#COMPILE EXE
#COMPILER PBWIN 9.05
#DIM ALL

FUNCTION ROT13(BYVAL a AS STRING) AS STRING
  LOCAL p AS BYTE PTR
  LOCAL n AS BYTE, e AS BYTE
  LOCAL res AS STRING

  res = a
  p = STRPTR(res)
  n = @p
  DO WHILE n
    SELECT CASE n
      CASE 65 TO 90
        e = 90
        n += 13
      CASE 97 TO 122
        e = 122
        n += 13
      CASE ELSE
        e = 255
    END SELECT

    IF n > e THEN
      n -= 26
    END IF
    @p = n
    INCR p
    n = @p
  LOOP
FUNCTION = res
END FUNCTION

'testing:

FUNCTION PBMAIN () AS LONG
#DEBUG PRINT ROT13("abc")
#DEBUG PRINT ROT13("nop")
END FUNCTION