classxapian_1_1bm25weight.tex

xapian 是一个“Xapian 是一个开源概率论信息检索库

\hypertarget{classXapian_1_1BM25Weight}{
\section{Xapian::BM25Weight Class Reference}
\label{classXapian_1_1BM25Weight}\index{Xapian::BM25Weight@{Xapian::BM25Weight}}
}
BM25 weighting scheme.  


{\tt \#include $<$enquire.h$>$}

Inheritance diagram for Xapian::BM25Weight:\begin{figure}[H]
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Collaboration diagram for Xapian::BM25Weight:\begin{figure}[H]
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\subsection*{Public Member Functions}
\begin{CompactItemize}
\item 
\hyperlink{classXapian_1_1BM25Weight_a348894a51d5b8f13ebf46fd39cfbe2a}{BM25Weight} (double k1\_\-, double k2\_\-, double k3\_\-, double b\_\-, double min\_\-normlen\_\-)
\begin{CompactList}\small\item\em Construct a BM25 weight. \item\end{CompactList}\item 
\hyperlink{classXapian_1_1BM25Weight}{BM25Weight} $\ast$ \hyperlink{classXapian_1_1BM25Weight_7b274a31997eb61c70f92418a9abc36c}{clone} () const 
\begin{CompactList}\small\item\em Return a new weight object of this type. \item\end{CompactList}\item 
std::string \hyperlink{classXapian_1_1BM25Weight_943fdc83495a97f7d6af1208f5b26052}{name} () const 
\begin{CompactList}\small\item\em Name of the weighting scheme. \item\end{CompactList}\item 
std::string \hyperlink{classXapian_1_1BM25Weight_2dd2eb60deb5690e471d68f4c7fc3203}{serialise} () const 
\begin{CompactList}\small\item\em Serialise object parameters into a string. \item\end{CompactList}\item 
\hyperlink{classXapian_1_1BM25Weight}{BM25Weight} $\ast$ \hyperlink{classXapian_1_1BM25Weight_8a9f0c48f4229aa0d62feff5f496dc78}{unserialise} (const std::string \&s) const 
\begin{CompactList}\small\item\em Create object given string serialisation returned by \hyperlink{classXapian_1_1BM25Weight_2dd2eb60deb5690e471d68f4c7fc3203}{serialise()}. \item\end{CompactList}\item 
\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} \hyperlink{classXapian_1_1BM25Weight_6fc34658a0e0bbf6904cbef7161ba3b0}{get\_\-sumpart} (\hyperlink{types_8h_6979d98ebbe67186bd31e5ee2a7585cb}{Xapian::termcount} wdf, \hyperlink{types_8h_7272583216038f810105df7dc927723c}{Xapian::doclength} len) const 
\begin{CompactList}\small\item\em Get a weight which is part of the sum over terms being performed. \item\end{CompactList}\item 
\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} \hyperlink{classXapian_1_1BM25Weight_f23907adc06e73cdf1228fc616a2fda2}{get\_\-maxpart} () const 
\begin{CompactList}\small\item\em Gets the maximum value that \hyperlink{classXapian_1_1BM25Weight_6fc34658a0e0bbf6904cbef7161ba3b0}{get\_\-sumpart()} may return. \item\end{CompactList}\item 
\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} \hyperlink{classXapian_1_1BM25Weight_85644662b31656561181f26fc4e3b190}{get\_\-sumextra} (\hyperlink{types_8h_7272583216038f810105df7dc927723c}{Xapian::doclength} len) const 
\begin{CompactList}\small\item\em Get an extra weight for a document to add to the sum calculated over the query terms. \item\end{CompactList}\item 
\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} \hyperlink{classXapian_1_1BM25Weight_7c91481891313e089eb38bf1a9a83029}{get\_\-maxextra} () const 
\begin{CompactList}\small\item\em Gets the maximum value that \hyperlink{classXapian_1_1BM25Weight_85644662b31656561181f26fc4e3b190}{get\_\-sumextra()} may return. \item\end{CompactList}\item 
bool \hyperlink{classXapian_1_1BM25Weight_0c54cede890ff405245757615d7bc651}{get\_\-sumpart\_\-needs\_\-doclength} () const 
\begin{CompactList}\small\item\em return false if the weight object doesn't need doclength \item\end{CompactList}\end{CompactItemize}


\subsection{Detailed Description}
BM25 weighting scheme. 

