// Rotates each letter in a string of text by 13 places right. Rot13
// is a simple method of encoding text. Encoding entails shifting each
// alphabetic character in a string of text right by thirteen places.
// If the new character isn't alphabetic then the shift wraps round to
// the start of the alphabet.
//
// Contributed by Derek (2010).
// Minor alterations by Thomas Larsen (2010).

option explicit

print "Rot13 is a simple method of encoding text. Encoding entails shifting"
print "each alphabetic character in a string of text right by thirteen"
print "places. If the new character isn't alphabetic then the shift wraps"
print "round to the start of the alphabet."
print
print "For example, A becomes N when shifted right by 13 places; M becomes Z"
print "when shifted right by 13 places; N becomes A when shifted right by 13"
print "places. So the Rot13 algorithm both \"encodes\" and \"decodes\" text."
print

// Main loop.
do
   line input "Enter a string of text to convert (blank to exit): " string$
   if string$ = "" exit
   print rot13$ (string$)
loop

// Rotate each character in a string of text by 13 places to the
// right, wrapping around to the start of the alphabet if necessary.
sub rot13$ (string$)
   local replace_ascii, character$, new_string$

   // Deal with each character in the string.
   for character = 1 to len (string$)
      character$ = mid$ (string$, character, 1)    // extract character
      replace_ascii = asc (character$)    // get ASCII value of character

      // If the character is upper-case...
      if character$ >= "A" and character$ <= "Z" then
         // Rotate the character 13 places to the right.
         replace_ascii = replace_ascii + 13

         // Wrap around to the start of the alphabet?
         if replace_ascii > asc ("Z") then
            replace_ascii = asc ("A") + ((replace_ascii - asc ("Z")) - 1)
         endif
      endif

      // If the character is lower-case...
      if character$ >= "a" and character$ <= "z" then
         // Rotate the character 13 places to the right.
         replace_ascii = replace_ascii + 13

         // Wrap around to the start of the alphabet?
         if replace_ascii > asc ("z") then
            replace_ascii = asc ("a") + ((replace_ascii - asc ("z")) - 1)
         endif
      endif

      // Add the character to the end of the new string.
      new_string$ = new_string$ + chr$ (replace_ascii)
   next

   // Return the new string.
   return new_string$
end sub