libcruft-util/view.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2015-2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "annotation.hpp"
#include "cast.hpp"
#include "debug/assert.hpp"
#include "maths.hpp"
#include "platform.hpp"
#include "types/traits.hpp"

#include <cstdlib>
#include <iosfwd>
#include <string>
#include <cstring>
#include <stdexcept>
#include <iterator>
#include <type_traits>

namespace cruft {
    template <typename BeginT, typename EndT = BeginT>
    struct view {
    public:
        using begin_type = BeginT;
        using end_type   = EndT;

        //---------------------------------------------------------------------
        using value_type = typename std::iterator_traits<
            remove_restrict_t<BeginT>
        >::value_type;

        using size_type = size_t;

        //---------------------------------------------------------------------
        constexpr
        view (const BeginT &first, const EndT &last) noexcept:
            m_begin (first),
            m_end   (last)
        {
            if constexpr (cruft::is_lteq_orderable_v<BeginT,EndT>) {
                CHECK_LE (m_begin, m_end);
            }
        }


        template <
            typename ContainerT,
            typename = std::void_t<
                decltype (std::declval<ContainerT&> ().begin ()),
                decltype (std::declval<ContainerT&> ().end   ())
            >
        >
        view (ContainerT &rhs) noexcept (
            noexcept (std::declval<ContainerT> ().begin ()) &&
            noexcept (std::declval<ContainerT> ().end   ())
        )
        : view (rhs.begin (), rhs.end ())
        { ; }


        template <
            typename ContainerT,
            typename = std::void_t<
                decltype (std::declval<ContainerT const&> ().begin ()),
                decltype (std::declval<ContainerT const&> ().end   ())
            >
        >
        view (const ContainerT &rhs):
            view (rhs.begin (), rhs.end ())
        { ; }


        //---------------------------------------------------------------------
        // Construction from pointer/size representations for ease of use with
        // legacy C code.
        template <
            typename CountT,
            typename = std::enable_if_t<std::is_integral_v<CountT>,void>
        >
        constexpr view (
            const BeginT &_begin,
            CountT _size
        ) : view (_begin, _begin + _size)
        { ; }


        //---------------------------------------------------------------------
        // implicit conversion from const pointer const views to const pointer views
        template <
            typename ValueT,
            typename = std::enable_if_t<
                std::is_same_v<BeginT, const ValueT**> &&
                std::is_same_v<EndT,   const ValueT**>
            >
        >
        view (const view<const ValueT*const*,const ValueT*const*> &rhs):
            view (
                const_cast<const ValueT**> (rhs.begin ()),
                const_cast<const ValueT**> (rhs.end   ())
            )
        { ; }

        //---------------------------------------------------------------------
        // implicit conversion from pointer views to const pointer views
        template <
            typename ValueT,
            typename = std::enable_if_t<
                std::is_same_v<BeginT, const ValueT*> &&
                std::is_same_v<EndT,   const ValueT*>
            >
        >
        view (const view<ValueT*,ValueT*> &rhs):
            view (rhs.begin (), rhs.end ())
        { ; }


        //---------------------------------------------------------------------
        // explicitly cater for the char array case so that we don't
        // accidentally include the trailing null in the data.
        template <std::size_t N>
        view (const char (&value)[N]):
            view {std::begin (value), std::begin (value) + N - 1}
        {
            static_assert (N > 0);
        }


        //---------------------------------------------------------------------
        view (const char *str):
            view { str, str + strlen (str) }
        { ; }


        //---------------------------------------------------------------------
        view (char *str):
            view (str, str + strlen (str))
        { ; }


        //---------------------------------------------------------------------
        template <std::size_t N>
        view (char (&value)[N]):
            view {std::begin (value), std::begin (value) + N - 1}
        {
            static_assert (N > 0);
        }


        //---------------------------------------------------------------------
        template <std::size_t N, typename ValueT>
        view (const ValueT(&value)[N]):
            view {std::begin (value), std::end (value)}
        { ; }


