perlfaq nodetype
faq_monk
<P>
Use
<CODE>pack()</CODE> and
<CODE>unpack(),</CODE> or else
<CODE>vec()</CODE> and the bitwise operations.
<P>
For example, this sets <U>$vec</U> to have bit
<FONT SIZE=-1>N</FONT> set if $ints[N] was set:
<P>
<PRE> $vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
</PRE>
<P>
And here's how, given a vector in $vec, you can get those bits into your
<CODE>@ints</CODE> array:
<P>
<PRE> sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/\0// / length $vec > 0.95) {
use integer;
my $i;
# This method is faster with mostly null-bytes
while($vec =~ /[^\0]/g ) {
$i = -9 + 8 * pos $vec;
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
}
} else {
# This method is a fast general algorithm
use integer;
my $bits = unpack "b*", $vec;
push @ints, 0 if $bits =~ s/^(\d)// && $1;
push @ints, pos $bits while($bits =~ /1/g);
}
return \@ints;
}
</PRE>
<P>
This method gets faster the more sparse the bit vector is. (Courtesy of Tim
Bunce and Winfried Koenig.)
<P>