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From: lwall@jpl-devvax.jpl.nasa.gov (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: flipping bits in perl
Message-ID: <1991May31.184927.167@jpl-devvax.jpl.nasa.gov>
Date: 31 May 91 18:49:27 GMT
References: <1991May30.231900.12417@com50.c2s.mn.org> <1991May31.162709.7069@iWarp.intel.com>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 40

In article <1991May31.162709.7069@iWarp.intel.com> merlyn@iWarp.intel.com (Randal L. Schwartz) writes:
: 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/

You can't give negative ranges like \377-\000.  You could construct one and
eval it, however.

: | didn't work neither did s/./~$&/g in various type of things.  I even

You'd have to say pack(C,~unpack(C,$&)), or some such.

: | 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;
: }

Note that there's a bug in vector ^ (in 4.003) that may prevent this
from working.  It might work to use the assignment operator, $_ ^= $ones,
however.

Larry