        //---------------------------------------------------------------------
        template <std::size_t N, typename ValueT>
        view (ValueT(&value)[N]):
            view {std::begin (value), std::end (value)}
        { ; }


        //---------------------------------------------------------------------
        constexpr view (const view &) noexcept (
            std::is_nothrow_copy_constructible_v<BeginT> && std::is_nothrow_copy_constructible_v<EndT>
        ) = default;


        constexpr view (view &&) noexcept (
            std::is_nothrow_move_constructible_v<BeginT> && std::is_nothrow_move_constructible_v<EndT>
        ) = default;


        view& operator= (view const &rhs) noexcept (
            std::is_nothrow_copy_assignable_v<BeginT> && std::is_nothrow_copy_assignable_v<EndT>
        ) = default;


        view& operator= (view &&rhs) noexcept (
            std::is_nothrow_move_assignable_v<BeginT> && std::is_nothrow_move_assignable_v<EndT>
        ) = default;


        //---------------------------------------------------------------------
        // allow null construction of views where IteratorT is constructible
        // from nullptr_t
        //
        // ideally we would avoid exposing this as it promotes use of nulls but
        // it simplifies construction of views that are data members of classes
        // when we may not immediately know the values we should contain.
        constexpr view (std::nullptr_t) noexcept:
            view {nullptr,nullptr}
        { ; }


        //---------------------------------------------------------------------
        template <typename CharT, typename Traits, typename Allocator>
        view (std::basic_string<CharT,Traits,Allocator> &val):
            view (std::data (val), std::size (val))
        { ; }


        //---------------------------------------------------------------------
        template <typename CharT, typename Traits, typename Allocator>
        view (const std::basic_string<CharT,Traits,Allocator> &val):
            view (std::data (val), std::size (val))
        { ; }


        //---------------------------------------------------------------------
        template <typename ValueT, typename AllocatorT>
        view (const std::vector<ValueT,AllocatorT> &rhs):
            view (std::data (rhs), std::size (rhs))
        { ; }


        //---------------------------------------------------------------------
        template <typename ValueT, typename AllocatorT>
        view (std::vector<ValueT,AllocatorT> &rhs):
            view (std::data (rhs), std::size (rhs))
        { ; }


        //---------------------------------------------------------------------
        template <typename ValueT, std::size_t N>
        view (std::array<ValueT,N> &rhs):
            view (std::data (rhs), std::size (rhs))
        { ; }


        //---------------------------------------------------------------------
        template <typename ValueT, std::size_t N>
        view (const std::array<ValueT,N> &rhs):
            view (std::data (rhs), std::size (rhs))
        { ; }


        ///////////////////////////////////////////////////////////////////////
        constexpr BeginT begin (void)       noexcept { return m_begin; }
        constexpr EndT   end   (void)       noexcept { return m_end;   }
        constexpr BeginT begin (void) const noexcept { return m_begin; }
        constexpr EndT   end   (void) const noexcept { return m_end; }

        //---------------------------------------------------------------------
        constexpr BeginT cbegin (void) const noexcept { return m_begin; }
        constexpr EndT   cend   (void) const noexcept { return m_end;   }

        //---------------------------------------------------------------------
        auto data (void)       noexcept { return begin (); }
        auto data (void) const noexcept { return begin (); }

        //---------------------------------------------------------------------
        auto& front (void)       noexcept { return *m_begin; }
        auto& front (void) const noexcept { return *m_begin; }

        //---------------------------------------------------------------------
        auto& back (void)       noexcept { return *(m_end - 1); }
        auto& back (void) const noexcept { return *(m_end - 1); }


        ///////////////////////////////////////////////////////////////////////
        /// Returns true if the size of the view is zero.
        constexpr bool
        empty (void) const noexcept
        {
            return m_begin == m_end;
        }


        ///--------------------------------------------------------------------
        /// Returns true if the view is not empty; ie, there is data remaining.
        constexpr operator bool () const noexcept
        {
            return not empty ();
        }


