How can I get the number of pack`ed items in Perl without actually unpacking?

Question

I have a string of packed values which was created sequentially using something like:

while (...) {
 ...
 $packed .= pack( 'L', $val );
}

In another program, after I load $packed, I wish to find out how many values were actually packed. I know how to do that after unpacking:

my @vals = unpack( 'L*', $packed );
print scalar(@vals);

But is it really necessary? If I only care about the number of values, can I do better and skip the unpacking?

Answer 1

Since you know the size of a packed value (L is an unsigned 32-bit int, or 4 bytes), just divide the length by the size:

my $count = length($packed) / 4;

If you don't want to hard code the size, you could also pack a sample value to calculate it. (Note that Perl's compile-time constant folding doesn't work with pack, at least not with 5.10.1, so you'd want to do that calculation only once.)

my $size = length(pack('L', 0));
...
my $count = length($packed) / $size;

Answer 2

Since L is merely groups of 32 bit values, you can simply count the number of bytes and divide by 4.

Answer 3

use constant LLength => length(pack("L", 0));
...
print length($packed)/LLength;

Check if LLength is really constant:

$ perl -MO=Deparse,-d -e'use constant L => length(pack("L", 0));print L, "\n";'
sub L () { 4 }
use constant ("L", length pack("L", 0));
print 4, "\n";
-e syntax OK