; ono-lines.lsp  11june2009  dmemos

( define ( vertical_line x y1 y2  )  
   ( for ( y y1 y2 ) ( point x y  )))
     
( define ( horizontal_line x1 x2 y  )
   ( for ( x x1 x2 ) ( point x y  )))

( define (normal_line  x1 y1 x2 y2 )
   ( set 'dm (div ( sub y2 y1 ) ( sub x2 x1 )))
   ( if (<=  (abs dm ) 1 )
     ( begin  ; flat line 
       ( set 'y y1 )
       ( if ( > x1 x2 ) ( set 'dm (sub dm )))
       ( for ( x x1 x2 )
         ( point x y )
         ( set 'y ( add y dm ))))
     ( begin ; steep line 
       ( set 'dm ( div 1 dm ))
       ( set 'x x1 )
       ( if ( > y1 y2 ) ( set 'dm (sub dm )))
       ( for ( y y1 y2 )
          ( point x y  )
          ( set 'x ( add x dm ))))))

( define ( line  x1 y1 x2 y2 )
     ( if ( = x1 x2 ) 
          ( if ( = y1 y2 )
                ( point x1 y1  )
                ( vertical_line x1 y1 y2  ))
          ( if ( = y1 y2 )
                ( horizontal_line x1 x2 y1 ) 
                ( normal_line x1 y1 x2 y2 )))) 

( define ( drawline plist )
    ( line ( plist 0 ) ( plist 1 ) ( plist 2 ) ( plist 3 )))

( define ( drawpath plist )
   ( for ( i 0 (- ( length plist ) 4)  2  )
      ( drawline ( i 4 plist ) )))

( define ( drawpolygon plist )
    ( drawpath plist )
    ( line ( plist -2) (plist -1 ) (plist 0 ) ( plist 1)))