#include <enquire.h>
Inheritance diagram for Xapian::TradWeight:
Public Member Functions | |
| TradWeight (double k) | |
| Construct a TradWeight. | |
| TradWeight * | clone () const |
| Return a new weight object of this type. | |
| std::string | name () const |
| Name of the weighting scheme. | |
| std::string | serialise () const |
| Serialise object parameters into a string. | |
| TradWeight * | unserialise (const std::string &s) const |
| Create object given string serialisation returned by serialise(). | |
| Xapian::weight | get_sumpart (Xapian::termcount wdf, Xapian::doclength len) const |
| Get a weight which is part of the sum over terms being performed. | |
| Xapian::weight | get_maxpart () const |
| Gets the maximum value that get_sumpart() may return. | |
| Xapian::weight | get_sumextra (Xapian::doclength len) const |
| Get an extra weight for a document to add to the sum calculated over the query terms. | |
| Xapian::weight | get_maxextra () const |
| Gets the maximum value that get_sumextra() may return. | |
| bool | get_sumpart_needs_doclength () const |
| return false if the weight object doesn't need doclength | |
This class implements the Traditional Probabilistic Weighting scheme, as described by the early papers on Probabilistic Retrieval. BM25 generally gives better results.
The Traditional weighting scheme formula is
![\[ \sum_{t}\frac{f_{t,d}}{k.L_{d}+f_{t,d}}.w_{t} \]](form_10.png)
where
is the termweight of term t
is the within document frequency of term t in document d
is the normalised length of document d
is a user specifiable parameterTradWeight(k) is equivalent to BM25Weight(k, 0, 0, 1, 0), except that the latter returns weights (k+1) times larger.
| Xapian::TradWeight::TradWeight | ( | double | k | ) | [inline, explicit] |
Construct a TradWeight.
| k | parameter governing the importance of within document frequency and document length - any non-negative number (0 meaning to ignore wdf and doc length when calculating weights). Default is 1. |
| TradWeight* Xapian::TradWeight::clone | ( | ) | const [virtual] |
Return a new weight object of this type.
A subclass called FooWeight taking parameters param1 and param2 should implement this as:
virtual FooWeight * clone() const { return new FooWeight(param1, param2); }
Implements Xapian::Weight.
| std::string Xapian::TradWeight::name | ( | ) | const [virtual] |
Name of the weighting scheme.
If the subclass is called FooWeight, this should return "Foo".
Implements Xapian::Weight.
| std::string Xapian::TradWeight::serialise | ( | ) | const [virtual] |
| TradWeight* Xapian::TradWeight::unserialise | ( | const std::string & | s | ) | const [virtual] |
| Xapian::weight Xapian::TradWeight::get_sumpart | ( | Xapian::termcount | wdf, | |
| Xapian::doclength | len | |||
| ) | const [virtual] |
Get a weight which is part of the sum over terms being performed.
This returns a weight for a given term and document. These weights are summed to give a total weight for the document.
| wdf | the within document frequency of the term. | |
| len | the (unnormalised) document length. |
Implements Xapian::Weight.
| Xapian::weight Xapian::TradWeight::get_maxpart | ( | ) | const [virtual] |
Gets the maximum value that get_sumpart() may return.
This is used in optimising searches, by having the postlist tree decay appropriately when parts of it can have limited, or no, further effect.
Implements Xapian::Weight.
| Xapian::weight Xapian::TradWeight::get_sumextra | ( | Xapian::doclength | len | ) | const [virtual] |
Get an extra weight for a document to add to the sum calculated over the query terms.
This returns a weight for a given document, and is used by some weighting schemes to account for influence such as document length.
| len | the (unnormalised) document length. |
Implements Xapian::Weight.
| Xapian::weight Xapian::TradWeight::get_maxextra | ( | ) | const [virtual] |
Gets the maximum value that get_sumextra() may return.
This is used in optimising searches.
Implements Xapian::Weight.
| bool Xapian::TradWeight::get_sumpart_needs_doclength | ( | ) | const [virtual] |