        ///--------------------------------------------------------------------
        /// Returns the number of items in the view.
        constexpr auto
        size (void) const noexcept
        {
            return static_cast<size_type> (std::distance (m_begin, m_end));
        }


        ///--------------------------------------------------------------------
        /// Returns a signed count of items in the view.
        constexpr ssize_t
        ssize (void) const noexcept
        {
            return std::distance (m_begin, m_end);
        }


        ///--------------------------------------------------------------------
        /// Returns a subview of the first `count` elements of this view.
        [[nodiscard]] constexpr auto
        redim (size_type count) const
        {
            assert (count > 0);
            if (count > size ())
                throw std::invalid_argument ("redim to higher size not allowed");
            return view { m_begin, m_begin + count };
        };


        ///--------------------------------------------------------------------
        /// Returns two subviews split at `pos`.
        ///
        /// The first view extends from `begin` to `pos`, and the second view
        /// extends from `pos` to `end`.
        [[nodiscard]] constexpr std::pair<
            view<BeginT,BeginT>,
            view<BeginT,EndT>
        >
        split (BeginT pos) const
        {
            CHECK_GE (pos, m_begin);
            CHECK_LE (pos, m_end  );

            return {
                { m_begin, pos },
                { pos, m_end }
            };
        }


        ///--------------------------------------------------------------------
        template <
            typename IndexT,
            typename = std::enable_if_t<std::is_integral_v<IndexT>>
        >
        [[nodiscard]] constexpr auto
        split (IndexT idx) const
        {
            // It's ok if `idx` points to the end iterator; this just means the
            // second element of the returned pair is an empty view.
            static_assert (
                std::numeric_limits<IndexT>::max () <= std::numeric_limits<size_type>::max ()
            );
            CHECK_GE (idx, IndexT {0});
            CHECK_LE (cruft::cast::lossless<size_type> (idx), size ());

            auto last = m_begin;
            std::advance (last, idx);
            return split (last);
        }


        //---------------------------------------------------------------------
        // slices a view using python indexing semantics. ie,
        // "abc".slice(0,  3) == "abc"
        // "abc".slice(0, -1) == "abc"
        // "abc".slice(0, -2) == "ab"
        template <typename IndexA, typename IndexB>
        [[nodiscard]] constexpr
        auto
        slice (IndexA a, IndexB b) const
        {
            CHECK_LIMIT (cruft::abs (a), IndexA {0}, cruft::cast::lossless<IndexA> (size ()));
            CHECK_LIMIT (cruft::abs (b), IndexB {0}, cruft::cast::lossless<IndexB> (size ()));

            auto first = m_begin;
            auto last  = m_begin;

            std::advance (first, a < 0 ? size () + a + 1 : a);
            std::advance (last,  b < 0 ? size () + b + 1 : b);

            return view { first, last };
        }


        //---------------------------------------------------------------------
        template <
            typename IndexT,
            typename = std::enable_if_t<std::is_integral_v<IndexT>>
        >
        [[nodiscard]] constexpr auto
        head (IndexT idx) const
        {
            return std::get<0> (split (idx));
        }


        //---------------------------------------------------------------------
        template <
            typename IndexT,
            typename = std::enable_if_t<std::is_integral_v<IndexT>>
        >
        [[nodiscard]] constexpr auto
        tail (IndexT idx) const
        {
            return std::get<1> (split (idx));
        }


        //---------------------------------------------------------------------
        template <
            typename IndexT,
            typename = std::enable_if_t<std::is_integral_v<IndexT>>
        >
        [[nodiscard]] constexpr auto
        consume (IndexT count) const
        {
            auto [a,b] = split (count);
            (void)a;
            return b;
        }


