Re: Can unpack add zero bytes before converting?

You can borrow missing (to count of 8) bytes from adjacent value and then throw them away with post-processing, applying a mask. Looks like it's faster to stay "numeric" at all stages if possible. The "unpack-pad-pack-unpack", (tybalt89's solution) is somewhere in the middle in terms of performance. Not that you said speed is the goal, but anyway:

use strict;
use warnings;
no warnings 'portable';
use Benchmark 'cmpthese';

my $bytes_per_value = 5;
my $count = 1_000;

my $value = 0xf_dead_beef_4;
my $bin_value = substr (pack ('Q', $value), 0, $bytes_per_value);
my $buffer = $bin_value x $count;

my $fmt = sprintf "(b%d)*", $bytes_per_value << 3;

my $pad_len = 8 - $bytes_per_value;
my $mask = ~ 0 >> $pad_len * 8;

cmpthese -1, {
    strings => sub {
        my @values = map { oct '0b'.reverse ($_)} 
            unpack ($fmt, $buffer);
        return \@values;
    },
    numbers => sub {
        my @values = unpack "(QX$pad_len)$count", 
            $buffer . "\0" x $pad_len;
        $_ &= $mask for @values;
        return \@values;
    },
};        

__END__

           Rate strings numbers
strings  2386/s      --    -77%
numbers 10274/s    331%      --
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Comment on Re: Can unpack add zero bytes before converting? Download Code

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How does backing up several bytes work with unpack? (a bug?) (was: Re^2: Can unpack add zero bytes before converting?)
by vr (Curate) on Sep 13, 2021 at 20:40 UTC

I had to explicitly include a count in template: "(QX$pad_len)$count", instead of simply "(QX$pad_len)*", because otherwise @values array would result in a very puzzling length of 2501 items instead of 1000 i.e. $count.

I suspect there's special code somewhere to prevent ((un?)documented(?) case of) endless loops with '(CX)*' or '(vX2)*' or similar, but still:

say for unpack '(VX2)*', "\1\1\1\1";    # why 2 items?
say for unpack '(VX3)*', "\1\1\1\1";

'X' outside of string in unpack         # how's that?
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and why 2501 items with '(QX3)*' template to begin with?

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