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https://issues.apache.org/jira/browse/LUCENENET-337?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12843812#action_12843812
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Michael Garski commented on LUCENENET-337:
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There is an alternate way to avoid having synonyms add to the length norm of a
field, which is currently in the trunk.
Rather than using the default SImilarity as-is, if you call
SetDiscountOverlaps(true) on DefaultSimilarity overlapping tokens such as
synonyms will not be considered for the length norm.
That being said, there are cases where the approach in the patch would work
better, such as when you wish to maintain positional information but specify
the length norm to be something other than the number of tokens such as using
synonyms with acronyms. This is a case I run into quite a bit with names of
musicians and bands.
> TokenAttribute for Selectively Including Tokens in Length Norm
> --------------------------------------------------------------
>
> Key: LUCENENET-337
> URL: https://issues.apache.org/jira/browse/LUCENENET-337
> Project: Lucene.Net
> Issue Type: Improvement
> Reporter: Michael Garski
> Priority: Minor
> Attachments: LengthNorm.patch
>
>
> This patch adds functionality to Lucene.Net that allow a TokenFilter to mark
> a Token as not to be included in the length norm calculation through the use
> of a new TokenAttribute interface LengthNormAttribute and a corresponding
> implementation LengthNormAttributeImpl. This functionality is useful to
> prevent the increase of the length norm during synonym injection,
> particularly in cases where there are a large number of synonyms in relation
> to the number of original tokens.
> Following is an example of how to use the new attribute.
> Within your custom TokenFilter, define a field to persist a reference to the
> attribute and set it's value in the constructor. When a the stream advances
> to a new Token within the call to IncrementToken() the value of the
> IncludeInLengthNorm property of the attribute is set to false for Tokens
> which should not be included in the length norm calculation. It defaults to
> true and is reset to true after each Token is consumed within
> DocInverterPerField.ProcessFields.
> {code:title=CustomTokenFilter.cs|borderStyle=solid}
> public class CustomTokenFilter : TokenFilter
> {
> private LengthNormAttribute lnAttribute;
>
> public CustomTokenFilter(TokenStream input) : base(input)
> {
> this.lnAttribute =
> (LengthNormAttribute)AddAttribute(typeof(LengthNormAttribute));
> }
>
> public override bool IncrementToken()
> {
> if (input.IncrementToken())
> {
> // make determination that the token is not to be
> // included in the length norm value
> // this example marks all tokens to not be
> // included in the length norm value
> this.lnAttribute.IncludeInLengthNorm = false;
> return true;
> }
> else
> {
> return false;
> }
> }
> }
> {code}
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