        //---------------------------------------------------------------------
        [[nodiscard]] constexpr view<BeginT,EndT>
        consume (view<BeginT,EndT> prefix) const
        {
            assert (prefix.begin () == begin ());
            assert (prefix.end () < end ());

            return { prefix.end (), end () };
        }


        [[nodiscard]] constexpr view<BeginT,EndT>
        consume (const BeginT pos) const
        {
            return { pos, end () };
        }


        ///////////////////////////////////////////////////////////////////////
        /// Explicitly cast to a view with different iterator types.
        ///
        /// The source and destination iterator types must be:
        ///   * pointers
        ///   * alignable
        ///
        /// It is undefined behaviour to cast with begin and/or end iterators
        /// that do not have natural alignment. Instrumented builds _may_
        /// provide diagnostics or assertions in this case.
        ///
        /// \tparam ValueT   The new iterator type
        /// \return          A view that uses the new iterator type
        template <
            typename ValueT,
            typename = std::enable_if_t<
                // We can only convert views that use pointer iterators
                std::is_pointer_v<BeginT>   &&
                std::is_pointer_v<EndT>     &&
                std::is_same_v<BeginT,EndT> &&
                std::is_pointer_v<ValueT>
            >
        >
        view<ValueT>
        cast (void) const
        {
            // The values they point to must allow for alignment in one
            // direction or another.
            //
            // We prefer a static_assert over SFINAE because it reduces the
            // header burden for users (they do not need to include the
            // implementation of the pointer values to satisfy
            // iterator_traits), and it is quite unlikely we want to disable
            // this only if alignment is incompatible).
            static_assert (
                sizeof (typename std::iterator_traits<BeginT>::value_type) %
                sizeof (typename std::iterator_traits<ValueT>::value_type) == 0 ||
                sizeof (typename std::iterator_traits<ValueT>::value_type) %
                sizeof (typename std::iterator_traits<BeginT>::value_type) == 0
            );

            return {
                cast::alignment<ValueT> (m_begin),
                cast::alignment<ValueT> (m_end)
            };
        }


        ///////////////////////////////////////////////////////////////////////
        constexpr auto&&
        operator[] (size_t idx) noexcept
        {
            CHECK_LIMIT (idx, 0u, size ());
            return *std::next (begin (), idx);
        }

        //---------------------------------------------------------------------
        constexpr auto&&
        operator[] (size_t idx) const noexcept
        {
            CHECK_LIMIT (idx, 0u, size ());
            return *std::next (begin (), idx);
        }


    private:
        ///////////////////////////////////////////////////////////////////////
        BeginT m_begin;
        EndT m_end;
    };


    //-------------------------------------------------------------------------
    template <typename ValueT, std::size_t N>
    view (ValueT(&)[N]) -> view<ValueT*,ValueT*>;


    //-------------------------------------------------------------------------
    view (const char*) -> view<const char*, const char*>;

    view (char*) -> view<char*>;


    //-------------------------------------------------------------------------
    template <
        typename IteratorT,
        typename SizeT,
        typename = std::enable_if_t<
            std::is_integral_v<SizeT>
        >
    >
    view (IteratorT, SizeT) -> view<IteratorT,IteratorT>;


    template <typename CharT, typename Traits, typename Allocator>
    view (std::basic_string<CharT,Traits,Allocator> &) -> view<typename Allocator::pointer>;


    template <typename CharT, typename Traits, typename Allocator>
    view (const std::basic_string<CharT,Traits,Allocator> &) -> view<typename Allocator::const_pointer>;

    template <typename ValueT, typename AllocatorT>
    view (std::vector<ValueT,AllocatorT>&) -> view<typename AllocatorT::pointer>;


    template <typename ValueT, typename AllocatorT>
    view (const std::vector<ValueT,AllocatorT>&) -> view<typename AllocatorT::const_pointer>;

    template <typename ValueT, std::size_t N>
    view (std::array<ValueT,N>&) -> view<ValueT*>;

    template <typename ValueT, std::size_t N>
    view (const std::array<ValueT,N>&) -> view<const ValueT*>;

    template <typename ContainerT>
    view (ContainerT&) -> view<
        decltype (std::declval<ContainerT&> ().begin ()),
        decltype (std::declval<ContainerT&> ().end   ())
    >;

    template <typename ContainerT>
    view (const ContainerT&) -> view<
        decltype (std::declval<ContainerT const&> ().begin ()),
        decltype (std::declval<ContainerT const&> ().end   ())
    >;

