From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <bb7edde3eee4fe672ae6c478b61d2642@quanstro.net>
From: erik quanstrom <quanstro@quanstro.net>
Date: Sat, 12 Jan 2008 14:30:07 -0500
To: 9fans@cse.psu.edu
Subject: Re: [9fans] snap checksum
In-Reply-To: <20080112190615.5E7541E8C3A@holo.morphisms.net>
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> > does anyone know if the function /sys/src/cmd/snap/take.c:/^sumr
> > can return 0 for a buffer that is not all zeros?
> > 
> > i think it can not, since for sum&1, the next sum at least
> > has 0x8000 set and otherwise if (sum&1) == 0, then either
> > sum == 0 or sum>>1 != 0.
> 
> >>		if(sum & 1)
> >>			sum = 0xffff & ((sum>>1)+*s+0x8000);
> >>		else
> >>			sum = 0xffff & ((sum>>1)+*s);
> 
> your logic does not account for the addition of *s,
> which can cause an overflow that clears the 0x8000 bit.
> ff 80 80 80 80 80 80 80 80 01.
> 

d'oh.  i must need more sleep. i forgot to add *s back in.

thanks for the historical perspective.

- erik