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From: merlyn@iWarp.intel.com (Randal L. Schwartz)
Newsgroups: comp.lang.perl
Subject: Re: flipping bits in perl
Message-ID: <1991May31.162709.7069@iWarp.intel.com>
Date: 31 May 91 16:27:09 GMT
References: <1991May30.231900.12417@com50.c2s.mn.org>
Sender: news@iWarp.intel.com
Reply-To: merlyn@iWarp.intel.com (Randal L. Schwartz)
Organization: Stonehenge; netaccess via Intel, Beaverton, Oregon, USA
Lines: 35
In-Reply-To: caa@com50.c2s.mn.org (Charles Anderson)
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In article <1991May30.231900.12417@com50.c2s.mn.org>, caa@com50 (Charles Anderson) writes:
| 
| Yesterday a coworker asked me if I could flip all of the bits in the
| font file he was using, and I decided to try using perl to do it.
| (I could easily do it in C but I'm trying to learn perl and thought
| I'd give it a go.)  I tried all sorts of things tr/\000-\377/\377-\000/
| didn't work neither did s/./~$&/g in various type of things.  I even
| tried unpacking it into an array and flipping from there but that wasn't
| very succesful either.  I but a vec() into the code (something like
| vec("blah", 0, 1) to see if the bit twiddling would start working after
| that but it didn't.  I'm stumped...anybody know how to do it.  All I
| really want is a bit negation or xor $ff.

Here's a really awfully fast program hand-tuned by Larry and me while
we were one-upping each other while writing the book.  It does exactly
what you asked for.  *How* it does it should be a useful puzzle to
figure out...

#!/usr/bin/perl
$bufsize = 16384;
vec($ones,0,8) = 255;
$ones x= $bufsize;
while(read(STDIN,$_,$bufsize)) {
	print STDOUT length != $bufsize ?
		substr($_ ^ $ones, 0, length) : $_ ^ $ones;
}

or, you can try it like this:

print pack("C*",grep(($_^=255)||1,unpack("C*",pack("H*","b58a8c8bdf9e91908b979a8ddfaf9a8d93df979e9c949a8dd3"))))
-- 
/=Randal L. Schwartz, Stonehenge Consulting Services (503)777-0095 ==========\
| on contract to Intel's iWarp project, Beaverton, Oregon, USA, Sol III      |
| merlyn@iwarp.intel.com ...!any-MX-mailer-like-uunet!iwarp.intel.com!merlyn |
\=Cute Quote: "Intel: putting the 'backward' in 'backward compatible'..."====/