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From:	arnold@cc.gatech.edu (Arnold Robbins)
Message-Id: <9209301833.AA01662@penfold.cc.gatech.edu>
Date:	Wed, 30 Sep 1992 14:33:15 -0400
In-Reply-To: Byron Rakitzis's 150-line message on Sep 28,  4:42pm
X-Ultrix: Just Say NO!
X-Important-Saying: Premature Optimization Is The Root Of All Evil.
X-Mailer: Mail User's Shell (7.2.3 5/22/91)
To:	byron@netapp.com (Byron Rakitzis)
Subject: Re: recent rc hacks
Cc:	rc@hawkwind.utcs.toronto.edu, david@cs.dal.ca

> Date: 	Mon, 28 Sep 1992 16:42:10 -0400
> From: byron@netapp.com (Byron Rakitzis)
> To: rc@hawkwind.utcs.toronto.edu
> Subject: recent rc hacks
> 
> Anyway, before I talk more about "tac", let me quote the "onlyfirst"
> filter here:
> 
> #!/usr/local/bin/gawk -f
> {
> 	if (hash[$0] == 0) {
> 		hash[$0] = 1
> 		strings[total++] = $0
> 	}
> }
> 
> END {
> 	for (i = 0; i < total; i++)
> 		print strings[i]
> }

This would probably be better written as

	{
		if (! ($0 in strings))
			strings[$0]++
	}

	END {
		for (i in strings)
			print i
	}

You could modify the print statement in the END block to print a count
of commands or whatever other info you might want.

> Anyway, I didn't realize tail -r was broken until very recently (it's
> actually documented under BUGS in the Sun man pages --- oops). My
> first pass was an awk program, but the performance was terrible.
> (I wonder if anyone maintaining gawk might care to see why it's
> so expensive to append elements to an array?)

Arrays are hash tables.  The current version uses a fixed size set
of buckets.  You probably used something like

	{ a[NR] = $0 }
	END { for (i = NR; i >= 1; i++) print a[i] }

to do tac -- gawk has to malloc storage to hold your whole file, and
also has to do lots of hash chain following.  I also expect that the
hash function isn't great on purely numeric strings.  I'd be curious what
the performance of this program was compared to SunOS nawk or mawk.

Arnold