, C> first() const {
        static_assert(C <= E, "Count <= Extent");
        return span<T, C>(s_.p, C);
    }
 
    template<std::size_t C>
    constexpr span<T, C> last() const {
        static_assert(C <= E, "Count <= Extent");
        return span<T, C>(s_.p + (s_.n - C), C);
    }
 
    template<std::size_t O, std::size_t C = dynamic_extent>
    constexpr typename std::enable_if<C == dynamic_extent,
        span<T, detail::span_sub<E, O>::value> >::type subspan() const {
        static_assert(O <= E, "Offset <= Extent");
        return span<T, detail::span_sub<E, O>::value>(s_.p + O, s_.n - O);
    }
 
    template<std::size_t O, std::size_t C = dynamic_extent>
    constexpr typename std::enable_if<C != dynamic_extent,
        span<T, C> >::type subspan() const {
        static_assert(O <= E && C <= E - O,
            "Offset <= Extent && Count <= Extent - Offset");
        return span<T, C>(s_.p + O, C);
    }
 
    constexpr span<T, dynamic_extent> first(size_type c) const {
        return span<T, dynamic_extent>(s_.p, c);
    }
 
    constexpr span<T, dynamic_extent> last(size_type c) const {
        return span<T, dynamic_extent>(s_.p + (s_.n - c), c);
    }
 
    constexpr span<T, dynamic_extent> subspan(size_type o,
        size_type c = dynamic_extent) const {
        return span<T, dynamic_extent>(s_.p + o,
            c == dynamic_extent ? s_.n - o : c);
    }
 
    constexpr size_type size() const noexcept {
        return s_.n;
    }
 
    constexpr size_type size_bytes() const noexcept {
        return s_.n * sizeof(T);
    }
 
    constexpr bool empty() const noexcept {
        return s_.n == 0;
    }
 
    constexpr reference operator[](size_type i) const {
        return s_.p[i];
    }
 
    constexpr reference front() const {
        return *s_.p;
    }
 
    constexpr reference back() const {
        return s_.p[s_.n - 1];
    }
 
    constexpr pointer data() const noexcept {
        return s_.p;
    }
 
    constexpr iterator begin() const noexcept {
        return s_.p;
    }
 
    constexpr iterator end() const noexcept {
        return s_.p + s_.n;
    }
 
    constexpr reverse_iterator rbegin() const noexcept {
        return reverse_iterator(s_.p + s_.n);
    }
 
    constexpr reverse_iterator rend() const noexcept {
        return reverse_iterator(s_.p);
    }
 
    constexpr const_iterator cbegin() const noexcept {
        return s_.p;
    }
 
    constexpr const_iterator cend() const noexcept {
        return s_.p + s_.n;
    }
 
    constexpr const_reverse_iterator crbegin() const noexcept {
        return const_reverse_iterator(s_.p + s_.n);
    }
 
    constexpr const_reverse_iterator crend() const noexcept {
        return const_reverse_iterator(s_.p);
    }
 
private:
    detail::span_store<T, E> s_;
};
 
#if defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template<class T, std::size_t E>
constexpr std::size_t span<T, E>::extent;
#endif
 
#ifdef __cpp_deduction_guides
template<class I, class L>
span(I*, L) -> span<I>;
 
template<class T, std::size_t N>
span(T(&)[N]) -> span<T, N>;
 
template<class T, std::size_t N>
span(std::array<T, N>&) -> span<T, N>;
 
template<class T, std::size_t N>
span(const std::array<T, N>&) -> span<const T, N>;
 
template<class R>
span(R&&) -> span<typename detail::span_data<R>::type>;
 
template<class T, std::size_t E>
span(span<T, E>) -> span<T, E>;
#endif
 
#ifdef __cpp_lib_byte
template<class T, std::size_t E>
inline span<const std::byte, detail::span_bytes<T, E>::value>
as_bytes(span<T, E> s) noexcept
{
    return span<const std::byte, detail::span_bytes<T,
        E>::value>(reinterpret_cast<const std::byte*>(s.data()),
            s.size_bytes());
}
 
template<class T, std::size_t E>
inline typename std::enable_if<!std::is_const<T>::value,
    span<std::byte, detail::span_bytes<T, E>::value> >::type
as_writable_bytes(span<T, E> s) noexcept
{
    return span<std::byte, detail::span_bytes<T,
        E>::value>(reinterpret_cast<std::byte*>(s.data()), s.size_bytes());
}
#endif
 
} /* boost */
 
namespace utils { using boost::span; }
 
#endif