Read weights from a value which is known to decrease as docid increases. More...

#include <postingsource.h>

Inheritance diagram for Xapian::DecreasingValueWeightPostingSource:

Public Member Functions
	DecreasingValueWeightPostingSource (Xapian::valueno slot_, Xapian::docid range_start_=0, Xapian::docid range_end_=0)
	Construct a DecreasingValueWeightPostingSource.

double	get_weight () const
	Return the weight contribution for the current document.

DecreasingValueWeightPostingSource *	clone () const
	Clone the posting source.

std::string	name () const
	Name of the posting source class.

std::string	serialise () const
	Serialise object parameters into a string.

DecreasingValueWeightPostingSource *	unserialise (const std::string &serialised) const
	Create object given string serialisation returned by serialise().

void	init (const Xapian::Database &db_)
	Set this PostingSource to the start of the list of postings.

void	next (double min_wt)
	Advance the current position to the next matching document.

void	skip_to (Xapian::docid min_docid, double min_wt)
	Advance to the specified docid.

bool	check (Xapian::docid min_docid, double min_wt)
	Check if the specified docid occurs.

std::string	get_description () const
	Return a string describing this object.

Public Member Functions inherited from Xapian::ValueWeightPostingSource
	ValueWeightPostingSource (Xapian::valueno slot_)
	Construct a ValueWeightPostingSource.

double	get_weight () const
	Return the weight contribution for the current document.

ValueWeightPostingSource *	clone () const
	Clone the posting source.

std::string	name () const
	Name of the posting source class.

std::string	serialise () const
	Serialise object parameters into a string.

ValueWeightPostingSource *	unserialise (const std::string &serialised) const
	Create object given string serialisation returned by serialise().

void	init (const Database &db_)
	Set this PostingSource to the start of the list of postings.

std::string	get_description () const
	Return a string describing this object.

Public Member Functions inherited from Xapian::ValuePostingSource
	ValuePostingSource (Xapian::valueno slot_)
	Construct a ValuePostingSource.

Xapian::doccount	get_termfreq_min () const
	A lower bound on the number of documents this object can return.

Xapian::doccount	get_termfreq_est () const
	An estimate of the number of documents this object can return.

Xapian::doccount	get_termfreq_max () const
	An upper bound on the number of documents this object can return.

void	next (double min_wt)
	Advance the current position to the next matching document.

void	skip_to (Xapian::docid min_docid, double min_wt)
	Advance to the specified docid.

bool	check (Xapian::docid min_docid, double min_wt)
	Check if the specified docid occurs.

bool	at_end () const
	Return true if the current position is past the last entry in this list.

Xapian::docid	get_docid () const
	Return the current docid.

Xapian::Database	get_database () const
	The database we're reading values from.

Xapian::valueno	get_slot () const
	The slot we're reading values from.

std::string	get_value () const
	Read current value.

void	done ()
	End the iteration.

bool	get_started () const
	Flag indicating if we've started (true if we have).

void	set_termfreq_min (Xapian::doccount termfreq_min_)
	Set a lower bound on the term frequency.

void	set_termfreq_est (Xapian::doccount termfreq_est_)
	An estimate of the term frequency.

void	set_termfreq_max (Xapian::doccount termfreq_max_)
	An upper bound on the term frequency.

Public Member Functions inherited from Xapian::PostingSource
	PostingSource ()
	Allow subclasses to be instantiated.

void	set_maxweight (double max_weight)
	Specify an upper bound on what get_weight() will return from now on.

double	get_maxweight () const
	Return the currently set upper bound on what get_weight() can return.

virtual PostingSource *	unserialise_with_registry (const std::string &serialised, const Registry &registry) const
	Create object given string serialisation returned by serialise().

PostingSource *	release ()
	Start reference counting this object.

const PostingSource *	release () const
	Start reference counting this object.

Protected Member Functions
void	skip_if_in_range (double min_wt)
	Skip the iterator forward if in the decreasing range, and weight is low.

Protected Attributes
Xapian::docid	range_start
	Start of range of docids for which weights are known to be decreasing.

Xapian::docid	range_end
	End of range of docids for which weights are known to be decreasing.

double	curr_weight
	Weight at current position.

bool	items_at_end
	Flag, set to true if there are docs after the end of the range.

Protected Attributes inherited from Xapian::ValuePostingSource
Xapian::Database &	db
	The database we're reading values from.

