This class implements the PL2 weighting scheme. More...
#include <weight.h>
Inheritance diagram for Xapian::PL2Weight:
Public Member Functions | |
| PL2Weight (double c) | |
| Construct a PL2Weight. More... | |
| std::string | name () const |
| Return the name of this weighting scheme. More... | |
| std::string | serialise () const |
| Return this object's parameters serialised as a single string. More... | |
| PL2Weight * | unserialise (const std::string &serialised) const |
| Unserialise parameters. More... | |
| double | get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen, Xapian::termcount uniqterms) const |
| Calculate the weight contribution for this object's term to a document. More... | |
| double | get_maxpart () const |
| Return an upper bound on what get_sumpart() can return for any document. More... | |
| double | get_sumextra (Xapian::termcount doclen, Xapian::termcount uniqterms) const |
| Calculate the term-independent weight component for a document. More... | |
| double | get_maxextra () const |
| Return an upper bound on what get_sumextra() can return for any document. More... | |
| Public Member Functions inherited from Xapian::Weight | |
| Weight () | |
| Default constructor, needed by subclass constructors. | |
| virtual | ~Weight () |
| Virtual destructor, because we have virtual methods. | |
| virtual Weight * | clone () const =0 |
| Clone this object. More... | |
| virtual std::string | name () const |
| Return the name of this weighting scheme. More... | |
| virtual std::string | serialise () const |
| Return this object's parameters serialised as a single string. More... | |
| virtual Weight * | unserialise (const std::string &serialised) const |
| Unserialise parameters. More... | |
| virtual double | get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen, Xapian::termcount uniqterms) const =0 |
| Calculate the weight contribution for this object's term to a document. More... | |
| virtual double | get_maxpart () const =0 |
| Return an upper bound on what get_sumpart() can return for any document. More... | |
| virtual double | get_sumextra (Xapian::termcount doclen, Xapian::termcount uniqterms) const =0 |
| Calculate the term-independent weight component for a document. More... | |
| virtual double | get_maxextra () const =0 |
| Return an upper bound on what get_sumextra() can return for any document. More... | |
Additional Inherited Members | |
| Public Types inherited from Xapian::Weight | |
| enum | type_smoothing { TWO_STAGE_SMOOTHING = 1 , DIRICHLET_SMOOTHING = 2 , ABSOLUTE_DISCOUNT_SMOOTHING = 3 , JELINEK_MERCER_SMOOTHING = 4 , DIRICHLET_PLUS_SMOOTHING = 5 } |
| Type of smoothing to use with the Language Model Weighting scheme. More... | |
| Protected Types inherited from Xapian::Weight | |
| enum | stat_flags { COLLECTION_SIZE = 1 , RSET_SIZE = 2 , AVERAGE_LENGTH = 4 , TERMFREQ = 8 , RELTERMFREQ = 16 , QUERY_LENGTH = 32 , WQF = 64 , WDF = 128 , DOC_LENGTH = 256 , DOC_LENGTH_MIN = 512 , DOC_LENGTH_MAX = 1024 , WDF_MAX = 2048 , COLLECTION_FREQ = 4096 , UNIQUE_TERMS = 8192 , TOTAL_LENGTH = COLLECTION_SIZE | AVERAGE_LENGTH } |
| 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. More... | |
| virtual void | init (double factor)=0 |
| Allow the subclass to perform any initialisation it needs to. More... | |
| Weight (const Weight &) | |
| Don't allow copying. More... | |
| 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 database. More... | |
| Xapian::termcount | get_doclength_lower_bound () const |
| A lower bound on the minimum length of any document in the database. More... | |
| Xapian::termcount | get_wdf_upper_bound () const |
| An upper bound on the wdf of this term. More... | |
| Xapian::totallength | get_total_length () const |
| Total length of all documents in the collection. | |
Detailed Description
This class implements the PL2 weighting scheme.
PL2 is a representative scheme of the Divergence from Randomness Framework by Gianni Amati.
This weighting scheme is useful for tasks that require early precision.
It uses the Poisson approximation of the Binomial Probabilistic distribution (P) along with Stirling's approximation for the factorial value, the Laplace method to find the aftereffect of sampling (L) and the second wdf normalization proposed by Amati to normalize the wdf in the document to the length of the document (H2).
For more information about the DFR Framework and the PL2 scheme, please refer to : Gianni Amati and Cornelis Joost Van Rijsbergen Probabilistic models of information retrieval based on measuring the divergence from randomness ACM Transactions on Information Systems (TOIS) 20, (4), 2002, pp. 357-389.
Constructor & Destructor Documentation
◆ PL2Weight()
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