BM25 weighting options : The BM25 formula is \[ \frac{k_{2}.n_{q}}{1+L_{d}}+\sum_{t}\frac{(k_{3}+1)q_{t}}{k_{3}+q_{t}}.\frac{(k_{1}+1)f_{t,d}}{k_{1}((1-b)+bL_{d})+f_{t,d}}.w_{t} \] where\begin{itemize}
\item $w_{t}$ is the termweight of term t\item $f_{t,d}$ is the within document frequency of term t in document d\item $q_{t}$ is the within query frequency of term t\item $L_{d}$ is the normalised length of document d\item $n_{q}$ is the size of the query\item $k_{1}$, $k_{2}$, $k_{3}$ and $b$ are user specified parameters \end{itemize}




\subsection{Constructor \& Destructor Documentation}
\hypertarget{classXapian_1_1BM25Weight_a348894a51d5b8f13ebf46fd39cfbe2a}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!BM25Weight@{BM25Weight}}
\index{BM25Weight@{BM25Weight}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[BM25Weight]{\setlength{\rightskip}{0pt plus 5cm}Xapian::BM25Weight::BM25Weight (double {\em k1\_\-}, double {\em k2\_\-}, double {\em k3\_\-}, double {\em b\_\-}, double {\em min\_\-normlen\_\-})\hspace{0.3cm}{\tt  \mbox{[}inline\mbox{]}}}}
\label{classXapian_1_1BM25Weight_a348894a51d5b8f13ebf46fd39cfbe2a}


Construct a BM25 weight. 

\begin{Desc}
\item[Parameters:]
\begin{description}
\item[{\em k1}]governs the importance of within document frequency. Must be $>$= 0. 0 means ignore wdf. Default is 1. \item[{\em k2}]compensation factor for the high wdf values in large documents. Must be $>$= 0. 0 means no compensation. Default is 0. \item[{\em k3}]governs the importance of within query frequency. Must be $>$= 0. 0 means ignore wqf. Default is 1. \item[{\em b}]Relative importance of within document frequency and document length. Must be $>$= 0 and $<$= 1. Default is 0.5. \item[{\em 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. \end{description}
\end{Desc}


\subsection{Member Function Documentation}
\hypertarget{classXapian_1_1BM25Weight_7b274a31997eb61c70f92418a9abc36c}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!clone@{clone}}
\index{clone@{clone}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[clone]{\setlength{\rightskip}{0pt plus 5cm}\hyperlink{classXapian_1_1BM25Weight}{BM25Weight}$\ast$ Xapian::BM25Weight::clone () const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_7b274a31997eb61c70f92418a9abc36c}


Return a new weight object of this type. 

A subclass called Foo\-Weight taking parameters param1 and param2 should implement this as:

virtual Foo\-Weight $\ast$ \hyperlink{classXapian_1_1BM25Weight_7b274a31997eb61c70f92418a9abc36c}{clone() const} \{ return new Foo\-Weight(param1, param2); \} 

Implements \hyperlink{classXapian_1_1Weight}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_7c91481891313e089eb38bf1a9a83029}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!get_maxextra@{get\_\-maxextra}}
\index{get_maxextra@{get\_\-maxextra}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[get\_\-maxextra]{\setlength{\rightskip}{0pt plus 5cm}\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} Xapian::BM25Weight::get\_\-maxextra () const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_7c91481891313e089eb38bf1a9a83029}


Gets the maximum value that \hyperlink{classXapian_1_1BM25Weight_85644662b31656561181f26fc4e3b190}{get\_\-sumextra()} may return. 

This is used in optimising searches. 

Implements \hyperlink{classXapian_1_1Weight_ca641ecc2f5645666820f864d243f46d}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_f23907adc06e73cdf1228fc616a2fda2}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!get_maxpart@{get\_\-maxpart}}
\index{get_maxpart@{get\_\-maxpart}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[get\_\-maxpart]{\setlength{\rightskip}{0pt plus 5cm}\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} Xapian::BM25Weight::get\_\-maxpart () const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_f23907adc06e73cdf1228fc616a2fda2}


Gets the maximum value that \hyperlink{classXapian_1_1BM25Weight_6fc34658a0e0bbf6904cbef7161ba3b0}{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 \hyperlink{classXapian_1_1Weight_f37e797fa12bb915f73b20b953a132b7}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_85644662b31656561181f26fc4e3b190}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!get_sumextra@{get\_\-sumextra}}
\index{get_sumextra@{get\_\-sumextra}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[get\_\-sumextra]{\setlength{\rightskip}{0pt plus 5cm}\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} Xapian::BM25Weight::get\_\-sumextra (\hyperlink{types_8h_7272583216038f810105df7dc927723c}{Xapian::doclength} {\em len}) const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_85644662b31656561181f26fc4e3b190}