Xapian::valueno &	slot
	The slot we're reading values from.

Xapian::ValueIterator &	value_it
	Value stream iterator.

bool &	started
	Flag indicating if we've started (true if we have).

Xapian::doccount &	termfreq_min
	A lower bound on the term frequency.

Xapian::doccount &	termfreq_est
	An estimate of the term frequency.

Xapian::doccount &	termfreq_max
	An upper bound on the term frequency.

Detailed Description

Read weights from a value which is known to decrease as docid increases.

This posting source can be used, like ValueWeightPostingSource, to add a weight contribution to a query based on the values stored in a slot. The values in the slot must be serialised as by sortable_serialise().

However, this posting source is additionally given a range of document IDs, within which the weight is known to be decreasing. ie, for all documents with ids A and B within this range (including the endpoints), where A is less than B, the weight of A is less than or equal to the weight of B. This can allow the posting source to skip to the end of the range quickly if insufficient weight is left in the posting source for a particular source.

By default, the range is assumed to cover all document IDs.

The ordering property can be arranged at index time, or by sorting an indexed database to produce a new, sorted, database.

Constructor & Destructor Documentation

◆ DecreasingValueWeightPostingSource()

Xapian::DecreasingValueWeightPostingSource::DecreasingValueWeightPostingSource	(	Xapian::valueno	slot_,
		Xapian::docid	range_start_ = `0`,
		Xapian::docid	range_end_ = `0`
	)

Construct a DecreasingValueWeightPostingSource.

Parameters

slot_	The value slot to read values from.
range_start_	Start of range of docids for which weights are known to be decreasing (default: first docid)
range_end_	End of range of docids for which weights are known to be decreasing (default: last docid)

Member Function Documentation

◆ check()

bool Xapian::DecreasingValueWeightPostingSource::check	(	Xapian::docid	did,
		double	min_wt
	)

virtual

Check if the specified docid occurs.

The caller is required to ensure that the specified document id did actually exists in the database. If it does, it must move to that document id, and return true. If it does not, it may either:

return true, having moved to a definite position (including "at_end"), which must be the same position as skip_to() would have moved to.

or

return false, having moved to an "indeterminate" position, such that a subsequent call to next() or skip_to() will move to the next matching position after did.

Generally, this method should act like skip_to() and return true if that can be done at little extra cost.

Otherwise it should simply check if a particular docid is present, returning true if it is, and false if it isn't.

The default implementation calls skip_to() and always returns true.

Xapian will always call init() on a PostingSource before calling this for the first time.

Note: in the case of a multi-database search, the docid specified is the docid in the single subdatabase relevant to this posting source. See the init() method for details.

Parameters

did	The document id to check.
min_wt	The minimum weight contribution that is needed (this is just a hint which subclasses may ignore).

Reimplemented from Xapian::PostingSource.

◆ clone()

DecreasingValueWeightPostingSource * Xapian::DecreasingValueWeightPostingSource::clone ( ) const

virtual

Clone the posting source.

The clone should inherit the configuration of the parent, but need not inherit the state. ie, the clone does not need to be in the same iteration position as the original: the matcher will always call init() on the clone before attempting to move the iterator, or read the information about the current position of the iterator.

This may return NULL to indicate that cloning is not supported. In this case, the PostingSource may only be used with a single-database search.

The default implementation returns NULL.

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

Reimplemented from Xapian::PostingSource.

◆ get_description()

std::string Xapian::DecreasingValueWeightPostingSource::get_description ( ) const

virtual

Return a string describing this object.

This default implementation returns a generic answer. This default it provided to avoid forcing those deriving their own PostingSource subclass from having to implement this (they may not care what get_description() gives for their subclass).

Reimplemented from Xapian::PostingSource.

◆ get_weight()

double Xapian::DecreasingValueWeightPostingSource::get_weight ( ) const

virtual

Return the weight contribution for the current document.

This default implementation always returns 0, for convenience when implementing "weight-less" PostingSource subclasses.

This method may assume that it will only be called when there is a "current document". In detail: Xapian will always call init() on a PostingSource before calling this for the first time. It will also only call this if the PostingSource reports that it is pointing to a valid document (ie, it will not call it before calling at least one of next(), skip_to() or check(), and will ensure that the PostingSource is not at the end by calling at_end()).

Reimplemented from Xapian::PostingSource.

