/// <summary>
/// Backward method for the Viterbi decoding algorithm.
/// </summary>
/// <param name="tagger"></param>
/// <param name="testWords"></param>
/// <param name="model"></param>
/// <param name="mode"></param>
private void BackwardAlgorithm(PartOfSpeechModel tagger, List <Tokenizer.WordTag> testWords, string model, string mode, int beam)
{
// right to left encoding - backward approach
bool startPoint = true;
int triPoz = -1;
for (int i = testWords.Count - 2; i >= -1; i--) // count - 2 is to start from the first word != "."
{
triPoz++;
if (i == -1) // we first check to see if we got to index -1
{
Backtrace(method: "backward");
startPoint = true;
continue;
}
if (testWords[i].tag == ".")
{
Backtrace(method: "backward");
startPoint = true;
continue;
}
PartOfSpeechModel.EmissionProbabilisticModel foundWord = tagger.WordCapitalizedTagsEmissionProbabilities.Find(x => x.Word == testWords[i].word);
if (foundWord == null)
{
foundWord = tagger.WordTagsEmissionProbabilities.Find(x => x.Word == testWords[i].word.ToLower());
}
if (startPoint)
{
triPoz = 0;
List <ViterbiNode> vList = new List <ViterbiNode>();
if (foundWord != null)
{
if (foundWord.TagFreq.Count == 1 && foundWord.TagFreq.ContainsKey("."))
{
foundWord = null;
}
}
if (foundWord == null)
{
UnknownWords.Add(testWords[i].word);
// we take the best transition case where first item is "."
var orderedTransitions = tagger.BigramTransitionProbabilities.OrderByDescending(x => x.Value).ToList();
double product = 0.0d;
string nodeTag = "NULL";
foreach (var item in orderedTransitions)
{
if (item.Key.Item2.Equals(".") && item.Key.Item1 != ".")
{
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(item.Key.Item1)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (item.Value * tagger.BgramLambda2);
double unknownProcent = tagger.GetValueWeightForUnknownWord(testWords[i].word, item.Key.Item1);
product = biTrans * unknownProcent;
nodeTag = item.Key.Item1;
ViterbiNode node = new ViterbiNode(product, nodeTag);
vList.Add(node);
}
}
}
else
{
foreach (var wt in foundWord.TagFreq)
{
if (wt.Key == ".")
{
continue;
}
double emissionFreqValue = wt.Value; // eg. Jane -> 0.1111 (NN)
Tuple <string, string> tuple = new Tuple <string, string>(wt.Key, ".");
double biTransition = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(tuple)).Value; // eg. NN->VB - 0.25
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(wt.Key)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (biTransition * tagger.BgramLambda2);
double product = (double)emissionFreqValue * biTrans;
ViterbiNode node = new ViterbiNode(product, wt.Key);
vList.Add(node);
}
}
this.ViterbiGraph.Add(vList);
startPoint = false;
}
else
{
List <ViterbiNode> vList = new List <ViterbiNode>();
if (foundWord != null)
{
if (foundWord.TagFreq.Count == 1 && foundWord.TagFreq.ContainsKey("."))
{
foundWord = null;
}
}
if (foundWord == null)
{
UnknownWords.Add(testWords[i].word);
for (int j = 0; j < this.ViterbiGraph[this.ViterbiGraph.Count - 1].Count; j++)
{
ViterbiNode elem = this.ViterbiGraph[this.ViterbiGraph.Count - 1][j];
ViterbiNode vGoodNode = new ViterbiNode(0.0d, "NULL");
// we take the best transition case where first item is "."
var orderedTransitions = tagger.BigramTransitionProbabilities.OrderByDescending(x => x.Value).ToList();
if (model == "trigram" && triPoz >= 2)
{
if (elem.NextNode == null)
{
continue;
}
ViterbiNode elem2 = elem.NextNode;
var orderedTransitionsTri = tagger.TrigramTransitionProbabilities.OrderByDescending(x => x.Value).ToList();
double product = 0.0d;
string nodeTag = "NULL_TRI";
foreach (var item in orderedTransitionsTri)
{
if (item.Key.Item3.Equals(elem2.CurrentTag) && item.Key.Item2.Equals(elem.CurrentTag) && item.Key.Item1 != ".")
