-
Notifications
You must be signed in to change notification settings - Fork 624
/
LMJelinekMercerSimilarity.cs
81 lines (73 loc) · 3.24 KB
/
LMJelinekMercerSimilarity.cs
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
using Lucene.Net.Support;
using System;
using System.Globalization;
namespace Lucene.Net.Search.Similarities
{
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/// <summary>
/// Language model based on the Jelinek-Mercer smoothing method. From Chengxiang
/// Zhai and John Lafferty. 2001. A study of smoothing methods for language
/// models applied to Ad Hoc information retrieval. In Proceedings of the 24th
/// annual international ACM SIGIR conference on Research and development in
/// information retrieval (SIGIR '01). ACM, New York, NY, USA, 334-342.
/// <para>The model has a single parameter, λ. According to said paper, the
/// optimal value depends on both the collection and the query. The optimal value
/// is around <c>0.1</c> for title queries and <c>0.7</c> for long queries.</para>
///
/// @lucene.experimental
/// </summary>
public class LMJelinekMercerSimilarity : LMSimilarity
{
/// <summary>
/// The λ parameter. </summary>
private readonly float lambda;
/// <summary>
/// Instantiates with the specified <paramref name="collectionModel"/> and λ parameter. </summary>
public LMJelinekMercerSimilarity(ICollectionModel collectionModel, float lambda)
: base(collectionModel)
{
this.lambda = lambda;
}
/// <summary>
/// Instantiates with the specified λ parameter. </summary>
public LMJelinekMercerSimilarity(float lambda)
{
this.lambda = lambda;
}
public override float Score(BasicStats stats, float freq, float docLen)
{
return stats.TotalBoost * (float)Math.Log(1 + ((1 - lambda) * freq / docLen) / (lambda * ((LMStats)stats).CollectionProbability));
}
protected internal override void Explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen)
{
if (stats.TotalBoost != 1.0f)
{
expl.AddDetail(new Explanation(stats.TotalBoost, "boost"));
}
expl.AddDetail(new Explanation(lambda, "lambda"));
base.Explain(expl, stats, doc, freq, docLen);
}
/// <summary>
/// Returns the λ parameter. </summary>
public virtual float Lambda => lambda;
public override string GetName()
{
return "Jelinek-Mercer(" + Number.ToString(Lambda) + ")";
}
}
}