rot13_test: procedure options(main);

    dcl (plain, encoded) char(127) varying;
    plain = 'The quick brown fox jumps over the lazy dog.';
    put skip list ('Plain text:', plain);
    encoded = rot13(plain);
    put skip list ('Encoded   :', encoded);
    put skip list ('Restored  :', rot13(encoded));

    stop;

rot13:
    procedure (s) returns (char(127) varying);
      dcl
        s char(127) varying,
        ch char(1),
        i fixed bin(15);
      do i = 1 to length(s);
        ch = substr(s,i,1);
        if (ch >= 'a' & ch <= 'm') | (ch >= 'A' & ch <= 'M')
           then ch = ascii(rank(ch) + 13);
        else if (ch >= 'n' & ch <= 'z') | (ch >= 'N' & ch <= 'Z')
           then ch = ascii(rank(ch) - 13);
        substr(s,i,1) = ch;
      end;
      return (s);
    end rot13;

end rot13_test;