Extraction of bits using unpack

[pip]

Hi all,

I am trying to extract some data from a binary file using unpack. All
is well where I have to extract simple data types such as shorts,
strings and ints but then I reach this in the docs I am using:

Here is the way a date is represented:

MSB LSB
__________________________________________________________
| year | Month | Day | Time |
| 12 bits | 4 bits | 5 bits | 11 bits |
|__________|____________|___________|____________________|

I have played with the hex string operator in unpack with little
joy. I also had a go at it with vec() but got nowhere.

Could someone please point me in the right direction for extracting
this data please?

Thanks,

pip

P.S if it makes a difference I am pulling the file from an x86 based
system and parsing it on an x86 system.

 

[Anno Siegel]

Hi all,

I am trying to extract some data from a binary file using unpack. All
is well where I have to extract simple data types such as shorts,
strings and ints but then I reach this in the docs I am using:

Here is the way a date is represented:

MSB LSB
__________________________________________________________
| year | Month | Day | Time |
| 12 bits | 4 bits | 5 bits | 11 bits |
|__________|____________|___________|____________________|

I have played with the hex string operator in unpack with little
joy. I also had a go at it with vec() but got nowhere.

Could someone please point me in the right direction for extracting
this data please?

You want the usual shifting and masking of integers, it seems. Get
the four bytes into an integer $x (unpack), and apply

my( $year, $month, $day, $time) = bit_split( $x, 12, 4, 5, 11);

with

sub bit_split {
my $bits = shift;
my @parts;
for ( reverse @_ ) {
my $mask = ( 1 << $_) - 1;
unshift @parts, $mask & $bits;
$bits >>= $_;
}
@parts;
}

Anno

 

[pip]

Anno,

Hi all,

I am trying to extract some data from a binary file using unpack. All
is well where I have to extract simple data types such as shorts,
strings and ints but then I reach this in the docs I am using:
[ ... snip ... ]

my( $year, $month, $day, $time) = bit_split( $x, 12, 4, 5, 11);

with

sub bit_split {
my $bits = shift;
my @parts;
for ( reverse @_ ) {
my $mask = ( 1 << $_) - 1;
unshift @parts, $mask & $bits;
$bits >>= $_;
}
@parts;
}

This worked a charm, I would never have worked this out without your
help. Now to go a read up on this bit shifting malarkey.

Cheers,

pip