libcruft-util/cruft/util/adapter.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 2015-2022 Danny Robson <danny@nerdcruft.net>
 */

#include "concepts/named.hpp"

#include <cstddef>
#include <iterator>
#include <tuple>

namespace cruft::adapter {
    namespace container {
        /// An identity proxy for a referenced container.
        ///
        /// Provided for symmetry with `reverse` and others, so as to allow a
        /// common template interface.
        template <typename ContainerT>
        class identity {
        public:
            identity (ContainerT &_target)
                : m_target (_target)
            { ; }

            decltype(auto) begin (void) & { return m_target.begin (); }
            decltype(auto) end   (void) & { return m_target.end (); }

            decltype(auto) begin (void) const& { return m_target.begin (); }
            decltype(auto) end   (void) const& { return m_target.end (); }

            decltype(auto) cbegin (void) const& { return m_target.cbegin (); }
            decltype(auto) cend   (void) const& { return m_target.cend (); }

        private:
            ContainerT &m_target;
        };


        /// Creates a reversed proxy for a referenced container.
        ///
        /// The target container must remain an l-value.
        ///
        /// It is the caller's job to ensure the targeted container remains
        /// valid for the duration of the proxy's lifetime.
        template <typename T>
        class reverse {
        public:
            explicit reverse (T &_target):
                m_target (_target)
            { ; }

            auto begin (void) & { return m_target.rbegin (); }
            auto end   (void) & { return m_target.rend (); }

            auto begin (void) const& { return m_target.rbegin (); }
            auto end   (void) const& { return m_target.rend (); }

            auto cbegin (void) const& { return m_target.crbegin (); }
            auto cend   (void) const& { return m_target.crend (); }

        private:
            T &m_target;
        };


        //---------------------------------------------------------------------
        template <typename Container>
        reverse (Container&) -> reverse<Container>;


        ///////////////////////////////////////////////////////////////////////
        /// Adapter's a container to return indices of the container's data
        /// rather than the values themselves.
        ///
        /// The targeted container must be an l-value.
        ///
        /// It is the caller's job to ensure the targeted container remains
        /// valid for the duration of the proxy's lifetime.
        template <typename T>
        class indices {
        public:
            using typename T::size_type;

            explicit indices (T &_target):
                m_target (_target)
            { ; }

            size_type begin (void)& { return 0; }
            size_type end   (void)& { return m_target.size (); }

        private:
            T &m_target;
        };


        template <
            typename ContainerT,
            typename FunctionT
        >
        requires (
            std::is_invocable_v<
                FunctionT,
                typename std::iterator_traits<typename ContainerT::iterator>::value_type
            >
        )
        class transform {
        public:
            using value_type = std::invoke_result_t<
                FunctionT,
                typename std::iterator_traits<typename ContainerT::iterator>::value_type
            >;
            using pointer = void;
            using reference = void;

            using difference_type = typename std::iterator_traits<
                typename ContainerT::iterator
            >::difference_type;


            template <typename _ContainerT, typename _FunctionT>
            transform (_ContainerT &&_container, _FunctionT &&_function)
                : m_container (std::forward<_ContainerT> (_container))
                , m_function  (std::forward< _FunctionT> (_function ))
            { ; }

            auto begin (void) const { return iterator (m_container.begin (), m_function); }
            auto end   (void) const { return iterator (m_container.end   (), m_function); }

            class iterator {
            private:
                using underlying = typename ContainerT::iterator;

            public:
                // HACK: We really should be using the underlying iterator's
                // category. However it's a massive PITA to write this code
                // right now.
                using iterator_category = std::forward_iterator_tag; //typename std::iterator_traits<underlying>::iterator_category;
                using difference_type = typename std::iterator_traits<underlying>::difference_type;
                using value_type = transform::value_type;
                using pointer = void;
                using reference = void;

                iterator (
                    typename ContainerT::iterator _cursor,
                    FunctionT const &_function
                )
                    : m_cursor (_cursor)
                    , m_function (_function)
                { ; }

                decltype(auto) operator* ()
                {
                    return m_function (*m_cursor);
                }

                iterator& operator++ ()&
                {
                    ++m_cursor;
                    return *this;
                }

                bool operator== (iterator const &rhs) const noexcept
                {
                    return m_cursor == rhs.m_cursor;
                }

            private:
                typename ContainerT::iterator m_cursor;
                FunctionT const &m_function;
            };

        private:
            ContainerT m_container;
            FunctionT  m_function;

        };

        template <typename ContainerT, typename FunctionT>
        transform (ContainerT, FunctionT) -> transform<
            std::remove_cvref_t<ContainerT>,
            std::remove_cvref_t< FunctionT>
        >;
    }


    namespace iterator {
        ///////////////////////////////////////////////////////////////////////
        /// Adapt's an iterator to return the n-th element of the tuple that
        /// corresponds to the underlying iterator::value_type when
        /// dereferenced.
        template <size_t I, typename IteratorT>
        struct scalar {
        public:
            using iterator_category = typename std::iterator_traits<IteratorT>::iterator_category;
            using value_type = typename std::tuple_element<
                I,
                typename std::iterator_traits<
                    IteratorT
                >::value_type
            >::type;
            using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
            using reference  = value_type&;
            using const_reference  = value_type const&;


            explicit scalar (IteratorT _inner):
                m_inner (_inner)
            { ; }


            const_reference operator* (void) const&
            { return std::get<I> (*m_inner); }

            reference operator* (void)&
            { return std::get<I> (*m_inner); }

            bool operator== (scalar<I,IteratorT> rhs) { return m_inner == rhs.m_inner; }
            bool operator!= (scalar<I,IteratorT> rhs) { return m_inner != rhs.m_inner; }


            scalar<I,IteratorT>& operator++ (void)
            { ++m_inner; return *this; }


        private:
            IteratorT m_inner;
        };
    }
}