[LUCENE-5005] Length norm value of DefaultSimilarity for a few terms

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Not A Problem
• Affects Version/s: 4.0
• Component/s: core/search
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Shingo Sasaki
• Created: 17/May/13 10:47
• Updated: 28/Aug/22 13:46
• Resolved: 15/May/15 06:05

Description

lengthNorm method of DefaultSimilarity is following:

  public float lengthNorm(FieldInvertState state) {
    final int numTerms;
    if (discountOverlaps)
      numTerms = state.getLength() - state.getNumOverlap();
    else
      numTerms = state.getLength();
   return state.getBoost() * ((float) (1.0 / Math.sqrt(numTerms)));
  }

The retrun value is decided by (1.0 / Math.sqrt(numTerms)).
The type is float, but this value is encoded to byte length by SmallFloat.floatToByte315.

term count	1/sqrt(numTerms)	1/sqrt(numTerms) to byte
1	1.000000	1.0000
2	0.707107	0.6250
3	0.577350	0.5000
4	0.500000	0.5000
5	0.447214	0.4375

The length norm of 3 terms is the same as that of 4 terms.