Wow, I totally should have seen that!

Okay so now that mutual correctness has been established, it is time for optimality checking. It turns out that your method is about 10% faster.

I called my method unroll, and your method rollahead.

Here are the benchtests:

Benchmark: timing 100000 iterations of rollahead, unroll...
rollahead: 18.3956 wallclock secs (17.94 usr + 0.00 sys = 17.94 CPU) @ 5575.07/s (n=100000)
unroll: 22.1357 wallclock secs (20.58 usr + 0.00 sys = 20.58 CPU) @ 4859.56/s (n=100000)
                         Rate unroll rollahead
unroll           4860/s       --          -13%
rollahead     5575/s    15%            --


and the code: 
use strict;

use Benchmark ':all', ':hireswallclock';

my $x = "#@##@###@####@#####@";
my $y = reverse $x;
my $z = "$x$x$x$y$y$x$x$y$y$y$y$y$x$x$x$x";

my $r = timethese(
    100000,
    {
        unroll    => sub {
           my @x = ([unroll($x)],[unroll($y)],[unroll($z)])
        },
        rollahead => sub {
           my @x = ([rollahead($x)],[rollahead($y)],[rollahead($z)])
        },
    }
);
cmpthese($r);

sub unroll {
    my @x = $_[0] =~ /(?:^|@)((?:##|#@|[^#@])*)/g;
    for(@x){
        $_ =~ s/##/#/g;
        $_ =~ s/#@/@/g;
    } @x
}

sub rollahead {
    my $x = shift;
    $x = reverse $x;
    my @x = reverse(split/\@(?=(?:##)*(?!#))/,$x,-1);  
    for(@x){
        $_ = reverse;
        $_ =~ s/##/#/g;
        $_ =~ s/#@/@/g;
    } @x
}

[download]
Is there a monk who could explain why rollahead is faster? I was surprised considering the number of calls to reverse. I can only guess that somewhere deep inside the guts of the Perl-Regex-Beasty, that the optimizer droids are nasty hardcore with lookaround automata but can't be bothered with alternation.

In reply to Re^4: Splitting on escapable delimiter by mobiusinversion
in thread Splitting on escapable delimiter by Daryn

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