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.

\begin{Desc}
\item[Parameters:]
\begin{description}
\item[{\em len}]the (unnormalised) document length. \end{description}
\end{Desc}


Implements \hyperlink{classXapian_1_1Weight_5b39302405b8d10911b26d796ba3f430}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_6fc34658a0e0bbf6904cbef7161ba3b0}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!get_sumpart@{get\_\-sumpart}}
\index{get_sumpart@{get\_\-sumpart}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[get\_\-sumpart]{\setlength{\rightskip}{0pt plus 5cm}\hyperlink{types_8h_167860ee249b702eb746dca0d245f808}{Xapian::weight} Xapian::BM25Weight::get\_\-sumpart (\hyperlink{types_8h_6979d98ebbe67186bd31e5ee2a7585cb}{Xapian::termcount} {\em wdf}, \hyperlink{types_8h_7272583216038f810105df7dc927723c}{Xapian::doclength} {\em len}) const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_6fc34658a0e0bbf6904cbef7161ba3b0}


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.

\begin{Desc}
\item[Parameters:]
\begin{description}
\item[{\em wdf}]the within document frequency of the term. \item[{\em len}]the (unnormalised) document length. \end{description}
\end{Desc}


Implements \hyperlink{classXapian_1_1Weight_cf94e6ef70e95adf04c70fee3f3c965b}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_0c54cede890ff405245757615d7bc651}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!get_sumpart_needs_doclength@{get\_\-sumpart\_\-needs\_\-doclength}}
\index{get_sumpart_needs_doclength@{get\_\-sumpart\_\-needs\_\-doclength}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[get\_\-sumpart\_\-needs\_\-doclength]{\setlength{\rightskip}{0pt plus 5cm}bool Xapian::BM25Weight::get\_\-sumpart\_\-needs\_\-doclength () const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_0c54cede890ff405245757615d7bc651}


return false if the weight object doesn't need doclength 



Reimplemented from \hyperlink{classXapian_1_1Weight_11a52ff08a2277bb765d579bf0ab77d8}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_943fdc83495a97f7d6af1208f5b26052}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!name@{name}}
\index{name@{name}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[name]{\setlength{\rightskip}{0pt plus 5cm}std::string Xapian::BM25Weight::name () const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_943fdc83495a97f7d6af1208f5b26052}


Name of the weighting scheme. 

If the subclass is called Foo\-Weight, this should return \char`\"{}Foo\char`\"{}. 

Implements \hyperlink{classXapian_1_1Weight_10f2a94ec786e68633012c41ecedc123}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_2dd2eb60deb5690e471d68f4c7fc3203}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!serialise@{serialise}}
\index{serialise@{serialise}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[serialise]{\setlength{\rightskip}{0pt plus 5cm}std::string Xapian::BM25Weight::serialise () const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_2dd2eb60deb5690e471d68f4c7fc3203}


Serialise object parameters into a string. 



Implements \hyperlink{classXapian_1_1Weight_a395a7cb4abd4ae0a3acc8b7d2510620}{Xapian::Weight}.\hypertarget{classXapian_1_1BM25Weight_8a9f0c48f4229aa0d62feff5f496dc78}{
\index{Xapian::BM25Weight@{Xapian::BM25Weight}!unserialise@{unserialise}}
\index{unserialise@{unserialise}!Xapian::BM25Weight@{Xapian::BM25Weight}}
\subsubsection[unserialise]{\setlength{\rightskip}{0pt plus 5cm}\hyperlink{classXapian_1_1BM25Weight}{BM25Weight}$\ast$ Xapian::BM25Weight::unserialise (const std::string \& {\em s}) const\hspace{0.3cm}{\tt  \mbox{[}virtual\mbox{]}}}}
\label{classXapian_1_1BM25Weight_8a9f0c48f4229aa0d62feff5f496dc78}


Create object given string serialisation returned by \hyperlink{classXapian_1_1BM25Weight_2dd2eb60deb5690e471d68f4c7fc3203}{serialise()}. 



Implements \hyperlink{classXapian_1_1Weight_c7406de40d11ffa4ef044d641a458484}{Xapian::Weight}.

The documentation for this class was generated from the following file:\begin{CompactItemize}
\item 
include/xapian/\hyperlink{enquire_8h}{enquire.h}\end{CompactItemize}