Class DirectSpellChecker

Simple automaton-based spellchecker.

Candidates are presented directly from the term dictionary, based on Levenshtein distance. This is an alternative to SpellChecker if you are using an edit-distance-like metric such as Levenshtein or JaroWinklerDistance.

A practical benefit of this spellchecker is that it requires no additional datastructures (neither in RAM nor on disk) to do its work.

Inheritance

System.Object

DirectSpellChecker

Inherited Members

System.Object.Equals(System.Object)

System.Object.Equals(System.Object, System.Object)

System.Object.GetHashCode()

System.Object.GetType()

System.Object.MemberwiseClone()

System.Object.ReferenceEquals(System.Object, System.Object)

System.Object.ToString()

Namespace: Lucene.Net.Search.Spell

Assembly: Lucene.Net.Suggest.dll

Syntax

public class DirectSpellChecker

Constructors

DirectSpellChecker()

Creates a DirectSpellChecker with default configuration values

Declaration

public DirectSpellChecker()

Fields

INTERNAL_LEVENSHTEIN

The default StringDistance, Damerau-Levenshtein distance implemented internally via LevenshteinAutomata.

Note: this is the fastest distance metric, because Damerau-Levenshtein is used to draw candidates from the term dictionary: this just re-uses the scoring.

Declaration

public static readonly IStringDistance INTERNAL_LEVENSHTEIN

Field Value

Type	Description
IStringDistance

Properties

Accuracy

Gets or sets the minimal accuracy required (default: 0.5f) from a StringDistance for a suggestion match.

Declaration

public virtual float Accuracy { get; set; }

Property Value

Type	Description
System.Single

Comparer

Gets or sets the comparer for sorting suggestions. The default is DEFAULT_COMPARER

Declaration

public virtual IComparer<SuggestWord> Comparer { get; set; }

Property Value

Type	Description
System.Collections.Generic.IComparer<SuggestWord>

Distance

Gets or sets the string distance metric. The default is INTERNAL_LEVENSHTEIN.

Note: because this spellchecker draws its candidates from the term dictionary using Damerau-Levenshtein, it works best with an edit-distance-like string metric. If you use a different metric than the default, you might want to consider increasing MaxInspections to draw more candidates for your metric to rank.

Declaration

public virtual IStringDistance Distance { get; set; }

Property Value

Type	Description
IStringDistance

LowerCaseTerms

True if the spellchecker should lowercase terms (default: true)

This is a convenience method, if your index field has more complicated analysis (such as StandardTokenizer removing punctuation), its probably better to turn this off, and instead run your query terms through your Analyzer first.

If this option is not on, case differences count as an edit!

Declaration

public virtual bool LowerCaseTerms { get; set; }

Property Value

Type	Description
System.Boolean

LowerCaseTermsCulture

Gets or sets the culture to use for lowercasing terms. Set to null (the default) to use System.Globalization.CultureInfo.CurrentCulture.

Declaration

public virtual CultureInfo LowerCaseTermsCulture { get; set; }

Property Value

Type	Description
System.Globalization.CultureInfo

MaxEdits

Gets or sets the maximum number of Levenshtein edit-distances to draw candidate terms from.This value can be 1 or 2. The default is 2.

Note: a large number of spelling errors occur with an edit distance of 1, by setting this value to 1 you can increase both performance and precision at the cost of recall.

Declaration

public virtual int MaxEdits { get; set; }

Property Value

Type	Description
System.Int32

MaxInspections

Get the maximum number of top-N inspections per suggestion.

Increasing this number can improve the accuracy of results, at the cost of performance.

Declaration

public virtual int MaxInspections { get; set; }

Property Value

Type	Description
System.Int32

MaxQueryFrequency

Gets or sets the maximum threshold (default: 0.01f) of documents a query term can appear in order to provide suggestions.

Very high-frequency terms are typically spelled correctly. Additionally, this can increase performance as it will do no work for the common case of correctly-spelled input terms.

This can be specified as a relative percentage of documents such as 0.5f, or it can be specified as an absolute whole document frequency, such as 4f. Absolute document frequencies may not be fractional.

Declaration

public virtual float MaxQueryFrequency { get; set; }

Property Value

Type	Description
System.Single

MinPrefix

Gets or sets the minimal number of characters that must match exactly.

This can improve both performance and accuracy of results, as misspellings are commonly not the first character.