    // base + count constructor
    template <typename BeginT>
    view (BeginT, std::uint64_t count) -> view<BeginT,BeginT>;

    template <typename BeginT>
    view (BeginT, std::uint32_t count) -> view<BeginT,BeginT>;


    ///////////////////////////////////////////////////////////////////////////
    template <typename ValueT, size_t N>
    auto
    make_view (const ValueT (&arr)[N])
    {
        return view<const ValueT*> (arr + 0, arr + N);
    }

    //-------------------------------------------------------------------------
    template <typename ContainerT>
    auto
    make_view (ContainerT &t)
    {
        return view { std::begin (t), std::end (t) };
    }

    //-------------------------------------------------------------------------
    template <typename ContainerT>
    auto
    make_view (const ContainerT &t)
    {
        return view { std::cbegin (t), std::cend (t) };
    }


    //-------------------------------------------------------------------------
    // disable the possibility of creating a view to a temporary. note that
    // this only works if an lval version has already been defined otherwise
    // universal reference rules will capture both lval and rval here.
    template <typename ContainerT>
    auto
    make_view (ContainerT&&) = delete;


    ///////////////////////////////////////////////////////////////////////////
    template <typename ContainerT>
    auto
    make_cview (const ContainerT &t)
    {
        return make_view (t);
        //return view<decltype(std::cbegin (t))> { std::cbegin (t), std::cend (t) };
    }


    //-------------------------------------------------------------------------
    template <typename BeginT, typename EndT>
    auto
    make_view (BeginT first, EndT last)
    {
        return view<BeginT, EndT> {first, last};
    }

    //-------------------------------------------------------------------------
    template <typename ValueT>
    auto
    make_cview (ValueT *first, ValueT *last)
    {
        return view<const ValueT*> {first, last};
    }

    ///////////////////////////////////////////////////////////////////////////
    inline
    view<const char*> make_view (const char *str)
    {
        return { str, str + strlen (str) };
    }


    //-------------------------------------------------------------------------
    inline
    view<char*> make_view (char *str)
    {
        return { str, str + strlen (str) };
    }


    //-------------------------------------------------------------------------
    template <typename CharT, typename TraitsT, typename AllocT>
    view<const CharT*>
    make_view (const std::basic_string<CharT,TraitsT,AllocT> &str)
    {
        return {
            std::data (str),
            std::data (str) + std::size (str)
        };
    }


    //-------------------------------------------------------------------------
    template <typename CharT, typename TraitsT, typename AllocT>
    view<CharT*>
    make_view (std::basic_string<CharT,TraitsT,AllocT> &str)
    {
        return {
            std::data (str),
            std::data (str) + std::size (str)
        };
    }


    //-------------------------------------------------------------------------
    template <typename CharT, typename TraitsT, typename AllocT>
    view<const CharT*>
    make_view (const std::basic_string<CharT,TraitsT,AllocT>&&) = delete;


    //-------------------------------------------------------------------------
    template <typename CharT, typename TraitsT, typename AllocT>
    view<CharT*>
    make_view (std::basic_string<CharT,TraitsT,AllocT>&&) = delete;


