#ifndef ALT_HPP_
#define ALT_HPP_

#include "symbol.hpp"

namespace detail
{

template< class StaticVisitor , class ...Symbols >
class apply_visitor_impl;


template< class ...Symbols >
class alt_impl;
template<>
class alt_impl<>
    : public symbol
{
public:
    template< class CharType >
    constexpr alt_impl( const_string< CharType > str , std::size_t begin = 0 , std::size_t end = invalid_index )
        : symbol( end )
    {
    }
    template< class StaticVisitor >
    constexpr typename StaticVisitor::result_type apply_visitor( StaticVisitor visitor )
    {
        return visitor.null();
    }
};
template< class Head , class ...Tail >
class alt_impl< Head , Tail... >
    : public alt_impl< Tail... >
{
public:
    template< class CharType >
    constexpr alt_impl( const_string< CharType > str , std::size_t begin = 0 , std::size_t end = symbol::invalid_index )
        : alt_impl< Tail... >
        (
            str ,
            begin ,
            end != symbol::invalid_index ?
                end :
                Head( str , begin ).valid() ?
                    Head( str , begin ).end() :
                    end
        ) ,
        obj( Head( str , begin ) )
    {
    }
    template< class StaticVisitor >
    constexpr typename StaticVisitor::result_type apply_visitor( StaticVisitor visitor )
    {
        return
        obj.valid() ?
            visitor( obj ) :
            alt_impl< Tail... >::apply_visitor( visitor );
    }
private:
public:
    Head obj;
};

template< class StaticVisitor >
struct apply_visitor_impl< StaticVisitor >
{
    static constexpr typename StaticVisitor::result_type impl( StaticVisitor , alt_impl<> )
    {
        return typename StaticVisitor::result_type();
    }
};
template< class StaticVisitor , class Head , class ...Tail >
struct apply_visitor_impl< StaticVisitor , Head , Tail... >
{
    static constexpr typename StaticVisitor::result_type impl( StaticVisitor visitor , alt_impl< Head , Tail... > a )
    {
        return
        a.obj.valid() ?
            visitor( a.obj ) :
            apply_visitor_impl< StaticVisitor , Tail... >::impl( visitor , a );
    }
};

}

template< class ...Symbols >
class alt
    : public detail::alt_impl< Symbols... >
{
public:
    template< class CharType >
    constexpr alt( const_string< CharType > str , std::size_t begin = 0 )
        : detail::alt_impl< Symbols... >( str , begin )
    {
    }
    template< class StaticVisitor >
    constexpr typename StaticVisitor::result_type apply_visitor( StaticVisitor visitor )
    {
        return detail::apply_visitor_impl< StaticVisitor , Symbols... >::impl( visitor , *this );
    }
};

template< class StaticVisitor , class ...Symbols >
constexpr typename StaticVisitor::result_type apply_visitor( StaticVisitor visitor , alt< Symbols... > a )
{
    return detail::apply_visitor_impl< StaticVisitor , Symbols... >::impl( visitor , a );
}

#endif