accessing bits in a file

andreyb051 has asked for the wisdom of the Perl Monks concerning the following question:

I have a file from which I read the first 3 bytes. The first 11 bits are irrelevant. Then I need to take remaining 13 bits. When the file is viewed in hex editor the first 3 bytes are 47 69 01 hex. Based on this the last 13 bits need to be 901 hex which is "01001 00000001" binary but I could not figure out how to do this.

my @z = unpack ("B11 B13", $buf);
[download]

gives "0000000100011" binary

I performed this operation by binary mathematics, but I can't use this as a solution because it is not flexible. I need a general solution, that will allow flexibility of choosing different offsets and field width easily. (In general I am parsing a bit aligned structure...) Thanks!

Comment on accessing bits in a file Download Code

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Re: accessing bits in a file by BrowserUk (Patriarch) on Apr 21, 2010 at 12:18 UTC
`sub nBitsAt{ my( $n, $offset, $bits ) = @_; return unpack "x[A$offset] A$n", unpack "B*", $bits; } print nBitsAt( 13, 11, "\x47\x69\x01" );; 0100100000001` [download]	[reply] [d/l]
Re: accessing bits in a file by almut (Canon) on Apr 21, 2010 at 15:05 UTC
`my $buf = "\x47\x69\x01"; print substr(unpack("B*", $buf), 11, 13); # 0100100000001` [download]	[reply] [d/l]
Re: accessing bits in a file by jwkrahn (Abbot) on Apr 21, 2010 at 14:53 UTC
`$ perl -le'my $buf = "\x47\x69\x01"; print unpack "B*", substr( $buf, +-2 ) & "\x1f\xff"' 0000100100000001` [download]	[reply] [d/l]
Re: accessing bits in a file by moritz (Cardinal) on Apr 21, 2010 at 12:03 UTC
Maybe vec is helpful for you?	[reply]
Re^2: accessing bits in a file by andreyb051 (Initiate) on Apr 21, 2010 at 13:08 UTC
Hi, I have tried to use vec, and it doesn't seem to solve my problem, since it doesn't return arbitrary number of bits at arbitrary offset. I can use any offset, but then I will get a single bit. Am I correct? I also tried to use: `new Parse::Binary::FixedFormat [ qw(sync:b8 tei:b1 pusi:b1 tp:b1 PID:b +13) ];` [download] This gives me 0000000001000 in PID instead of expected 0100100000001. I wonder why? Thanks, Andrey.	[reply] [d/l]
Re^3: accessing bits in a file by moritz (Cardinal) on Apr 21, 2010 at 13:22 UTC
Hi, I have tried to use vec, and it doesn't seem to solve my problem, since it doesn't return arbitrary number of bits at arbitrary offset You can call it multiple times in a loop. Perl 6 - links to (nearly) everything that is Perl 6.	[reply]