{
Tuple <string, string> biTuple = new Tuple <string, string>(item.Key.Item1, elem.CurrentTag);
double biVal = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(biTuple)).Value;
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(item.Key.Item1)).Value;
double triTransition = (double)(tagger.TgramLambda3 * item.Value) + (tagger.TgramLambda2 * biVal) + (tagger.TgramLambda1 * uniVal);
double unknownProcent = tagger.GetValueWeightForUnknownWord(testWords[i].word, item.Key.Item1);
product = (double)elem.value * triTransition * unknownProcent;
nodeTag = item.Key.Item1;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, nodeTag, NextNode: elem);
}
}
}
}
else
{
double product = 0.0d;
string nodeTag = "NULL_BI";
foreach (var item in orderedTransitions)
{
if (item.Key.Item2.Equals(elem.CurrentTag) && item.Key.Item1 != ".")
{
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(item.Key.Item1)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (item.Value * tagger.BgramLambda2);
double unknownProcent = tagger.GetValueWeightForUnknownWord(testWords[i].word, item.Key.Item1);
product = (double)elem.value * biTrans * unknownProcent;
nodeTag = item.Key.Item1;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, nodeTag, NextNode: elem);
}
}
}
}
vList.Add(vGoodNode);
}
}
else
{
foreach (var tf in foundWord.TagFreq)
{
if (tf.Key == ".")
{
continue;
}
ViterbiNode vGoodNode = new ViterbiNode(0.0d, "NULL");
foreach (ViterbiNode vn in this.ViterbiGraph[this.ViterbiGraph.Count - 1])
{
if (model == "trigram" && triPoz >= 2)
{
if (vn.NextNode == null)
{
continue;
}
Tuple <string, string, string> triTuple = new Tuple <string, string, string>(tf.Key, vn.CurrentTag, vn.NextNode.CurrentTag);
double triVal = tagger.TrigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(triTuple)).Value;
Tuple <string, string> biTuple = new Tuple <string, string>(tf.Key, vn.CurrentTag);
double biVal = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(biTuple)).Value;
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(tf.Key)).Value;
double triTransition = (double)(tagger.TgramLambda3 * triVal) + (tagger.TgramLambda2 * biVal) + (tagger.TgramLambda1 * uniVal);
double product = (double)vn.value * triTransition * tf.Value;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, tf.Key, NextNode: vn);
}
}
else
{
Tuple <string, string> tuple = new Tuple <string, string>(tf.Key, vn.CurrentTag);
double biTransition = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(tuple)).Value; // eg. NN->VB - 0.25
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(tf.Key)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (biTransition * tagger.BgramLambda2);
double product = (double)vn.value * biTrans * tf.Value;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, tf.Key, NextNode: vn);
}
}
}
vList.Add(vGoodNode);
}
}
this.ViterbiGraph.Add(vList);
}
this.ViterbiGraph[this.ViterbiGraph.Count - 1] = this.ViterbiGraph[this.ViterbiGraph.Count - 1].OrderByDescending(x => x.value).ToList();
if (beam != 0)
{
this.ViterbiGraph[this.ViterbiGraph.Count - 1] = this.ViterbiGraph[this.ViterbiGraph.Count - 1].Take(beam).ToList();
}
}
if (mode == "backward")
{
this.PredictedTags = new List <string>();
List <ViterbiNode> historyCopy = new List <ViterbiNode>(BackwardHistory);
for (int i = 0; i < historyCopy.Count; i++)
{
List <string> tagsViterbi = new List <string>();
while (true)
{
if (historyCopy[i].CurrentTag != ".")
{
tagsViterbi.Add(historyCopy[i].CurrentTag);
}
if (historyCopy[i].NextNode == null)
{
break;
}
historyCopy[i] = historyCopy[i].NextNode;
}
this.PredictedTags.AddRange(tagsViterbi);
}
}
}
/// <summary>
/// Forward method for the Viterbi decoding algorithm.
