Xapian::BM25PlusWeight Class Reference

Xapian::Weight subclass implementing the BM25+ probabilistic formula. More...

`#include <weight.h>`

Inheritance diagram for Xapian::BM25PlusWeight:
[legend]

## Public Member Functions

| BM25PlusWeight (double k1, double k2, double k3, double b, double min_normlen, double delta)
| Construct a BM25PlusWeight.
std::string | name () const
| Return the name of this weighting scheme, e.g.
std::string | serialise () const
| Return this object's parameters serialised as a single string.
BM25PlusWeight * | unserialise (const std::string &serialised) const
| Unserialise parameters.
double | get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen, Xapian::termcount uniqterms, Xapian::termcount wdfdocmax) const
| Calculate the weight contribution for this object's term to a document.
double | get_maxpart () const
| Return an upper bound on what get_sumpart() can return for any document.
double | get_sumextra (Xapian::termcount doclen, Xapian::termcount uniqterms, Xapian::termcount wdfdocmax) const
| Calculate the term-independent weight component for a document.
double | get_maxextra () const
| Return an upper bound on what get_sumextra() can return for any document.
BM25PlusWeight * | create_from_parameters (const char *params) const
| Create from a human-readable parameter string.
Public Member Functions inherited from Xapian::Weight
| Weight ()
| Default constructor, needed by subclass constructors.
virtual | ~Weight ()
| Virtual destructor, because we have virtual methods.

## Additional Inherited Members

Static Public Member Functions inherited from Xapian::Weight
static const Weight * | create (const std::string &scheme, const Registry &reg=Registry())
| Return the appropriate weighting scheme object.
Protected Types inherited from Xapian::Weight

enum

stat_flags {
COLLECTION_SIZE = 0 , RSET_SIZE = 0 , AVERAGE_LENGTH = 4 , TERMFREQ = 1 ,
RELTERMFREQ = 1 , QUERY_LENGTH = 0 , WQF = 0 , WDF = 2 ,
DOC_LENGTH = 8 , DOC_LENGTH_MIN = 16 , DOC_LENGTH_MAX = 32 , WDF_MAX = 64 ,
COLLECTION_FREQ = 1 , UNIQUE_TERMS = 128 , TOTAL_LENGTH = 256 , WDF_DOC_MAX = 512 ,
UNIQUE_TERMS_MIN = 1024 , UNIQUE_TERMS_MAX = 2048 , DB_DOC_LENGTH_MIN = 4096 , DB_DOC_LENGTH_MAX = 8192 ,
DB_UNIQUE_TERMS_MIN = 16384 , DB_UNIQUE_TERMS_MAX = 32768 , DB_WDF_MAX = 65536 , IS_BOOLWEIGHT_ = static_cast<int>(0x80000000)
}

| Stats which the weighting scheme can use (see need_stat()). More...
Protected Member Functions inherited from Xapian::Weight
void | need_stat (stat_flags flag)
| Tell Xapian that your subclass will want a particular statistic.
| Weight (const Weight &)
| Don't allow copying.
Xapian::doccount | get_collection_size () const
| The number of documents in the collection.
Xapian::doccount | get_rset_size () const
| The number of documents marked as relevant.
Xapian::doclength | get_average_length () const
| The average length of a document in the collection.
Xapian::doccount | get_termfreq () const
| The number of documents which this term indexes.
Xapian::doccount | get_reltermfreq () const
| The number of relevant documents which this term indexes.
Xapian::termcount | get_collection_freq () const
| The collection frequency of the term.
Xapian::termcount | get_query_length () const
| The length of the query.
Xapian::termcount | get_wqf () const
| The within-query-frequency of this term.
Xapian::termcount | get_doclength_upper_bound () const
| An upper bound on the maximum length of any document in the shard.
Xapian::termcount | get_doclength_lower_bound () const
| A lower bound on the minimum length of any document in the shard.
Xapian::termcount | get_wdf_upper_bound () const
| An upper bound on the wdf of this term in the shard.
Xapian::totallength | get_total_length () const
| Total length of all documents in the collection.
Xapian::termcount | get_unique_terms_upper_bound () const
| A lower bound on the number of unique terms in any document in the shard.
Xapian::termcount | get_unique_terms_lower_bound () const
| An upper bound on the number of unique terms in any document in the shard.
Xapian::termcount | get_db_doclength_upper_bound () const
| An upper bound on the maximum length of any document in the database.
Xapian::termcount | get_db_doclength_lower_bound () const
| A lower bound on the minimum length of any document in the database.
Xapian::termcount | get_db_unique_terms_upper_bound () const
| A lower bound on the number of unique terms in any document in the database.
Xapian::termcount | get_db_unique_terms_lower_bound () const
| An upper bound on the number of unique terms in any document in the database.
Xapian::termcount | get_db_wdf_upper_bound () const
| An upper bound on the wdf of this term in the database.

