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Reducing the Swelling of the Phone Bill with DIF and SSED
November 17, 1981
Chuck Forsberg
Computer Development Inc
Beaverton OR
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	Lately (if not sooner) it has become obvious that there must be
a better and cheaper way to distribute software updates to changing
programs than to transmit all of the new files in their totality,
even though only a few lines in each have been changed.

	For some years the Unix differential file print program diff(1)
(the (1) refers to the section of the Unix Programmers Manual in which
it is described) has had a -e flag which provides a set of ed
commands suitable for transforming the first file to the second.

	With these tools, only an update file need be transmitted,
provided, of course, that both the sender and the receiver had
copies of the same antecdent file.

	I have written a "new" diff called dif.c which manages to operate
in the primitive CP/M environment.
The editing commands output in response to the -e option refrence sequential
lines in the source files, so they (the commands) can be executed by a
stream editor.
(The Unix diff(1) creates difference files with non-forward-sequential
commands.)

	To generate a difference file, the command is

	dif -e oldfile newfile >file.dif

	The >file.dif redirects the standard output to the file.
A + may be susbtituted for > if simultaneous console output is
desired.

	The receiver then invokes:

	ssed oldfile <file.dif >newfile

	Which will result in newfile being created identical to the
oroginal newfile.
Well, not precisely identical, but identical up to and including the
EOF (^Z) character.
The dribble after that may change, so CRCK may say they are different.
To check, compare the two files with dif.

	Unix folks with 14 character file names and modification times
stored by the filesystem have little trouble keeping the files
synchronized.
(If the antecedent files are different, there's no telling what the
output file will look like!)
For us poor CP/M folks (verrry) patiently awaiting something like
Unix to appear magically on out desktops, I propose that the revision
or revision date of the antecedent file be placed in the new file
adjacent to the new revision or date, preferably on the same line.
This way the user may easily verify that he has the correct
antecedent.

	Dif Versions 1.10 and later place hash indices of the RETAINED lines
of the antecedent file in the difference output. This allows ssed
1.10 or later to verify correctness of the antecedent file.
the new .dif files are compatible with the old ssed, but, alas, not
with Unix ed or sed.

	The array sizes in dif.c may have to be shrunk somewhat to run
on a 48k system.

	For testing, give

	dif -e filea fileb |ssed filea >filec
	dif fileb filec

	(fileb and filec should be identical)

	It ought to work if you said
	dif -e filea fileb |ssed filea |dif fileb
	and it does, with version 2.0.

	Version 2.0 of dif.c adds a -u flag which will unsqueeze filea before
comparing it to fileb.

	Thus you can say
	sq filea
	dif -eu filea.qqq fileb |ssed filea |dif fileb

	Or you can say
	dif -eu filea.qqq fileb |ssed -u filea.qqq |dif fileb
	to test dif and ssed.
(Be sure dif and ssed are exactly where you say
they are, or else pipes will be broken.)

	Restriction:  Since the BDS Standard I/O library and the Directed
I/O package are somewhat confused about translation between CP/M's cr/lf
terminated lines and **nixs' \n terminated lines, dif was written to strip
cr's from the input in order that only one cr appear on the output.
As a result, lines terminated by cr/lf, lf, and lf/cr all come out the same!
This would munge files where lf/cr has a special meaning (MBASIC continuation
lines) or where embedded cr's are used (RTTY art).

	Unix is a trademark of WECO, CP/M of Digital Research.
