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# Xapian::BM25Weight Class Reference

BM25 weighting scheme. More...

Inheritance diagram for Xapian::BM25Weight:

[legend]
List of all members.

## Public Member Functions
| BM25Weight (double k1_, double k2_, double k3_, double b_, double min_normlen_)
| Construct a BM25 weight.
BM25Weight * | 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.
BM25Weight * | 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

## Detailed Description

BM25 weighting scheme.

BM25 weighting options : The BM25 formula is

where

- is the termweight of term t
- is the within document frequency of term t in document d
- is the within query frequency of term t
- is the normalised length of document d
- is the size of the query
- , , and are user specified parameters

## Constructor & Destructor Documentation

Xapian::BM25Weight::BM25Weight | ( | double | k1_,
|  | double | k2_,
|  | double | k3_,
|  | double | b_,
|  | double | min_normlen_ |
| ) |  |  | `[inline]`

Construct a BM25 weight.

Parameters:

  | k1 | governs the importance of within document frequency. Must be >= 0. 0 means ignore wdf. Default is 1.
  | k2 | compensation factor for the high wdf values in large documents. Must be >= 0. 0 means no compensation. Default is 0.
  | k3 | governs the importance of within query frequency. Must be >= 0. 0 means ignore wqf. Default is 1.
  | b | Relative importance of within document frequency and document length. Must be >= 0 and <= 1. Default is 0.5.
  | min_normlen | specifies a cutoff on the minimum value that can be used for a normalised document length - smaller values will be forced up to this cutoff. This prevents very small documents getting a huge bonus weight. Default is 0.5.

## Member Function Documentation

BM25Weight* Xapian::BM25Weight::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::BM25Weight::name | ( |  | ) | const`[virtual]`

Name of the weighting scheme.

If the subclass is called FooWeight, this should return "Foo".

Implements Xapian::Weight.

std::string Xapian::BM25Weight::serialise | ( |  | ) | const`[virtual]`

Serialise object parameters into a string.

Implements Xapian::Weight.

BM25Weight* Xapian::BM25Weight::unserialise | ( | const std::string & | s | ) | const`[virtual]`

Create object given string serialisation returned by serialise().

Implements Xapian::Weight.

Xapian::weight Xapian::BM25Weight::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.

Parameters:

  | wdf | the within document frequency of the term.
  | len | the (unnormalised) document length.

Implements Xapian::Weight.

Xapian::weight Xapian::BM25Weight::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::BM25Weight::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.

Parameters:

  | len | the (unnormalised) document length.

Implements Xapian::Weight.

Xapian::weight Xapian::BM25Weight::get_maxextra | ( |  | ) | const`[virtual]`

Gets the maximum value that get_sumextra() may return.

This is used in optimising searches.

Implements Xapian::Weight.

bool Xapian::BM25Weight::get_sumpart_needs_doclength | ( |  | ) | const`[virtual]`

return false if the weight object doesn't need doclength

Reimplemented from Xapian::Weight.

The documentation for this class was generated from the following file:

- xapian/enquire.h