! Copyright (C) 2013 Your name.
! See http://factorcode.org/license.txt for BSD license.
USING: arrays kernel locals math prettyprint sequences ;

IN: dragon

:: zig ( p1 p2 -- p3 )
    p1 first2 :> ( x1 y1 )
    p2 first2 :> ( x2 y2 )
    x1 x2 + y1 y2 - + 2 / ! (x1+x2+y1-y2)/2
    x2 x1 - y1 y2 + + 2 / ! (x2-x1+y1+y2)/2
    2array ;

:: zag ( p1 p2 -- p3 )
    p1 first2 :> ( x1 y1 )
    p2 first2 :> ( x2 y2 )
    x1 x2 + y1 y2 + - 2 / ! (x1+x2-y1+y2)/2
    x1 x2 - y1 y2 + + 2 / ! (x1-x2+y1+y2)/2
    2array ;

:: (dragon) ( p1 p2 p3 d -- seq )
    d 0 = [
        p1 p2
        2array
    ] [
        p1 p1 p2 zig p2 d 1 - (dragon)
        p2 p2 p3 zig p3 d 1 - (dragon)
        append
    ] if ;

:: dragon ( -- )
    { 100 100 } :> p1
    { 356 100 } :> p2
    p1 p1 p2 zig p2 3 (dragon)
    . ;

MAIN: dragon