/// <summary>
/// Caculate P(word|tag)
/// </summary>
/// <param name="tag"></param>
public double GetProbabilityTagGivenWord(Tag tag)
{
if (AssociatedTagDict.ContainsKey(tag))
{
return (double)AssociatedTagDict[tag] / WordCount;
}
return 0;
}
public void AddTag(Tag tag)
{
if (AssociatedTagDict.ContainsKey(tag))
{
//add 1 count
AssociatedTagDict[tag]++;
}
else
{
//and new tag into dictionary
AssociatedTagDict.Add(tag, 1);
}
WordCount++;
}
public Tagger()
{
WordDict = new Dictionary<string, Word>();
TagDict = new Dictionary<string, Tag>();
TrainingSentenceList = new List<string>();
TrainingTaggedSentenceList = new List<string>();
Seq2TagDict = new Dictionary<string, Sequence2Tag>();
Seq3TagDict = new Dictionary<string, Sequence3Tag>();
TagDescDict = new Dictionary<string, string>();
//set default value for lamda
Lamda1 = (double)1 / 3;
Lamda2 = (double)1 / 3;
Lamda3 = (double)1 / 3;
//add blank tag
DumpTag = new Tag() { Value = "Dump" };
//get tag dict desc
MakeTagDescDictionary();
}
/// <summary>
/// Caculate P(word|tag)
/// </summary>
/// <param name="word"></param>
/// <param name="tag"></param>
private double GetProbabilityWordGivenTag(Word word, Tag tag)
{
return (tag.GetProbabilityWordGivenTag(word));
}
/// <summary>
/// Caculate P(word|tag)
/// </summary>
/// <param name="word"></param>
/// <param name="tag"></param>
private double GetProbabilityTagGivenWord(Word word, Tag tag)
{
return (word.GetProbabilityTagGivenWord(tag));
}
/// <summary>
/// add to sequence 3 tag to dictionary
/// </summary>
/// <param name="tag1"></param>
/// <param name="tag2"></param>
/// <param name="tag3"></param>
private void AddSequence3Tag(Tag tag1, Tag tag2, Tag tag3)
{
var key = tag1.Value + SeparteStr + tag2.Value + SeparteStr + tag3.Value;
if (Seq3TagDict.ContainsKey(key))
{
Seq3TagDict[key].Count++;
}
else
{
var seq = new Sequence3Tag()
{
Tag1 = tag1,
Tag2 = tag2,
Tag3 = tag3,
Count = 1
};
Seq3TagDict.Add(key, seq);
}
}
/// <summary>
/// get tagged list
/// </summary>
/// <param name="inputWordList"></param>
/// <param name="isSuccessful"></param>
/// <param name="message"></param>
/// <returns></returns>
public List<Tag> GetTagList(List<Word> inputWordList, out bool isSuccessful, out string message)
{
//make Viterbi table as array
var viterbiArr = new List<ViterbiObject>[inputWordList.Count];
#region initilization
//add into first position
viterbiArr[0] = inputWordList[0].AssociatedTagDict.Keys.Select(tag3 => new ViterbiObject()
{
Tag1 = DumpTag,
Tag2 = DumpTag,
Tag3 = tag3,
Pi = Math.Log(GetProbabilitySequence3Tag(DumpTag, DumpTag, tag3))
+ Math.Log(GetProbabilityWordGivenTag(inputWordList[0], tag3))//use log to store probabilities
}).ToList();
//add into second position
if (inputWordList.Count >= 2)
{
viterbiArr[1] = (from tag2 in inputWordList[0].AssociatedTagDict.Keys
from tag3 in inputWordList[1].AssociatedTagDict.Keys
let prevViterbi = viterbiArr[0].FirstOrDefault(m => m.Tag3 == tag2)
where prevViterbi != null
select new ViterbiObject()
{
Tag1 = DumpTag,
Tag2 = tag2,
Tag3 = tag3,
Pi = prevViterbi.Pi
+ Math.Log(GetProbabilitySequence2Tag(tag2, tag3))
+ Math.Log(GetProbabilityWordGivenTag(inputWordList[1], tag3))
}).ToList();
}
#endregion
#region calculate Pi and generate viterbi table
//add from 3 to n
for (var i = 2; i < inputWordList.Count; i++)
{
var viterbiList = new List<ViterbiObject>();
foreach (var tag3 in inputWordList[i].AssociatedTagDict.Keys)
{
foreach (var tag2 in inputWordList[i - 1].AssociatedTagDict.Keys)
{
ViterbiObject maxValue = null;