Declaration

public virtual int MinPrefix { get; set; }

Property Value

Type	Description
System.Int32

MinQueryLength

Gets or sets the minimum length of a query term (default: 4) needed to return suggestions.

Very short query terms will often cause only bad suggestions with any distance metric.

Declaration

public virtual int MinQueryLength { get; set; }

Property Value

Type	Description
System.Int32

ThresholdFrequency

Gets or sets the minimal threshold of documents a term must appear for a match.

This can improve quality by only suggesting high-frequency terms. Note that very high values might decrease performance slightly, by forcing the spellchecker to draw more candidates from the term dictionary, but a practical value such as 1 can be very useful towards improving quality.

This can be specified as a relative percentage of documents such as 0.5f, or it can be specified as an absolute whole document frequency, such as 4f. Absolute document frequencies may not be fractional.

Declaration

public virtual float ThresholdFrequency { get; set; }

Property Value

Type	Description
System.Single

Methods

SuggestSimilar(Term, Int32, IndexReader)

Calls SuggestSimilar(Term, Int32, IndexReader, SuggestMode) SuggestSimilar(term, numSug, ir, SuggestMode.SUGGEST_WHEN_NOT_IN_INDEX)

Declaration

public virtual SuggestWord[] SuggestSimilar(Term term, int numSug, IndexReader ir)

Parameters

Type	Name	Description
Lucene.Net.Index.Term	term
System.Int32	numSug
Lucene.Net.Index.IndexReader	ir

Returns

Type	Description
SuggestWord[]

SuggestSimilar(Term, Int32, IndexReader, SuggestMode)

Calls SuggestSimilar(Term, Int32, IndexReader, SuggestMode, Single) SuggestSimilar(term, numSug, ir, suggestMode, this.accuracy)

Declaration

public virtual SuggestWord[] SuggestSimilar(Term term, int numSug, IndexReader ir, SuggestMode suggestMode)

Parameters

Type	Name	Description
Lucene.Net.Index.Term	term
System.Int32	numSug
Lucene.Net.Index.IndexReader	ir
SuggestMode	suggestMode

Returns

Type	Description
SuggestWord[]

SuggestSimilar(Term, Int32, IndexReader, SuggestMode, Single)

Suggest similar words.

Unlike SpellChecker, the similarity used to fetch the most relevant terms is an edit distance, therefore typically a low value for numSug will work very well.

Declaration

public virtual SuggestWord[] SuggestSimilar(Term term, int numSug, IndexReader ir, SuggestMode suggestMode, float accuracy)

Parameters

Type	Name	Description
Lucene.Net.Index.Term	term	Term you want to spell check on
System.Int32	numSug	the maximum number of suggested words
Lucene.Net.Index.IndexReader	ir	IndexReader to find terms from
SuggestMode	suggestMode	specifies when to return suggested words
System.Single	accuracy	return only suggested words that match with this similarity

Returns

Type	Description
SuggestWord[]	sorted list of the suggested words according to the comparer

Exceptions

Type	Condition
System.IO.IOException	If there is a low-level I/O error.

SuggestSimilar(Term, Int32, IndexReader, Int32, Int32, Single, CharsRef)

Provide spelling corrections based on several parameters.

Declaration

protected virtual ICollection<DirectSpellChecker.ScoreTerm> SuggestSimilar(Term term, int numSug, IndexReader ir, int docfreq, int editDistance, float accuracy, CharsRef spare)

Parameters

Type	Name	Description
Lucene.Net.Index.Term	term	The term to suggest spelling corrections for
System.Int32	numSug	The maximum number of spelling corrections
Lucene.Net.Index.IndexReader	ir	The index reader to fetch the candidate spelling corrections from
System.Int32	docfreq	The minimum document frequency a potential suggestion need to have in order to be included
System.Int32	editDistance	The maximum edit distance candidates are allowed to have
System.Single	accuracy	The minimum accuracy a suggested spelling correction needs to have in order to be included
Lucene.Net.Util.CharsRef	spare	a chars scratch

Returns

Type	Description
System.Collections.Generic.ICollection<DirectSpellChecker.ScoreTerm>	a collection of spelling corrections sorted by ScoreTerm 's natural order.

Exceptions

Type	Condition
System.IO.IOException	If I/O related errors occur

See Also

LevenshteinAutomata

FuzzyTermsEnum