Re^4: how to unpack a C struct with length array preceding data array

[AnomalousMonk]

... the absolute positioning solution ... is not really extensible to more than 2 len/data pairs.

I disagree, with a potential caveat dependent on the exact meaning of the word 'really'. I think I have shown below that an absolute positioning solution can easily (for some definition of 'easy') be generalized to any number of data items using any data length 'type'.

But not all things that are easy are wise, and I continue to agree with you and ig that a two-step approach is almost certainly best.

[BrowserUk]

This is what I mean by "not really extensible":

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];


our $I //= -1;
our $N //= 4;
my @ls =  map int( 2+rand 10 ), 1 .. $N;
our $str = pack 'S*', @ls;
$str .= 'x' x ($_-1) . ' ' for @ls;

cmpthese $I, {
    a => q[
# line 1 "a"
        my $templ = "S$N" . join( ' ', map{
            "\@0 x[S$_] S \@0" .
            "S$_ x[S${ \( $N-$_ )}]" .
            '/x' x $_ .
            '/a'
        } 0 .. $N-1 );
        my( @d ) = unpack $templ, $str;
        shift @d for 1 .. $N;
        $I == 1 and print 'a ', join '|', @d;
    ],
    b => q[
# line 1 "b"
        my @l = unpack "S$N", $str;
        my $templ = "x[S$N]" . join( ' a', '', @l );
        my @d = unpack $templ, $str;
        $I == 1 and print 'b ', join '|', @d;
    ],
};

__END__
C:\test>junk -I=-1 -N=1
      Rate    a    b
a 128551/s   -- -59%
b 312873/s 143%   --

C:\test>junk -I=-1 -N=10
     Rate    a    b
a 19086/s   -- -67%
b 57153/s 199%   --

C:\test>junk -I=-1 -N=100
    Rate    a    b
a  894/s   -- -89%
b 7979/s 793%   --

You can clean that up by not returning the counts -- which are redundant -- but it is still doing a shit load of extra work in order to avoid two calls to unpack.