use strict;
use Benchmark qw(timethese);

# Input data

my @fields = (
    [ name1 => { start =>  0, len => 10 } ],
    [ name2 => { start =>  8, len =>  3 } ],
    [ name3 => { start => 11, len => 14 } ],
);

# This was the first idea I had.
my $pos = 0;
my $template_1;

for my $field (@fields) {
    my $mismatch = $field->[1]{start} - $pos;
    if ($mismatch > 0) {
        $template_1 .= "x$mismatch";
    } elsif ($mismatch < 0) {
        $mismatch = 0 - $mismatch;
        $template_1 .= "X$mismatch";
    }
    $template_1 .= "A" . $field->[1]{len};
    $pos = $field->[1]{start} + $field->[1]{len};
}

# This was the idea I had very shortly after, while looking at -f pack
my $template_2 = join '', map { "\@$_->[1]{start}A$_->[1]{len}" } @fie
+lds;

# Benchmark it

print "t1: $template_1\nt2: $template_2\n";
my $str = "0123456789abcdefghijklmnopqrstuvwxyz";

timethese( 3_000_000, {
        t1 => sub {
            my @a = unpack ($template_1, $str)
        },
        t2 => sub {
            my @a = unpack ($template_2, $str)
        }
    }
);

# Templates and Benchmark results
# t1: A10X2A3A14
# t2: @0A10@8A3@11A14
# Benchmark: timing 3000000 iterations of t1, t2...
# t1:  8 wallclock secs ( 8.82 usr +  0.02 sys =  8.84 CPU) @ 339366.5
+2/s (n=3000000)
# t2:  9 wallclock secs ( 9.46 usr +  0.00 sys =  9.46 CPU) @ 317124.7
+4/s (n=3000000)
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