// Boost.Geometry (aka GGL, Generic Geometry Library)

// Copyright (c) 2009-2012 Mateusz Loskot, London, UK.

// Use, modification and distribution is subject to the Boost Software License,
// Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#ifndef BOOST_GEOMETRY_EXTENSIONS_ARITHMETIC_CROSS_PRODUCT_HPP
#define BOOST_GEOMETRY_EXTENSIONS_ARITHMETIC_CROSS_PRODUCT_HPP


#include <cstddef>

#include <boost/concept/requires.hpp>

#include <boost/geometry/core/access.hpp>
#include <boost/geometry/geometries/concepts/point_concept.hpp>
#include <boost/geometry/util/select_coordinate_type.hpp>

namespace boost { namespace geometry
{

#ifndef DOXYGEN_NO_DETAIL
namespace detail
{

template <typename P1, typename P2, std::size_t Dimension>
struct cross_product
{
    // We define cross product only for 2d (see Wolfram) and 3d.
    // In Math, it is also well-defined for 7-dimension.
    // Generalisation of cross product to n-dimension is defined as
    // wedge product but it is not direct analogue to binary cross product.
};

template <typename P1, typename P2>
struct cross_product<P1, P2, 2>
{
    typedef P1 return_type;

    static inline return_type apply(P1 const& p1, P2 const& p2)
    {
        assert_dimension<P1, 2>();
        assert_dimension<P2, 2>();

        // For 2-dimensions, analog of the cross product U(x,y) and V(x,y) is
        // Ux * Vy - Uy * Vx
        // which is returned as 0-component (or X) of 2d vector, 1-component is undefined.
        return_type v;
        set<0>(v, get<0>(p1) * get<1>(p2) - get<1>(p1) * get<0>(p2));
        return v;
    }
};

template <typename P1, typename P2>
struct cross_product<P1, P2, 3>
{
    typedef P1 return_type;

    static inline return_type apply(P1 const& p1, P2 const& p2)
    {
        assert_dimension<P1, 3>();
        assert_dimension<P2, 3>();

        return_type v;
        set<0>(v, get<1>(p1) * get<2>(p2) - get<2>(p1) * get<1>(p2));
        set<1>(v, get<2>(p1) * get<0>(p2) - get<0>(p1) * get<2>(p2));
        set<2>(v, get<0>(p1) * get<1>(p2) - get<1>(p1) * get<0>(p2));
        return v;
    }
};

} // namespace detail
#endif // DOXYGEN_NO_DETAIL


// TODO: This is a simple draft. If relevant, it can be extended to:
// - accept vectors of different coordinate_type, but common coordinate_system
// - if vectors are of mixed 2d and 3d, lower dimension is used
// - define result_type that will generate type of vector based on:
// -- select_coordinate_type
// -- selection of lower dimension

/*!
    \brief Computes the cross product of two vector.
    \details Both vectors shall be of the same type.
             This type also determines type of result vector.
    \ingroup arithmetic
    \param p1 first vector
    \param p2 second vector
    \return the cross product vector
 */
template <typename P1, typename P2>
inline P1 cross_product(P1 const& p1, P2 const& p2)
{
    BOOST_CONCEPT_ASSERT( (concept::ConstPoint<P1>) );
    BOOST_CONCEPT_ASSERT( (concept::ConstPoint<P2>) );

    return detail::cross_product
        <
            P1, P2,
            dimension<P1>::type::value
        >::apply(p1, p2);
}

}} // namespace boost::geometry

#endif // BOOST_GEOMETRY_EXTENSIONS_ARITHMETIC_CROSS_PRODUCT_HPP