foreach (var tag1 in inputWordList[i - 2].AssociatedTagDict.Keys)
{
var prevViterbi = viterbiArr[i - 1].FirstOrDefault(m => m.Tag3 == tag2 && m.Tag2 == tag1);
if (prevViterbi == null) continue;
var currentPi = prevViterbi.Pi
+ Math.Log(GetProbabilitySequence3Tag(tag1, tag2, tag3))
+ Math.Log(GetProbabilityWordGivenTag(inputWordList[i], tag3));
if (maxValue == null || maxValue.Pi < currentPi)
{
maxValue = new ViterbiObject()
{
Tag1 = tag1,
Tag2 = tag2,
Tag3 = tag3,
Pi = currentPi
};
}
}
viterbiList.Add(maxValue);
}
}
viterbiArr[i] = viterbiList;
}
#endregion
#region backtrack to find sequence tag
var tagArr = new Tag[inputWordList.Count];
//find last tag (n)th
tagArr[inputWordList.Count - 1] = viterbiArr[inputWordList.Count - 1].MaxBy(m => m.Pi).Tag3;
//find (n-1)th tag
if (inputWordList.Count > 1)
{
tagArr[inputWordList.Count - 2] = viterbiArr[inputWordList.Count - 1].MaxBy(m => m.Pi).Tag2;
}
//find (n-2)th tag
if (inputWordList.Count > 2)
{
tagArr[inputWordList.Count - 3] = viterbiArr[inputWordList.Count - 1].MaxBy(m => m.Pi).Tag1;
}
//find remaing tag
for (var i = inputWordList.Count - 4; i >= 0; i--)
{
var viterbiObj =
viterbiArr[i + 2].FirstOrDefault(m => m.Tag3 == tagArr[i + 2] && m.Tag2 == tagArr[i + 1]);
if (viterbiObj != null)
{
tagArr[i] = viterbiObj.Tag1;
}
}
#endregion
isSuccessful = true;
message = "";
return tagArr.ToList();
}
/// <summary>
/// Caculate P(tag3|tag1,tag2)
/// Equation: P(tag3|tag1,tag2)
/// = lamda1 * (Count(tag1, tag2,tag3)/Count(tag1,tag2))
/// + lamda2 * (Count(tag2,tag3)/Count(tag2))
/// + lamda3 * (Count(tag3)/Count(total tags))
/// </summary>
/// <param name="tag1"></param>
/// <param name="tag2"></param>
/// <param name="tag3"></param>
/// <returns></returns>
public double GetProbabilitySequence3Tag(Tag tag1, Tag tag2, Tag tag3)
{
if (tag3 == null) return 0;
double firstParam = 0;
double secondParam = 0;
//calculate first, second param
if (tag2 != null)
{
if (tag1 != null)
{
var key12 = tag1.Value + SeparteStr + tag2.Value;
if (Seq2TagDict.ContainsKey(key12))
{
var key123 = tag1.Value + SeparteStr + tag2.Value + SeparteStr + tag3.Value;
if (Seq3TagDict.ContainsKey(key123))
{
firstParam = Lamda1 * Seq3TagDict[key123].Count / Seq2TagDict[key12].Count;
}
}
}
var key23 = tag2.Value + SeparteStr + tag3.Value;
if (Seq2TagDict.ContainsKey(key23))
{
if (tag2 == DumpTag)
{
secondParam = Lamda2*Seq2TagDict[key23].Count / TrainingSentenceList.Count;
}
else
{
secondParam = Lamda2 * Seq2TagDict[key23].Count / TagDict[tag2.Value].TagCount;
}
}
}
//calculate second param
var thirdParam = Lamda3 * tag3.TagCount / WordCount;
return (firstParam + secondParam + thirdParam) / (Lamda1 + Lamda2 + Lamda3);
}
/// <summary>
/// Caculate P(tag2|tag1)
/// Equation: P(tag2|tag1)
/// = lamda2 * (Count(tag1,tag2)/Count(tag1))
/// + lamda3 * (Count(tag2)/Count(total tags))
/// </summary>
/// <param name="tag1"></param>
/// <param name="tag2"></param>
/// <returns></returns>
public double GetProbabilitySequence2Tag(Tag tag1, Tag tag2)
{
if (tag2 == null) return 0;
double firstParam = 0;
//calculate first param
if (tag1 != null)
{
var key = tag1.Value + SeparteStr + tag2.Value;
if (Seq2TagDict.ContainsKey(key))
{
firstParam = Lamda2 * Seq2TagDict[key].Count / TagDict[tag1.Value].TagCount;
}
}
//calculate second param
var secondParam = Lamda3 * tag2.TagCount / WordCount;
return (firstParam + secondParam) / (Lamda2 + Lamda3);
}
public Tag AddTag(string tag)
{
if (TagDict.ContainsKey(tag))
{
return TagDict[tag];
}
var newTag = new Tag { Value = tag };
TagDict.Add(tag, newTag);
return newTag;
}