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

Dear monks,

Is there a way to give names to bits as in a byte bit field whose every bit would have a meaning that could be set by a subroutine, like: 

  setStatus(powerOn);

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... which would set the powerOn bit in a 8-bit status byte. This bit could be the first one, bit 0. Then we could have: 

  setStatus(lightOn);

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Which would set another status bit in the same status byte while not modifying the other set/unset bits. So on so forth.

Is there some kind of enum type in Perl to do that ? Or can it be done using a hash ?

Thanks.

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Re: Giving names to bits
by Tanktalus (Canon) on Feb 23, 2007 at 03:11 UTC

    A bit of creativity with constant and vec would probably be the way I'd go about it if I wanted precisely the code you had. More likely, though, would be that I'd take strings, and convert to a number in a hash, and then use vec. More likely still, I'd just leave everything as a hash (yeah, waste a lot of memory), and then pack it all together at the last minute when I actually had to send it somewhere in binary format (if such a requirement was actually there). No, I don't concern myself with memory too much these days.

    Example 1: 

    use constant powerOn => 0;
use constant lightOn => 1;
{ my $status;
sub setStatus
{
  my $bit = shift;
  vec($status, $bit, 1) = 1;
}
sub resetStatus
{
  my $bit = shift;
  vec($status, $bit, 1) = 0;
}
sub getStatus
{
  my $bit = shift;
  vec($status, $bit, 1);
}

}

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    Example 2: 
    { my $status;
my %types = ( powerOn => 0, lightOn => 1 );
sub _bit { if (exists $types{$_[0]}) { $types{$_[0]} } else { die "Unk
+nown bit $_[0]"} }
sub setStatus
{
  my $bit = _bit(shift);
  vec($status, $bit, 1) = 1;
}
# etc.
}
# call as...
setStatus('powerOn');

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    I don't think the last one needs an example.

    Update: missing close-brace, as pointed out by fenLisesi.

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      Thanks, that works great. The only thing to do there was to initially set the status to 0 (so that vec and by ricochet logic does not complain) and then to add the actual hash set/unset to each subroutine so that it's possible to use the hash for showing the status of the individual bits easily.

      Since a hash is used, wouldn't it be possible to simply use it and let go of the $status variable i.e. when time to do so, to pack the hash 1/0 values in a byte ?

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        Serves me right... that was my last option, the one I didn't give an example for ;-)

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Re: Giving names to bits
by Zaxo (Archbishop) on Feb 23, 2007 at 03:14 UTC

    The vec function is an lvalue, as well as possessing a setter form, so you can set bitfields with it.

    Added: I was going to add an example, but Tanktalus++ beat me to it.

    After Compline,
    Zaxo

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Re: Giving names to bits
by varian (Chaplain) on Feb 23, 2007 at 09:15 UTC
    You could trick Perl a bit by passing the status identifiers as if they were subroutine names. It looks cool yet requires that you turn off strict on subs: 
    #!/usr/bin/perl

use warnings;
use strict;
no strict "subs";

my $Status=0;  # holds bitwise status

sub SetStatus{
    my (@bits) = @_;
    my %StatusBits= ( PowerOn=>1, LightOn =>2, TurboOn=>4 );
    $Status |= $StatusBits{$_} foreach (@bits);
}

SetStatus(PowerOn,TurboOn);
print "status = $Status \n";

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