/*
Copyright 2019 Glen Joseph Fernandes
(glenjofe@gmail.com)

Distributed under the Boost Software License, Version 1.0.
(http://www.boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_CORE_SPAN_HPP
#define BOOST_CORE_SPAN_HPP

#include <array>
#include <iterator>
#include <type_traits>
#include <cstddef>

namespace boost {

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>
struct span_uncvref {
    typedef typename std::remove_cv<typename
        std::remove_reference<T>::type>::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, class = void>
struct span_data { };

template<class T>
struct span_data<T,
    typename std::enable_if<std::is_pointer<decltype(std::declval<T
        &>().data())>::value>::type> {
    typedef typename std::remove_pointer<decltype(std::declval<T
        &>().data())>::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<typename span_uncvref<R>::type>::value &&
        !span_is_array<typename span_uncvref<R>::type>::value &&
        !std::is_array<typename span_uncvref<R>::type>::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(r.data()) && noexcept(r.size()))
        : s_(r.data(), 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(r.data()) &&
        noexcept(r.size()))
        : s_(r.data(), 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);
    }

    friend constexpr iterator begin(span s) noexcept {
        return s.begin();
    }

    friend constexpr iterator end(span s) noexcept {
        return s.end();
    }

private:
    detail::span_store<T, E> s_;
};

template<class T, std::size_t E>
constexpr std::size_t span<T, E>::extent;

#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 */

#endif