/*
 * 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 2021, Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include <iterator>
#include <tuple>
#include <type_traits>
#include <compare>


///////////////////////////////////////////////////////////////////////////////
namespace cruft::iterator {
    /// An iterator adapter that returns the tuple_element at IndexV for the
    /// underlying iterator when dereferenced.
    template <std::size_t IndexV, typename IteratorT>
    class tuple_picker {
    public:
        using iterator_category = typename std::iterator_traits<IteratorT>::iterator_category;
        using value_type = std::tuple_element_t<
            IndexV,
            // Ideally we'd use iterator_traits::value_type but that removes
            // constness from value_type for pointers to const, and we need to
            // retain const semantics to support some existing cases (like
            // map::keys queries).
            std::remove_reference_t<decltype(*std::declval<IteratorT> ())>
        >;

        using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
        using pointer = value_type*;
        using reference = value_type&;

        tuple_picker (IteratorT _cursor)
            : m_cursor (_cursor)
        { ; }

        reference operator* () { return std::get<IndexV> (*m_cursor); }
        tuple_picker& operator++ () { ++m_cursor; return *this; }

        auto operator<=> (tuple_picker const &rhs) const = default;

    private:
        IteratorT m_cursor;
    };


    /// A convenience constructor for tuple_picker.
    ///
    /// Provided in-lieu of a user deduction guide so that partial template
    /// parameters can be provided. ie, make_tuple_picker<1> (iter).
    template <std::size_t IndexV, typename IteratorT>
    auto
    make_tuple_picker (IteratorT &&iterator)
    {
        return tuple_picker<
            IndexV,
            std::remove_reference_t<IteratorT>
        > (
            std::forward<IteratorT> (iterator)
        );
    }
}