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  Tips for writing a good JavaCC lexical specification



 Tips for writing a good JavaCC lexical specification


There are many ways to write the lexical specification for a grammar. But
the performance of the generated token manager varies significantly depending
on how you do this.  Here are a few tips:



 Try to specify as many String literals as possible. These are recognized
   by a Deterministic Finite Automata (DFA), which is much faster than the Nondeterministic Finite Automata (NFA) needed to recognize other kinds
   of complex regular expressions. For example, to skip blanks/tabs/newlines,
    SKIP : { " " | "\t" | "\n" }

   is more efficient than doing
    SKIP : { < ([" ", "\t", "\n"])+ > }


   because in the first case you only have string literals, it will generate
   a DFA whereas for the second case it will generate an NFA.


 Try to use the pattern ~[] just by itself as much as possible. For example,
   doing a

    MORE : { < ~[] > }


   is better than doing

      TOKEN : { < (~[])+ > }


   of course, if your grammar dictates that one of these cannot be used, then
   you don't have a choice, but try to use < ~[] > as much as possible.

 Specify all the String literals in the order of increasing length, i.e.,
   all shorter string literals before longer ones. This will help optimizing
   the bit vectors needed for string literals.

 Try to minimize the use of lexical states. When using these, try to move
   all your complex regular expressions into a single lexical state, leaving
   others to just recognize simple string literals.

 Try to use IGNORE_CASE judiciously. Best thing to do is to set this option
   at the grammar level. If that is not possible, then try to have it set for
   *all* regular expressions in a lexical state. There is heavy performance
   penalty for setting IGNORE_CASE for some regular expressions and not for others in the
   same lexical state.

 Try to SKIP as much possible, if you don't care about certain patterns.
   Here, you have to be a bit careful about EOF. seeing an EOF after SKIP
   is fine whereas, seeing an EOF after a MORE is a lexical error.

 Try to avoid specifying lexical actions with MORE specifications. Generally
   every MORE should end up in a TOKEN (or SPECIAL_TOKEN) finally so you can
   do the action there at the TOKEN level, if it is possible.

 Also try to avoid lexical actions and lexical state changes with SKIP
   specifications (especially for single character SKIP's like " ", "\t",
   "\n" etc.). For such cases, a simple loop is generated to eat up the
   SKIP'ed single characters. So obviously, if there is a lexical action
   or state change associated with this, it is not possible to it this way.

Try to avoid having a choice of String literals for the same token, e.g.
      < NONE : "\"none\"" | "\'none\'" >


   Instead, have two different token kinds for this and use a nonterminal which
   is a choice between those choices. The above example can be written as :

        < NONE1 : "\"none\"" >
      |
        < NONE2 : "\'none'\" >


   and define a nonterminal called None() as :

      void None() : {} { <NONE1> | <NONE2> }


   This will make recognition much faster. Note however, that if the choice is
   between two complex regular expressions, it is OK to have the choice.