/// </summary>
/// <param name="tagger"></param>
/// <param name="testWords"></param>
/// <param name="model"></param>
private void ForwardAlgorithm(PartOfSpeechModel tagger, List <Tokenizer.WordTag> testWords, string model, int beam)
{
// left to right encoding - forward approach
bool startPoint = true;
int triPoz = -1;
for (int i = 0; i < testWords.Count; i++) // starting from left (0 index)
{
triPoz++;
if (testWords[i].tag == ".") // we can verify word instead of tag here
{
Backtrace(method: "forward"); // decompress method, going from right to left using prev nodes, applied only when '.' is met
startPoint = true;
continue;
}
PartOfSpeechModel.EmissionProbabilisticModel foundWord = tagger.WordCapitalizedTagsEmissionProbabilities.Find(x => x.Word == testWords[i].word);
if (foundWord == null)
{
foundWord = tagger.WordTagsEmissionProbabilities.Find(x => x.Word == testWords[i].word.ToLower());
}
if (startPoint) // first node (start)
{
triPoz = 0;
List <ViterbiNode> vList = new List <ViterbiNode>();
if (foundWord != null)
{
if (foundWord.TagFreq.Count == 1 && foundWord.TagFreq.ContainsKey(".")) // case where the only tag is '.'
{
foundWord = null;
}
}
if (foundWord == null)
{
UnknownWords.Add(testWords[i].word);
// we take the best transition case where first item is "."
// case 2: all the transitions
var orderedTransitions = tagger.BigramTransitionProbabilities.OrderByDescending(x => x.Value).ToList();
double product = 0.0d;
string nodeTag = "NULL";
foreach (var item in orderedTransitions)
{
if (item.Key.Item1.Equals(".") && item.Key.Item2 != ".")
{
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(item.Key.Item2)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (item.Value * tagger.BgramLambda2);
double unknownProcent = tagger.GetValueWeightForUnknownWord(testWords[i].word, item.Key.Item2);
product = biTrans * unknownProcent;
nodeTag = item.Key.Item2;
ViterbiNode node = new ViterbiNode(product, nodeTag);
vList.Add(node);
}
}
}
else
{
foreach (var wt in foundWord.TagFreq)
{
if (wt.Key == ".")
{
continue;
}
double emissionFreqValue = wt.Value; // eg. Jane -> 0.1111 (NN)
Tuple <string, string> tuple = new Tuple <string, string>(".", wt.Key);
double biTransition = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(tuple)).Value; // eg. NN->VB - 0.25
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(wt.Key)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (biTransition * tagger.BgramLambda2);
double product = (double)emissionFreqValue * biTrans;
ViterbiNode node = new ViterbiNode(product, wt.Key);
vList.Add(node);
}
}
this.ViterbiGraph.Add(vList);
startPoint = false;
}
else
{
List <ViterbiNode> vList = new List <ViterbiNode>();
if (foundWord != null)
{
if (foundWord.TagFreq.Count == 1 && foundWord.TagFreq.ContainsKey("."))
{
foundWord = null;
}
}
if (foundWord == null)
{
UnknownWords.Add(testWords[i].word);
for (int j = 0; j < this.ViterbiGraph[this.ViterbiGraph.Count - 1].Count; j++)
{
ViterbiNode vGoodNode = new ViterbiNode(0.0d, "NULL");
ViterbiNode elem = this.ViterbiGraph[this.ViterbiGraph.Count - 1][j];
// we take the best transition case where first item is "."
var orderedTransitions = tagger.BigramTransitionProbabilities.OrderByDescending(x => x.Value).ToList();
if (model == "trigram" && triPoz >= 2)
{
if (elem.PrevNode == null)
{
continue;
}
ViterbiNode elem2 = elem.PrevNode;
var orderedTransitionsTri = tagger.TrigramTransitionProbabilities.OrderByDescending(x => x.Value).ToList();
double product = 0.0d;
string nodeTag = "NULL_TRI";
foreach (var item in orderedTransitionsTri)
{
if (item.Key.Item1.Equals(elem2.CurrentTag) && item.Key.Item2.Equals(elem.CurrentTag) && item.Key.Item3 != ".")
