! Copyright (C) 2013 Your name.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel ;
IN: dragon

CONSTANT: d 15

: zig ( p1 p2 -- p1 p3 p2 ) ;
: zag ( p1 p2 -- p1 p3 p2 ) ;

: is-zero? ( p1 p2 p3 d -- ? p1 p2 p3 d )
    [ [ [ 0 = ]
          keep swap ]
        dip swap ]
      dip swap ;

: (dragon) ( p1 p2 p3 d -- seq )
    is-zero? [
        2array { } swap append
    ] [
        [ 1 - ] 3dip
        [ zig (dragon) ] 4keep
          zag (dragon)
    ] if
    ;

: dragon ( -- seq )
    { }
    d { 0 0 } { 400 400 }
    zig (dragon)
    append
    ;