How to decode hex into separate bits in Perl? - perl

Question

A byte stored as two hex numbers contains a set of flags. I need to extract these flags, as 0's and 1's. Iterating over entries in a file with:
foreach(<>)
{
#line = split(/ /,$_);
$torR = !!((hex $line[4]) & 0x3); # bit 0 or 1
$torY = !!((hex $line[4]) & 0x4); # bit 2
$torG = !!((hex $line[4]) & 0x8); # bit 3
print "$torR,$torY,$torG\n";
}
run on data file:
796.129 [1f/01] len:7< 02 01 D5 01 8B 0A 8E
796.224 [1f/01] len:7< 02 01 D4 03 09 A9 B8
796.320 [1f/01] len:7< 00 01 D4 03 07 49 5A
796.415 [1f/01] len:7< 00 01 D4 00 11 A0 EE
796.515 [1f/01] len:7< 00 01 D4 00 00 31 4C
796.627 [1f/01] len:7< 02 01 D4 01 89 C1 FD
796.724 [1f/01] len:7< 02 01 D3 03 06 39 FD
796.820 [1f/01] len:7< 08 01 D4 03 08 40 6F
796.915 [1f/01] len:7< 08 01 D5 00 13 3D A4
797.015 [1f/01] len:7< 08 01 D4 00 00 34 04
Actual Result -
1,,
1,,
,,
,,
,,
1,,
1,,
,,1
,,1
,,1
Desired result:
1,0,0
1,0,0
0,0,0
0,0,0
0,0,0
1,0,0
1,0,0
0,0,1
0,0,1
0,0,1
Seems like 'false' gets stored as empty string instead of '0'.
Is there a neat trick to get this right at once, or do I need to convert empty strings to zeros "manually"?

Answer 1

If you want the true/false values to be numeric, you need to coerce them to be numeric:
$torR = 0 + !!((hex $line[4]) & 0x3); # bit 0 or 1
$torY = 0 + !!((hex $line[4]) & 0x4); # bit 2
$torG = 0 + !!((hex $line[4]) & 0x8); # bit 3
Keep in mind that the empty string '' is also a false value.
On the other hand, I might be inclined to write that as:
my (#ryg) = map 0 + !!((hex $line[4]) & $_), 0x3, 0x4, 0x5;
print join(', ', #ryg), "\n";
In addition, you would probably benefit from not using plain numbers in your program. Consider, for example, having a %FLAG structure that gives names to these constants, and a %COL structure that gives names to the columns you are interested in. Using the data you posted:
use Const::Fast;
const my %FLAG => (
torR => 0x3,
torY => 0x4,
torG => 0x5,
);
const my %COL => (
# ...
tor => 4,
);
while (my $line = <DATA>) {
my #line = split ' ', $line;
my %set_flags = map +($_ => 0 + !!((hex $line[$COL{tor}]) & $FLAG{$_})), qw(torR torY torG);
print join(', ', #set_flags{qw(torR torY torG)}), "\n";
}
__DATA__
796.129 [1f/01] len:7< 02 01 D5 01 8B 0A 8E
796.224 [1f/01] len:7< 02 01 D4 03 09 A9 B8
796.320 [1f/01] len:7< 00 01 D4 03 07 49 5A
796.415 [1f/01] len:7< 00 01 D4 00 11 A0 EE
796.515 [1f/01] len:7< 00 01 D4 00 00 31 4C
796.627 [1f/01] len:7< 02 01 D4 01 89 C1 FD
796.724 [1f/01] len:7< 02 01 D3 03 06 39 FD
796.820 [1f/01] len:7< 08 01 D4 03 08 40 6F
796.915 [1f/01] len:7< 08 01 D5 00 13 3D A4
797.015 [1f/01] len:7< 08 01 D4 00 00 34 04

Answer 2

I think I would use split and unpack to turn each value into an array of zeroes and ones, and then examine them individually
use strict;
use warnings 'all';
for my $val ( qw/ 02 02 00 00 00 01 01 08 08 08 / ) {
my #bits = split //, unpack 'b8', chr hex $val;
my $torR = $bits[0] || $bits[1] ? 1 : 0;
my $torY = $bits[2] ? 1 : 0;
my $torG = $bits[3] ? 1 : 0;
print "$torR,$torY,$torG\n";
}
output
1,0,0
1,0,0
0,0,0
0,0,0
0,0,0
1,0,0
1,0,0
0,0,1
0,0,1
0,0,1
Or here's a way using the Bit::Vector which produces the same result
use strict;
use warnings 'all';
use Bit::Vector;
for my $val ( qw/ 02 02 00 00 00 01 01 08 08 08 / ) {
my $vec = Bit::Vector->new_Hex(8, $val);
my $torR = $vec->Chunk_Read(2, 0) ? 1 : 0;
my $torY = $vec->Chunk_Read(1, 2) ? 1 : 0;
my $torG = $vec->Chunk_Read(1, 3) ? 1 : 0;
print "$torR,$torY,$torG\n";
}