namespace CGAL {

/*!
\ingroup kernel_classes2

An object `r` of the data type `Ray_2` is a directed 
straight ray in the two-dimensional Euclidean plane \f$ \E^2\f$. It starts 
in a point called the <I>source</I> of `r` and goes to infinity. 

\sa `Kernel::Ray_2` 

*/
template< typename Kernel >
class Ray_2 {
public:

/// \name Creation 
/// @{

/*!
introduces a ray `r` 
with source `p` and passing through point `q`. 
*/ 
Ray_2(const Point_2<Kernel> &p, const Point_2<Kernel>&q); 

/*!
introduces a ray `r` starting at source `p` with 
direction `d`. 
*/ 
Ray_2(const Point_2<Kernel> &p, const Direction_2<Kernel> &d); 

/*!
introduces a ray `r` starting at source `p` with 
the direction of `v`. 
*/ 
Ray_2(const Point_2<Kernel> &p, const Vector_2<Kernel> &v); 

/*!
introduces a ray `r` starting at source `p` with 
the same direction as `l`. 
*/ 
Ray_2(const Point_2<Kernel> &p, const Line_2<Kernel> &l); 

/// @} 

/// \name Operations 
/// @{

/*!
Test for equality: two rays are equal, iff they have the same 
source and the same direction. 
*/ 
bool operator==(const Ray_2<Kernel> &h) const; 

/*!
Test for inequality. 
*/ 
bool operator!=(const Ray_2<Kernel> &h) const; 

/*!
returns the source of `r`. 
*/ 
Point_2<Kernel> source() const; 

/*!
returns a point on `r`. `point(0)` is the source, 
`point(i)`, with `i>0`, is different from the 
source. \pre \f$ i \geq0\f$. 
*/ 
Point_2<Kernel> point(int i) const; 

/*!
returns the direction of `r`. 
*/ 
Direction_2<Kernel> direction() const; 

/*!
returns a vector giving the direction of `r`. 
*/ 
Vector_2<Kernel> to_vector() const; 

/*!
returns the line supporting `r` which has the same direction. 
*/ 
Line_2<Kernel> supporting_line() const; 

/*!
returns the ray with the same source and the opposite direction. 
*/ 
Ray_2<Kernel> opposite() const; 

/// @} 

/// \name Predicates 
/// @{

/*!
ray `r` is degenerate, if the source and the second defining 
point fall together (that is if the direction is degenerate). 
*/ 
bool is_degenerate() const; 

/*!

*/ 
bool is_horizontal() const; 

/*!

*/ 
bool is_vertical() const; 

/*!
A point is on `r`, iff it is equal to the source 
of `r`, or if it is in the interior of `r`. 
*/ 
bool has_on(const Point_2<Kernel> &p) const; 

/*!
checks if point `p` is on `r`. This function is faster 
than function `has_on()` if the precondition 
checking is disabled. 
\pre `p` is on the supporting line of `r`. 
*/ 
bool collinear_has_on(const Point_2<Kernel> &p) const; 

/// @} 

/// \name Miscellaneous 
/// @{

/*!
returns the ray obtained by applying `t` on the source 
and on the direction of `r`. 
*/ 
Ray_2<Kernel> transform(const Aff_transformation_2<Kernel> &t) const; 

/// @}

}; /* end Ray_2 */
} /* end namespace CGAL */