◆ init()

void Xapian::DecreasingValueWeightPostingSource::init ( const Xapian::Database & db )

virtual

Set this PostingSource to the start of the list of postings.

This is called automatically by the matcher prior to each query being processed.

If a PostingSource is used for multiple searches, init() will therefore be called multiple times, and must handle this by using the database passed in the most recent call.

Parameters

db	The database which the PostingSource should iterate through.

Note: in the case of a multi-database search, a separate PostingSource will be used for each database (the separate PostingSources will be obtained using clone()), and each PostingSource will be passed one of the sub-databases as the db parameter here. The db parameter will therefore always refer to a single database. All docids passed to, or returned from, the PostingSource refer to docids in that single database, rather than in the multi-database.

Implements Xapian::PostingSource.

◆ name()

std::string Xapian::DecreasingValueWeightPostingSource::name ( ) const

virtual

Name of the posting source class.

This is used when serialising and unserialising posting sources; for example, for performing remote searches.

If the subclass is in a C++ namespace, the namespace should be included in the name, using "::" as a separator. For example, for a PostingSource subclass called "FooPostingSource" in the "Xapian" namespace the result of this call should be "Xapian::FooPostingSource".

This should only be implemented if serialise() and unserialise() are also implemented. The default implementation returns an empty string.

If this returns an empty string, Xapian will assume that serialise() and unserialise() are not implemented.

Reimplemented from Xapian::PostingSource.

◆ next()

void Xapian::DecreasingValueWeightPostingSource::next ( double min_wt )

virtual

Advance the current position to the next matching document.

The PostingSource starts before the first entry in the list, so next(), skip_to() or check() must be called before any methods which need the context of the current position.

Xapian will always call init() on a PostingSource before calling this for the first time.

Parameters

min_wt The minimum weight contribution that is needed (this is just a hint which subclasses may ignore).

Implements Xapian::PostingSource.

◆ serialise()

std::string Xapian::DecreasingValueWeightPostingSource::serialise ( ) const

virtual

Serialise object parameters into a string.

The serialised parameters should represent the configuration of the posting source, but need not (indeed, should not) represent the current iteration state.

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

Reimplemented from Xapian::PostingSource.

◆ skip_to()

void Xapian::DecreasingValueWeightPostingSource::skip_to	(	Xapian::docid	did,
		double	min_wt
	)

virtual

Advance to the specified docid.

If the specified docid isn't in the list, position ourselves on the first document after it (or at_end() if no greater docids are present).

If the current position is already the specified docid, this method will leave the position unmodified.

If the specified docid is earlier than the current position, the behaviour is unspecified. A sensible behaviour would be to leave the current position unmodified, but it is also reasonable to move to the specified docid.

The default implementation calls next() repeatedly, which works but skip_to() can often be implemented much more efficiently.

Xapian will always call init() on a PostingSource before calling this for the first time.

Note: in the case of a multi-database search, the docid specified is the docid in the single subdatabase relevant to this posting source. See the init() method for details.

Parameters

did	The document id to advance to.
min_wt	The minimum weight contribution that is needed (this is just a hint which subclasses may ignore).

Reimplemented from Xapian::PostingSource.

◆ unserialise()

DecreasingValueWeightPostingSource * Xapian::DecreasingValueWeightPostingSource::unserialise ( const std::string & serialised ) const

virtual

Create object given string serialisation returned by serialise().

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

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

Parameters

serialised A serialised instance of this PostingSource subclass.

Reimplemented from Xapian::PostingSource.

Member Data Documentation

◆ range_end

Xapian::docid Xapian::DecreasingValueWeightPostingSource::range_end

protected

End of range of docids for which weights are known to be decreasing.

0 => last docid.

◆ range_start

Xapian::docid Xapian::DecreasingValueWeightPostingSource::range_start

protected

Start of range of docids for which weights are known to be decreasing.

0 => first docid.

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

xapian/postingsource.h

Public Member Functions

Protected Member Functions

Protected Attributes

Detailed Description

Constructor & Destructor Documentation

◆ DecreasingValueWeightPostingSource()

Member Function Documentation

◆ check()

◆ clone()

◆ get_description()

◆ get_weight()

◆ init()

◆ name()

◆ next()

◆ serialise()

◆ skip_to()

◆ unserialise()

Member Data Documentation

◆ range_end

◆ range_start