{
Tuple <string, string> biTuple = new Tuple <string, string>(elem.CurrentTag, item.Key.Item3);
double biVal = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(biTuple)).Value;
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(item.Key.Item3)).Value;
double triTransition = (double)(tagger.TgramLambda3 * item.Value) + (tagger.TgramLambda2 * biVal) + (tagger.TgramLambda1 * uniVal);
double unknownProcent = tagger.GetValueWeightForUnknownWord(testWords[i].word, item.Key.Item3);
product = (double)elem.value * triTransition * unknownProcent;
nodeTag = item.Key.Item3;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, nodeTag, PrevNode: elem);
}
}
}
}
else
{
double product = 0.0d;
string nodeTag = "NULL_BI";
foreach (var item in orderedTransitions)
{
if (item.Key.Item1.Equals(elem.CurrentTag) && item.Key.Item2 != ".")
{
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(item.Key.Item2)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (item.Value * tagger.BgramLambda2);
double unknownProcent = tagger.GetValueWeightForUnknownWord(testWords[i].word, item.Key.Item2);
product = (double)elem.value * biTrans * unknownProcent;
nodeTag = item.Key.Item2;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, nodeTag, PrevNode: elem);
}
}
}
}
vList.Add(vGoodNode);
}
}
else
{
foreach (var tf in foundWord.TagFreq)
{
if (tf.Key == ".")
{
continue;
}
ViterbiNode vGoodNode = new ViterbiNode(0.0d, "NULL");
foreach (ViterbiNode vn in this.ViterbiGraph[this.ViterbiGraph.Count - 1])
{
if (model == "trigram" && triPoz >= 2)
{
if (vn.PrevNode == null)
{
continue;
}
Tuple <string, string, string> triTuple = new Tuple <string, string, string>(vn.PrevNode.CurrentTag, vn.CurrentTag, tf.Key);
double triVal = tagger.TrigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(triTuple)).Value;
Tuple <string, string> biTuple = new Tuple <string, string>(vn.CurrentTag, tf.Key);
double biVal = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(biTuple)).Value;
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(tf.Key)).Value;
double triTransition = (double)(tagger.TgramLambda3 * triVal) + (tagger.TgramLambda2 * biVal) + (tagger.TgramLambda1 * uniVal);
double product = (double)vn.value * triTransition * tf.Value;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, tf.Key, PrevNode: vn);
}
}
else
{
Tuple <string, string> tuple = new Tuple <string, string>(vn.CurrentTag, tf.Key);
double biTransition = tagger.BigramTransitionProbabilities.FirstOrDefault(x => x.Key.Equals(tuple)).Value; // eg. NN->VB - 0.25
double uniVal = tagger.UnigramProbabilities.FirstOrDefault(x => x.Key.Equals(tf.Key)).Value;
double biTrans = (double)(uniVal * tagger.BgramLambda1) + (biTransition * tagger.BgramLambda2);
double product = (double)vn.value * biTrans * tf.Value;
if (product >= vGoodNode.value)
{
vGoodNode = new ViterbiNode(product, tf.Key, PrevNode: vn);
}
}
}
vList.Add(vGoodNode);
}
}
this.ViterbiGraph.Add(vList);
}
this.ViterbiGraph[this.ViterbiGraph.Count - 1] = this.ViterbiGraph[this.ViterbiGraph.Count - 1].OrderByDescending(x => x.value).ToList();
if (beam != 0)
{
this.ViterbiGraph[this.ViterbiGraph.Count - 1] = this.ViterbiGraph[this.ViterbiGraph.Count - 1].Take(beam).ToList();
}
}
}
public ViterbiNode NextNode; // + bidirectionality
public ViterbiNode(double value, string CurrentTag, ViterbiNode PrevNode = null, ViterbiNode NextNode = null)
{
this.value = value;
this.CurrentTag = CurrentTag;
this.PrevNode = PrevNode;
this.NextNode = NextNode;
}
