[pacman-dev] [PATCH 2/3] pacsearch with repo-agnostic coloring

[Pierre Neidhardt]

On 14-01-16 10:13:18, Allan McRae wrote:
> On 16/01/14 02:03, Martti Kühne wrote:
> > On Wed, Jan 15, 2014 at 9:50 AM, Allan McRae <allan at archlinux.org> wrote:
> > [...]
> >>
> >>> +     # hash function (x*2+1) is completely arbitrary.
> >>> +     my $repohash = $v[0];
> >>> +     $repohash =~ s/(.)/ord($1)*2+1/ge;
> >>
> >> I have very little perl knowledge, so I have no idea what that hash is
> >> doing.  Can someone explain to me so I can see if that "hash" is reasonable.
> >>
> > 
> > Replace each character with its [0] ascii index times two plus one?
> > 'g' is group regexes, 'e' is eval expressions [1], as to utilize the
> > result of the calculation.
> > 
> > cheers!
> > mar77i
> > 
> > [0] http://perldoc.perl.org/functions/ord.html
> > [1] http://stackoverflow.com/questions/6082219/perl-regex-e-eval-modifier-with-s
> > 
> 
> Does that mean the answer can only be odd?

Ooopsy! Looks like I forgot to sum the result! Actually I didn't notice the flaw
since it still works. The reason is that the resulting numbers are so big they
are casted and rounded to float or sth equivalent. This is _terrible_ code
indeed!

In the first place this was an attempt to use Perl features for a quick,
one-line hash of strings, but since I'm not a Perl guru this may be doomed to
fail. Any better suggestion from a Perl champion?

A more traditional way to do it:

	sub hash_string {
		my $sum = 0;
		foreach my $l (split //, $_[0]) {
			$sum = $sum + 31*ord($l) + 5;
		}
		return $sum;
	}
	
    [...]	
	my $repo_hash = hash_string($v[0]);

This is not a very good hash since a permutation will yield the same
result. The following is much better

	$sum = $sum + 31*ord($l)^$pos + 7;

but do we really need this? This is just for repo names after all, the extra
exponentiation is superfluous in my opinion.

The offset (e.g. '+5') can be patched by other distributions make sure their
repo have different colors.

-- 
Pierre Neidhardt

"Now this is a totally brain damaged algorithm.  Gag me with a smurfette."
		-- P. Buhr, Computer Science 354