## Detailed Description

Xapian::Weight subclass implementing the BM25+ probabilistic formula.

## Constructor & Destructor Documentation

## BM25PlusWeight()

Xapian::BM25PlusWeight::BM25PlusWeight | ( | double | k1,
|  | double | k2,
|  | double | k3,
|  | double | b,
|  | double | min_normlen,
|  | double | delta )

inline

Construct a BM25PlusWeight.

Parameters

  k1 | A non-negative parameter controlling how influential within-document-frequency (wdf) is. k1=0 means that wdf doesn't affect the weights. The larger k1 is, the more wdf influences the weights. (default 1)
  k2 | A non-negative parameter which controls the strength of a correction factor which depends upon query length and normalised document length. k2=0 disable this factor; larger k2 makes it stronger. The paper which describes BM25+ ignores BM25's document-independent component (so implicitly k2=0), but we support non-zero k2 too. (default 0)
  k3 | A non-negative parameter controlling how influential within-query-frequency (wqf) is. k3=0 means that wqf doesn't affect the weights. The larger k3 is, the more wqf influences the weights. (default 1)
  b | A parameter between 0 and 1, controlling how strong the document length normalisation of wdf is. 0 means no normalisation; 1 means full normalisation. (default 0.5)
  min_normlen | A parameter specifying a minimum value for normalised document length. Normalised document length values less than this will be clamped to this value, helping to prevent very short documents getting large weights. (default 0.5)
  delta | A parameter for pseudo tf value to control the scale of the tf lower bound. Delta(δ) can be tuned for example from 0.0 to 1.5 but BM25+ can still work effectively across collections with a fixed δ = 1.0. (default 1.0)

References Xapian::Weight::AVERAGE_LENGTH, Xapian::Weight::COLLECTION_SIZE, Xapian::Weight::DOC_LENGTH, Xapian::Weight::DOC_LENGTH_MIN, Xapian::Weight::need_stat(), Xapian::Weight::QUERY_LENGTH, Xapian::Weight::RELTERMFREQ, Xapian::Weight::RSET_SIZE, Xapian::Weight::TERMFREQ, Xapian::Weight::WDF, Xapian::Weight::WDF_MAX, and Xapian::Weight::WQF.

Referenced by create_from_parameters(), and unserialise().

## Member Function Documentation

## create_from_parameters()

BM25PlusWeight * Xapian::BM25PlusWeight::create_from_parameters | ( | const char * | params | ) | const

virtual

Create from a human-readable parameter string.

Parameters

  params | string containing weighting scheme parameter values.

Since
  2.0.0

Reimplemented from Xapian::Weight.

References BM25PlusWeight().

## get_maxextra()

double Xapian::BM25PlusWeight::get_maxextra | ( |  | ) | const

virtual

Return an upper bound on what get_sumextra() can return for any document.

The default implementation always returns 0 (in Xapian < 2.0.0 this was a pure virtual method).

This information is used by the matcher to perform various optimisations, so strive to make the bound as tight as possible.

Reimplemented from Xapian::Weight.

## get_maxpart()

double Xapian::BM25PlusWeight::get_maxpart | ( |  | ) | const

virtual

Return an upper bound on what get_sumpart() can return for any document.

This information is used by the matcher to perform various optimisations, so strive to make the bound as tight as possible.

