Re: Strange speed-increase by separating "if"s

-----Original Message-----
From: Anno Siegel
Sent:  21.07.2005 12:22

Wolfram Humann  <w.n.humann.removethis@xxxxxxxxxxx> wrote in comp.lang.perl.misc:

I have a script for processing certain eps-files. What it basically does is going through the file looking for "setgray"-lines. If it finds one, it checks if it's followed by lines matching the values in @head and then @dot (plus some coordinate-checks). If all matches, @head remains in the file while @dot is discarded. If the match fails, the file remains unchanged. Here is the script:

#!/usr/local/bin/perl -w
use strict;

my @head = (
  '^N(\s+\d+)(\s+\d+)(\s+\d+) 0 360 arc sf N$',
  '^\d+\s+slw$',
);
my @dot = (
  '^(\d+)\s+(\d+)\s+M$',
  ('^(\d+)\s+(\d+)\s+D$') x 72,
);

my @c_head = map qr/$_/, @head;
my @c_dot  = map qr/$_/, @dot;
my ($x, $y, $r);

while(<>)
{
  print;
  if(/^[0-9.]+\s+setgray\s+$/)
  {
    my ($h, $d) = (0,0);
    my $l = "";
    while(<>)
    {
      if ($head[$h])
      {
        print;
        last unless $_ =~ $c_head[$h];
        ($x, $y, $r) = ($1, $2, $3) if defined $3;
        $h++;
      }
      elsif ($dot[$d])
      {
        $l .= $_;
        if ($_ !~ /$c_dot[$d]/ or
            $1 < $x - $r - 2 or
            $1 > $x + $r + 2 or
            $2 < $y - $r - 2 or
            $2 > $y + $r + 2 )
        {
          print $l;
          last;
        }
        $d++;
      }
      else
      {
        print $l if /^\d+\s+\d+\s+D$/;
        print;
        last;
      }
    }
  }
}

I was surprised how long it took and profiled with Devel::SmallProf. This showed that most time is spent in "if (($_ !~ /$c_dot[$d]/) or...". To see if the pattern match or the comparisons took so long, I split the "if" like this:

if ($_ !~ /$c_dot[$d]/)
{
    if ($1 < $x - $r - 2 or
        $1 > $x + $r + 2 or
        $2 < $y - $r - 2 or
        $2 > $y + $r + 2 )
    {
        print $l;
        last;
    }
}



The logic of the modified part is different from the original, and
makes no sense.  You are making sure the regex *doesn't* match, and
then go on to use $1 and $2.  In the original, you use them when the
regex *does* match.  If you had warnings switched on, you'd have
noticed.

Of course you're right. I had the idea that splitting an "if" in two parts like that was obvious (when indeed it was just stupid). I do have warnings and strict on, but with the file I use, the inner "if" is never hit...
If my brain isn't totally drained already the correctly split version should be:

if ($_ =~ /$c_dot[$d]/)
{
    if ($1 < $x - $r - 2 or
        $1 > $x + $r + 2 or
        $2 < $y - $r - 2 or
        $2 > $y + $r + 2)
    {
        print $l;
        last;
    }
}
else
{
    print $l;
    last;
}

Naturally, with this the timing is more or less back to what it was. According to profiling, the four comparisons take about as much time as the pattern match -- hence the speed increase when they never executed :-)

Thanks for the help!
.

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