PROGRAM koch
! improved output 8/9/95 by computing aspect ratio
! generate triadic Koch curve using recursion
LIBRARY "csgraphics"
CALL initial(x1,y1,x2,y2,n)
! draw Koch curve for different number of iterations
DO
   LET k = 0
   CALL draw(x1,y1,x2,y2,n)
   DO                             ! pause until any key is hit
      GET KEY k
   LOOP UNTIL k <> 0
   LET n = n + 1                  ! number of stages of generation
   CLEAR
LOOP UNTIL k = ord("s")
END

SUB initial(x1,y1,x2,y2,n)
    LET n = 0                     ! number of iterations
    LET x1 = 1                    ! coordinates at left end of line
    LET y1 = 5                    ! arbitrary units
    LET x2 = 10                   ! coordinates at right end of line
    LET y2 = 5
    CALL compute_aspect_ratio(8,xwin,ywin)
    SET WINDOW 0,xwin,0,ywin
    ASK MAX COLOR mc
    IF mc > 2 then SET COLOR "blue"
END SUB

SUB draw(x1,y1,x2,y2,n)
    IF n > 0 then
       LET dx = (x2 - x1)/3
       LET dy = (y2 - y1)/3
       LET x1n = x1 + dx
       LET y1n = y1 + dy
       LET x2n = x1 + 2*dx
       LET y2n = y1 + 2*dy
       ! rotate line segment (dx,dy) by 60 degs and add to (x1n,y1n)
       LET xmid = 0.5*dx - 0.866*dy + x1n
       LET ymid = 0.5*dy + 0.866*dx + y1n
       CALL draw(x1,y1,x1n,y1n,n-1)
       CALL draw(x1n,y1n,xmid,ymid,n-1)
       CALL draw(xmid,ymid,x2n,y2n,n-1)
       CALL draw(x2n,y2n,x2,y2,n-1)
    ELSE
       PLOT LINES: x1,y1;x2,y2
    END IF
END SUB