From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
Date: Wed, 16 Jan 2008 09:43:04 +0000
From: "Douglas A. Gwyn" <DAGwyn@null.net>
Message-ID: <478D1CF9.47DE6163@null.net>
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References: <20080112190615.5E7541E8C3A@holo.morphisms.net>
Subject: Re: [9fans] snap checksum
Topicbox-Message-UUID: 2ef1e320-ead3-11e9-9d60-3106f5b1d025

> sumr computes the same function as
> plan 9's sum -r, which is also the same function as
> seventh edition's sum.  i have always assumed the -r
> stood for "research" as in research unix, although
> the plan 9 manual describes it as "system v's sum -r" and
> other manuals describe it as the "historical bsd algorithm."
> it does seem to have appeared first in v7.

The "-r" option was added to the System V "sum", which
by default actually does a true modulo 2^16 sum of the bytes,
to obtain the Research version.  However, I think the "r"
may have been meant as a mnemonic for "rotating" rather
than "Research"; the "-r" algorithm rotates the accumulator
as the bytes are added to it.

Of course neither of these is as reliable as a polynomial-
based (feedback shift register) checksum for actual
checksumming applications.