Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!yale!mintaka!spdcc!iecc!compilers-sender
From: plx!plxsun!evan@Sun.COM (Evan Bigall)
Newsgroups: comp.compilers
Subject: Re: Hash specifics
Keywords: design
Message-ID: <EVAN.90Dec15155336@plxsun.uucp>
Date: 15 Dec 90 20:53:36 GMT
References: <9012111512.AA14873@iecc.cambridge.ma.us>
	<1990Dec12.221425.569@zoo.toronto.edu> <2980@lupine.NCD.COM>
Sender: compilers-sender@iecc.cambridge.ma.us
Reply-To: plx!plxsun!evan@Sun.COM (Evan Bigall)
Organization: Plexus Software Inc.
Lines: 35
Approved: compilers@iecc.cambridge.ma.us

In article <2980@lupine.NCD.COM> rfg@ncd.com (Ron Guilmette) writes:
   Henry's point is well taken, however there are *some* places where I think
   that it is probably worth the effort to try to find a *perfect* hash
   function.  Keyword tables in compilers and opcode tables in assemblers
   come to mind.

The time when I always find myself looking for a perfect hash function, is
when I am parsing long keyword options (ie: -ansi etc...) or some other case
when I have a small set of possibilities but want to avoid using:

	if (!strcmp("hello", arg))
	  {}
	else if (!strcmp("there", arg))
	  {}
	else if (!strcmp("world", arg))
	  {}
        else 
	  error;

Maybe I'm just being silly, but the above code, and the tiny hand crafted
lexers I usually end up both make me gag.

Because these situations often come up several times within one program I'd
much rather put in a quick perfect hash, then have to put in the code to deal
with collisions. 

I have one program where I ended up having 15 or so different instances of
this situation.  I ended up giving them start conditions and just calling
the (already terribly complicated) flex lexer, but that seems like overkill
for everyday use.

/Evan
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