# FAQ 4.53 How do I manipulate arrays of bits?

Discussion in 'Perl Misc' started by PerlFAQ Server, Mar 24, 2011.

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4.53: How do I manipulate arrays of bits?

Use "pack()" and "unpack()", or else "vec()" and the bitwise operations.

For example, you don't have to store individual bits in an array (which
would mean that you're wasting a lot of space). To convert an array of
bits to a string, use "vec()" to set the right bits. This sets \$vec to
have bit N set only if \$ints[N] was set:

@ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
\$vec = '';
foreach( 0 .. \$#ints ) {
vec(\$vec,\$_,1) = 1 if \$ints[\$_];
}

The string \$vec only takes up as many bits as it needs. For instance, if
you had 16 entries in @ints, \$vec only needs two bytes to store them
(not counting the scalar variable overhead).

Here's how, given a vector in \$vec, you can get those bits into your
@ints array:

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;
}

This method gets faster the more sparse the bit vector is. (Courtesy of
Tim Bunce and Winfried Koenig.)

You can make the while loop a lot shorter with this suggestion from
Benjamin Goldberg:

while(\$vec =~ /[^\0]+/g ) {
push @ints, grep vec(\$vec, \$_, 1), \$-[0] * 8 .. \$+[0] * 8;
}

Or use the CPAN module "Bit::Vector":

\$vector = Bit::Vector->new(\$num_of_bits);
\$vector->Index_List_Store(@ints);

"Bit::Vector" provides efficient methods for bit vector, sets of small
integers and "big int" math.

Here's a more extensive illustration using vec():

# vec demo
\$vector = "\xff\x0f\xef\xfe";
print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", \$vector), "\n";
\$is_set = vec(\$vector, 23, 1);
print "Its 23rd bit is ", \$is_set ? "set" : "clear", ".\n";
pvec(\$vector);

set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);

set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);

set_vec(0,32,17);
set_vec(1,32,17);

sub set_vec {
my (\$offset, \$width, \$value) = @_;
my \$vector = '';
vec(\$vector, \$offset, \$width) = \$value;
print "offset=\$offset width=\$width value=\$value\n";
pvec(\$vector);
}

sub pvec {
my \$vector = shift;
my \$bits = unpack("b*", \$vector);
my \$i = 0;
my \$BASE = 8;

print "vector length in bytes: ", length(\$vector), "\n";
@bytes = unpack("A8" x length(\$vector), \$bits);
print "bits are: @bytes\n\n";
}

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PerlFAQ Server, Mar 24, 2011