    ///////////////////////////////////////////////////////////////////////////
    /// Calculates a word oriented view over an arbitrary type
    ///
    /// Useful for passing in memory structures to file descriptors and the
    /// like. but the consequences of endian conversion is on the user...
    ///
    /// We have to be careful that rval-references and other temporaries aren't
    /// accepted in this signature.
    template <
        typename WordT = std::byte const,
        typename T
    >
    cruft::view<WordT*>
    make_byte_view (T &t)
    {
        static_assert (sizeof (T) % sizeof (WordT) == 0);
        static_assert (std::is_const_v<T> ? std::is_const_v<WordT> : true);

        return view {
            cast::alignment<WordT*> (&t),
            sizeof (T) / sizeof (WordT)
        };
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Returns a reference to a value of the designated type at the front of
    /// the word-view. if there is insufficient data for the extraction an
    /// exception will be thrown.
    ///
    /// There are no validity or other checks performed on the returned data
    /// this is deliberate, so that the function is safe to call on user
    /// supplied data during parsing routines. it is up to the user to ensure
    /// the object is valid.
    ///
    /// The buffer object is advanced in place so that it no longer covers
    /// the extract value
    ///
    /// It is assumed the user has taken care of alignment concerns
    template <
        typename ValueT,
        typename WordT,
        // Only allow calls if the value is a multiple of the word size. It's
        // useful to allow non-unit words for areas like TCP/IP which tend to
        // operate on u16 words.
        typename = std::enable_if_t<sizeof (ValueT) % sizeof (WordT) == 0>
    >
    ValueT const&
    extract (view<WordT const*> &buffer)
    {
        if (unlikely (sizeof (ValueT) > buffer.size () * sizeof (WordT)))
            throw std::runtime_error ("insufficient data for extraction");

        auto const ptr = cast::alignment<ValueT const*> (buffer.data ());
        buffer = buffer.consume (sizeof (ValueT) / sizeof (WordT));
        return *ptr;
    }


    ///////////////////////////////////////////////////////////////////////////
    /// extracts an object of a specified type from the front of a byte-view.
    ///
    /// in contrast to 'extract' this will always copy the bytes out from the
    /// view, making the operation alignment safe.
    template <
        typename ValueT,
        typename WordT,
        typename = std::enable_if_t<sizeof (ValueT) % sizeof(WordT) == 0>
    >
    ValueT
    read (view<WordT*> &buffer)
    {
        // We're going to use memcpy which requires that the type is
        // trivially copyable.
        static_assert (std::is_trivially_copyable_v<ValueT>);

        if (unlikely (sizeof (ValueT) > buffer.size () * sizeof (WordT)))
            throw std::runtime_error ("insufficient data for extraction");

        std::aligned_storage_t<sizeof(ValueT),alignof(ValueT)> bytes;
        memcpy (&bytes, buffer.data (), sizeof (ValueT));
        buffer = buffer.consume (sizeof (ValueT) / sizeof (WordT));
        return *reinterpret_cast<ValueT const*> (&bytes);
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Tests whether an iterator falls within a given view.
    template <typename IteratorT>
    constexpr bool
    intersects (view<IteratorT> a, IteratorT b)
    {
        return b >= a.begin () && b < a.end ();
    }


    ///------------------------------------------------------------------------
    /// Tests whether view `a` inclusively contains view `b`.
    template <typename IteratorA, typename IteratorB>
    constexpr bool
    covers (
        view<IteratorA,IteratorB> const &a,
        view<IteratorA,IteratorB> const &b
    ) {
        return a.begin () <= b.begin () && a.end () >= b.end ();
    }


    ///////////////////////////////////////////////////////////////////////////
    template <
        typename BeginA, typename EndA,
        typename BeginB, typename EndB
    >
    constexpr bool
    equal (const view<BeginA,EndA> &a, const view<BeginB,EndB> &b)
    {
        return a.size () == b.size () &&
               std::equal (std::begin (a), std::end (a), std::begin (b));
    }


    //-------------------------------------------------------------------------
    // defer equality to the view/view operator by way of make_view
    template <
        typename IteratorA,
        typename IteratorB,
        typename ValueT,
        typename = std::enable_if_t<
            !std::is_same_v<ValueT, view<IteratorA,IteratorB>>,
            void
        >
    >
    constexpr bool
    equal (const view<IteratorA,IteratorB> &a, const ValueT &b)
    {
        return equal (a, make_view (b));
    }


