// Copyright (c) 2014
// INRIA Saclay-Ile de France (France)
//
// This file is part of CGAL (www.cgal.org)
//
// $URL: https://github.com/CGAL/cgal/blob/v6.1/STL_Extension/include/CGAL/transforming_iterator.h $
// $Id: include/CGAL/transforming_iterator.h b26b07a1242 $
// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-Commercial
//
// Author(s)     : Marc Glisse

#ifndef CGAL_TRANSFORMING_ITERATOR_H
#define CGAL_TRANSFORMING_ITERATOR_H
#include <boost/iterator/iterator_adaptor.hpp>
#include <boost/mpl/or.hpp>
#include <CGAL/Default.h>
#include <utility>

// Inspired by the boost version, but more compact and
// without any iterator_category games.

namespace CGAL {
namespace internal {

// non-empty case
template<class T,bool=std::is_empty<T>::value> struct Functor_as_base {
        Functor_as_base(){}
        Functor_as_base(T const& t):f(t){}
        //template<class T2> Functor_as_base(Functor_as_base<T2> const&g):f(g.functor()){}
        T const& functor()const{return f;}
        T &      functor()     {return f;}
        private:
                T f;
};

// empty case
template<class T> struct Functor_as_base<T,true> : public T {
        Functor_as_base(){}
        Functor_as_base(T const& t):T(t){}
        //template<class T2> Functor_as_base(Functor_as_base<T2> const&g):T(g.functor()){}
        T const& functor()const{return *this;}
        T &      functor()     {return *this;}
};

template <typename Derived, typename F, typename Iter, typename Ref, typename Val>
class transforming_iterator_helper
{
        typedef std::iterator_traits<Iter> Iter_traits;
        typedef typename Iter_traits::reference Iter_ref;
        typedef typename Default::Get<Ref,
                decltype(std::declval<F>()(std::declval<Iter_ref>()))
                        >::type reference_;

        typedef typename Default::Get<Val,std::remove_cv_t<std::remove_reference_t<reference_>>>::type value_type;

        // Crappy heuristic. If we have *it that returns a Weighted_point and F that returns a reference to the Point contained in the Weighted_point it takes as argument, we do NOT want the transformed iterator to return a reference to the temporary *it. On the other hand, if *it returns an int n, and F returns a reference to array[n] it is not so good to lose the reference. This probably should be done elsewhere and should at least be made optional...
        typedef std::conditional_t<std::is_reference_v<Iter_ref> || std::is_integral_v<Iter_ref>,
                                   reference_, value_type> reference;

        public:
        typedef boost::iterator_adaptor<
                Derived,
                Iter,
                value_type,
                typename Iter_traits::iterator_category,
                reference
                        > type;
};
}

template <typename F, typename Iter, typename Ref=Default, typename Val=Default>
class transforming_iterator
: public internal::transforming_iterator_helper<transforming_iterator<F,Iter,Ref,Val>,F,Iter,Ref,Val>::type,
private internal::Functor_as_base<F>
{
        friend class boost::iterator_core_access;
        typedef typename internal::transforming_iterator_helper<transforming_iterator,F,Iter,Ref,Val>::type Base;
        typedef internal::Functor_as_base<F> Functor_base;
        typename Base::reference dereference()const{
                return functor()(*this->base_reference());
        }
        public:
        using Functor_base::functor;
        transforming_iterator(){}
        explicit transforming_iterator(Iter i,F const& f=F())
                :Base(i),Functor_base(f){}
        template<class F2,class I2,class R2,class V2>
        transforming_iterator(
                transforming_iterator<F2,I2,R2,V2> const&i,
                std::enable_if_t<std::is_convertible<I2, Iter>::value>* = 0,
                std::enable_if_t<std::is_convertible<F2, F>::value>* = 0)
                : Base(i.base()),Functor_base(i.functor()) {}

};

template <typename F, typename Iter> inline
transforming_iterator<F,Iter> make_transforming_iterator(Iter i, F const&f=F()) {
        return transforming_iterator<F,Iter>(i,f);
}

}

#endif // CGAL_TRANSFORMING_ITERATOR_H

