module Bifid_cipher (f, code$) {
   n=sqrt(len(code$))
   print #f,"  ";
   for i=1 to n
      print #f, i+" ";
   next
   print #f
   for i=0 to n-1
      Print #f, (i+1)+" "+STR$(mid$(code$,1+i*n, n),STRING$("@ ", 5))
   next
   tables=lambda (a$)->{
      n=sqrt(len(a$))
      a=list
      for i=1 to len(a$)
         append a,  mid$(a$,i, 1):=((i-1) mod n+1, (i-1) div n+1 )
      next
      b=list
      m=each(a)
      while m
         z=eval(m)
         append b, z#val$(1)+z#val$(0):=eval$(m!)
      end while
      =a, b
   }(code$)
   encode= lambda (a, b)->{
      =lambda a,b (mess as string) -> {
         document code$
         for n=1 to 0
            for i=1 to len(mess)
               q=mid$(mess, i,1)
               if not exist(a, q) then
                  if q="J" then q="I" else q="A"
               end if
               code$=a(q)#val$(n)
            next
         next
         document final$
         for i=1 to len(code$) step 2
            final$=b$(mid$(code$, i, 2))
         next
         = final$
      }
   }(!tables)
   decode= lambda (a, b)->{
      =lambda a,b (final as string) -> {
         document code$, mess$
         for i=1 to len(final)
            q=a(mid$(final, i, 1))
            code$=q#val$(1)+q#val$(0)
         next
         offset=len(code$) div 2
         for i=1 to offset
            mess$=b$(mid$(code$,i,1)+mid$(code$,i+offset,1))
         next
         = mess$
      }
   }(!tables)
   Print #f, encode("ATTACKATDAWN")
   Print #f, decode(encode("ATTACKATDAWN"))="ATTACKATDAWN"
   Print #f, encode(ucase$(filter$("The invasion will start on the first of January", " ")))
   Print #f, decode(encode(ucase$(filter$("The invasion will start on the first of January", " "))))
}
open "out.txt" for output as #a
Bifid_cipher a, "ABCDEFGHIKLMNOPQRSTUVWXYZ"
Bifid_cipher a, "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
Bifid_cipher a, "BGWKZQPNDSIOAXEFCLUMTHYVR"
close #a
win "out.txt"