Implements Xapian::Weight.

## get_sumextra()

double Xapian::BM25PlusWeight::get_sumextra | ( | Xapian::termcount | doclen,
|  | Xapian::termcount | uniqterms,
|  | Xapian::termcount | wdfdocmax ) const

virtual

Calculate the term-independent weight component for a document.

The default implementation always returns 0 (in Xapian < 2.0.0 this was a pure virtual method).

The parameter gives information about the document which may be used in the calculations:

Parameters

  doclen | The document's length (unnormalised). You need to call need_stat(DOC_LENGTH) if you use this value.
  uniqterms | Number of unique terms in the document. You need to call need_stat(UNIQUE_TERMS) if you use this value.
  wdfdocmax | Maximum wdf value in the document. You need to call need_stat(WDF_DOC_MAX) if you use this value.

Since
  The wdfdocmax parameter was added in Xapian 2.0.0.

Reimplemented from Xapian::Weight.

## get_sumpart()

double Xapian::BM25PlusWeight::get_sumpart | ( | Xapian::termcount | wdf,
|  | Xapian::termcount | doclen,
|  | Xapian::termcount | uniqterms,
|  | Xapian::termcount | wdfdocmax ) const

virtual

Calculate the weight contribution for this object's term to a document.

The parameters give information about the document which may be used in the calculations:

Parameters

  wdf | The within document frequency of the term in the document. You need to call need_stat(WDF) if you use this value.
  doclen | The document's length (unnormalised). You need to call need_stat(DOC_LENGTH) if you use this value.
  uniqterms | Number of unique terms in the document. You need to call need_stat(UNIQUE_TERMS) if you use this value.
  wdfdocmax | Maximum wdf value in the document. You need to call need_stat(WDF_DOC_MAX) if you use this value.

You can rely of wdf <= doclen if you call both need_stat(WDF) and need_stat(DOC_LENGTH) - this is trivially true for terms, but Xapian also ensure it's true for OP_SYNONYM, where the wdf is approximated.

Since
  The wdfdocmax parameter was added in Xapian 2.0.0.

Implements Xapian::Weight.

## name()

std::string Xapian::BM25PlusWeight::name | ( |  | ) | const

virtual

Return the name of this weighting scheme, e.g.

"bm25+".

This is the name that the weighting scheme gets registered under when passed to Xapian:Registry::register_weighting_scheme().

As a result:

- this is the name that needs to be used in Weight::create() to create a Weight object from a human-readable string description.
- it is also used by the remote backend where it is sent (along with the serialised parameters) to the remote server so that it knows which class to create.

For 1.4.x and earlier we recommended returning the full namespace-qualified name of your class here, but now we recommend returning a just the name in lower case, e.g. "foo" instead of "FooWeight", "bm25+" instead of "Xapian::BM25PlusWeight".

If you don't want to support creation via Weight::create() or the remote backend, you can use the default implementation which simply returns an empty string.

Reimplemented from Xapian::Weight.

## serialise()

std::string Xapian::BM25PlusWeight::serialise | ( |  | ) | const

virtual

Return this object's parameters serialised as a single string.

If you don't want to support the remote backend, you can use the default implementation which simply throws Xapian::UnimplementedError.

Reimplemented from Xapian::Weight.

## unserialise()

BM25PlusWeight * Xapian::BM25PlusWeight::unserialise | ( | const std::string & | serialised | ) | const

virtual

Unserialise parameters.

This method unserialises parameters serialised by the serialise() method and allocates and returns a new object initialised with them.

If you don't want to support the remote backend, you can use the default implementation which simply throws Xapian::UnimplementedError.

Note that the returned object will be deallocated by Xapian after use with "delete". If you want to handle the deletion in a special way (for example when wrapping the Xapian API for use from another language) then you can define a static `operator delete` method in your subclass as shown here: https://trac.xapian.org/ticket/554#comment:1

Parameters

  serialised | A string containing the serialised parameters.

Reimplemented from Xapian::Weight.

References BM25PlusWeight().

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

- xapian/weight.h