marvin at rectangular
Apr 3, 2007, 2:34 PM
Post #17 of 22
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On Apr 3, 2007, at 9:10 AM, Chris Nandor wrote:
> We need to cache the BitVector of a Filter on a per-reader basis.
> That
> BitVector should be destroyed when the Reader is.
>
> A Filter generally does not change, but it can, particularly in the
> case of
> a PolyFilter. When the Filter changes, the BitVector should no
> longer be
> used, and, ideally, it should be cleaned up, as it is not likely to
> be used
> again.
>
>
> That's basically it. Then there's implementation.
Well stated.
> [.NB: we still need a way to generate hash_code for PolyFilter.]
That could be as simple as adding the hashcodes of all component
filters, plus some extra factor for each logic.
> We could also use weak references. I generally think weak refs are
> a hack,
> but then again, the above is also turning into a hack. :)
Weak refs are a hack, but sometimes they enable elegant solutions.
Tell me what you think of the following, which doesn't even involve
IndexReader:
# Filter.pm
# Store a cached BitVector associated with a particular reader.
Store a weak
# reference to the Reader as an indicator of cache validity.
sub store_cached_bits {
my ( $self, $reader, $bits ) = @_;
my $pair = { reader => $reader, bits => $bits };
weaken( $pair->{reader} );
$self->{cached_bits}{$reader} = $pair;
}
# Retrived a cached BitVector associated with a particular reader.
As a side
# effect, clear away any BitVectors which are no longer valid because
their
# readers have gone away.
sub fetch_cached_bits {
my ( $self, $reader ) = @_;
my $cached_bits = $self->{cached_bits};
# sweep
while ( my ( $stringified, $pair ) = each %$cached_bits ) {
# if weak ref has decomposed into undef, reader is gone...
so delete
next if defined $pair->{reader};
delete $cached_bits->{$stringified};
}
# fetch
my $pair = $cached_bits->{$reader};
return $pair->{bits} if defined $pair;
return;
}
> However, apart from creating a dispose() method, we would also need to
> uniquely identify the *Reader*, instead of the Filter, unless we
> think the
> current method of using a stringified reference is sufficient.
Stringification is sufficient for the above, because the undef-ness
of the decomposed weak reference protects us against a second reader
impersonating one that's dead and gone.
FWIW, we might consider adding this to KinoSearch::Util::Class:
sub hash_code { refaddr(shift) }
Then we could use $reader->hash_code in place of "$reader", above.
But it doesn't really matter for this particular application.
Marvin Humphrey
Rectangular Research
http://www.rectangular.com/
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