game/sdlpp/sdlpp_geometry_line.hpp

#ifndef SDLPP_HPP_GEOMETRY_LINE
#define SDLPP_HPP_GEOMETRY_LINE

#include "sdlpp_common.hpp"
#include "sdlpp_vector.hpp"
#include "sdlpp_line.hpp"
#include <algorithm>
#include <iostream>

namespace SDLPP {
template < typename T >
Vec2D< T > pointProjectionOnLine( const Vec2D< T > &point,
                                  const Line< T > &line ) {
    /* from here -
     * https://stackoverflow.com/questions/849211/shortest-distance-between-a-point-and-a-line-segment
     */
    auto length_squared = line.lengthSquared();
    if ( length_squared == 0.0 )
        return point;
    auto t =
        std::max( 0.0, std::min( 1.0, ( ( point - line.getStart() ) *
                                        ( line.getEnd() - line.getStart() ) ) /
                                          length_squared ) );
    return line.getStart() + t * ( line.getEnd() - line.getStart() );
}
template < typename T >
double pointLineDistance( const Vec2D< T > &point, const Line< T > &line ) {
    return vecDistance( point, pointProjectionOnLine( point, line ) );
}
template < typename T >
int _determinant( const Line< T > &line, const Vec2D< T > &point ) {
    auto res = ( line.getEnd().getX() - line.getStart().getX() ) *
                   ( point.getY() - line.getStart().getY() ) -
               ( line.getEnd().getY() - line.getStart().getY() ) *
                   ( point.getX() - line.getStart().getX() );
    if ( res < 0 )
        return -1;
    if ( res > 0 )
        return 1;
    return 0;
}
template < typename T >
bool linesCross( const Line< T > &a, const Line< T > &b ) {
    auto det_a_s = _determinant( a, b.getStart() );
    auto det_a_e = _determinant( a, b.getEnd() );
    if ( det_a_s == det_a_e && det_a_s != 0 )
        return false;

    // det_a_s == det_a_e == 0, means that lines have the same vector,
    // we need to find out if they overlap
    if ( det_a_s == det_a_e ) {
        if ( a.getStart().getX() == a.getEnd().getX() ) {
            // vertical lines
            return a.bottomost().getY() > b.topmost().getY() &&
                   a.topmost().getY() < b.bottomost().getY();
        } else {
            // horizontal lines
            return a.leftmost().getX() < b.rightmost().getX() &&
                   a.rightmost().getX() > b.leftmost().getX();
        }
    }

    auto det_b_s = _determinant( b, a.getStart() );
    auto det_b_e = _determinant( b, a.getEnd() );
    if ( det_b_s == det_b_e )
        return false;
    return true;
}
} // namespace SDLPP

#endif