    //-------------------------------------------------------------------------
    // reverse the arguments and forward to the above operator. we formumlate
    // equality this way to avoid implementing the operator twice for each
    // weird case.
    template <
        typename IteratorA,
        typename IteratorB,
        typename ValueT,
        typename = std::enable_if_t<
            !std::is_same_v<ValueT, view<IteratorA,IteratorB>>,
            void
        >
    >
    constexpr bool
    equal (const ValueT &a, const view<IteratorA,IteratorB> &b)
    {
        return equal (b, a);
    }


    ///////////////////////////////////////////////////////////////////////////
    template <typename IteratorA, typename IteratorB>
    constexpr bool
    operator== (const view<IteratorA,IteratorB> &a, const view<IteratorA,IteratorB> &b)
    {
        return a.begin () == b.begin () && a.end () == b.end ();
    }


    //-------------------------------------------------------------------------
    template <typename IteratorA, typename IteratorB>
    constexpr bool
    operator!= (const view<IteratorA,IteratorB> &a, const view<IteratorA,IteratorB> &b)
    {
        return !(a == b);
    }


    //-------------------------------------------------------------------------
    template <
        typename IteratorA,
        typename IteratorB,
        typename ValueT,
        typename = std::enable_if_t<
            !std::is_same_v<ValueT, view<IteratorA,IteratorB>>,
            void
        >
    >
    constexpr bool
    operator!= (const view<IteratorA,IteratorB> &a, const ValueT &b)
    {
        return !(a == b);
    }


    //-------------------------------------------------------------------------
    template <
        typename IteratorA,
        typename IteratorB,
        typename ValueT,
        typename = std::enable_if_t<
            !std::is_same_v<ValueT, view<IteratorA,IteratorB>>,
            void
        >
    >
    constexpr bool
    operator!= (const ValueT &a, const view<IteratorA,IteratorB> &b)
    {
        return !(a == b);
    }


    ///////////////////////////////////////////////////////////////////////////
    template <typename CharT>
    bool
    operator== (
        view<const CharT*> lhs,
        const std::basic_string<CharT> &rhs
    ) {
        return lhs.size () == rhs.size () && std::equal (lhs.cbegin (), lhs.cend (), rhs.cbegin ());
    }


    inline bool
    operator== (view<const char*> lhs, const char *rhs)
    {
        return lhs.size () == strlen (rhs) && std::equal (lhs.cbegin (), lhs.cend (), rhs);
    }


    ///////////////////////////////////////////////////////////////////////////
    template <typename BeginT, typename EndT>
    std::ostream&
    operator<< (std::ostream &os, view<BeginT, EndT> val)
    {
        std::copy (
            std::cbegin (val),
            std::cend   (val),
            std::ostream_iterator<typename decltype(val)::value_type> (os)
        );

        return os;
    }


    ///////////////////////////////////////////////////////////////////////////
    /// a basic stringlike comparison operator that behaves as
    /// std::string::compare would.
    ///
    /// provided so that the common case of stringlike views can be used in a
    /// std::map and similar without a great deal of work.
    inline bool
    operator< (view<const char*> a, view<const char*> b)
    {
        const auto la = std::size (a);
        const auto lb = std::size (b);

        const auto res = strncmp (
            std::data (a),
            std::data (b),
            min (la, lb)
        );

        return res < 0 || (res == 0 && la < lb);
    }
}


namespace cruft::debug {
    ///////////////////////////////////////////////////////////////////////////
    template <typename IteratorT, typename ...ArgsT>
    struct validator<
        view<IteratorT>,
        ArgsT&&...
    > {
        static bool
        is_valid (view<IteratorT> data, ArgsT &&...args)
        {
            return std::all_of (
                std::begin (data),
                std::end   (data),
                [&] (auto const &i)
            {
                return ::cruft::debug::is_valid (i, args...);
            });
        }
    };
};