Omega: Missing support for newer weighting schemes

[Vivek Pal]

> No, use Xapian::Registry to find the weighting scheme from the name
> like how Weight::unserialise() does (otherwise every caller would need
> code similar to that above).

Okay, I looked into Xapian::Registry and it seems you are referring to using
the get_weighting_scheme method? (which expects a string e.g. "bm25" i.e. the
name of a weighting scheme.)

And, Weight::unserialise() looks like throwing a Xapian::UnimplementedError
error. I wonder if you meant the same method?

> The code in omega would just be:
>
>   enq.set_weighting_scheme(Xapian::Weight::parse_params(scheme));

I wonder if we could do something more like:

enq.set_weighting_scheme(Xapian::Registry::get_weighting_scheme(name).parse_params(params));

where, Xapian::Registry::get_weighting_scheme(name) returns a weighting scheme
object e.g. BM25Weight object and then calling parse_params method on that
to return a BM25Weight object now with parameters values as found in params
string.

It should work given that we first separate the "scheme" string into two
substrings namely "name" and "params" as used above. Also, we'd check if
params is not null e.g. in the case of scheme = "bm25" and then we'd just
call:

enq.set_weighting_scheme(Xapian::Registry::get_weighting_scheme(name));

Also, regarding Xapian::Weight::parse_params(scheme) -- I thought we were to
define parse_params method in each Weight subclass e.g. BM25Weight, LMWeight
etc.?

> There's probably a much better name than parse_params() though.

Yes, I agree. May be get_parameterized_weighting_scheme sound better? (if it
isn't too long to read :))

Thanks,
Vivek