/// <summary>
/// Backward algorithm (without scaling)
/// </summary>
/// <param name="observations">A sequence of observations.</param>
/// <returns></returns>
private double backward(int[] observations)
{
if (observations == null)
{
throw new ArgumentNullException("observations");
}
if (tempInstancts == null)
{
throw new ArgumentNullException("tempInstancts");
}
int T = observations.Length;
// For beta variables, I use the same scale factors
// for each time t as I used for alpha variables.
// 1. Initialization
for (int i = 0; i < States; i++)
{
tempInstancts[T - 1].Beta[i] = 1.0 / tempInstancts[T - 1].c;
}
// 2. Induction
for (int t = T - 2; t >= 0; t--)
{
for (int i = 0; i < States; i++)
{
double sum = 0.0;
for (int j = 0; j < States; j++)
{
sum += (Transitions.getValue(i, j) * Emissions.getValue(j, observations[t + 1]) * tempInstancts[t + 1].Beta[j]);
}
tempInstancts[t].Beta[i] += sum / tempInstancts[t].c;
}
}
// 3. Termination
double POGivenLambdaScaled = 0.0;
for (int i = 0; i < Probabilities.Length; i++)
{
POGivenLambdaScaled += Probabilities[i] * Emissions.getValue(i, observations[0]) * tempInstancts[0].Beta[i];
}
double scaling = 1.0;
for (int t = 0; t < T; t++)
{
scaling *= tempInstancts[t].c;
}
var POGivenLambda = POGivenLambdaScaled * scaling;
return(POGivenLambda);
}
/// <summary>
/// Determining the frequency of the transition-emission pair values
/// and dividing it by the probability of the entire string.
///
/// Using the scaled values, I don't need to divide it for the
/// probability of the entire string anymore
/// </summary>
/// <param name="t">Current time.</param>
/// <param name="nextObservation">Observation at time t + 1.</param>
/// <returns>Probability of transition from each state to any other at time t</returns>
private MySparse2DMatrix calcXi(int t, int nextObservation)
{
if (tempInstancts == null)
{
throw new ArgumentNullException("tempInstancts");
}
if (tempInstancts[t].Alpha == null || tempInstancts[t].Beta == null)
{
throw new ArgumentNullException("Alpha and Beta aren't set");
}
MySparse2DMatrix currentXi = new MySparse2DMatrix();
double s = 0;
for (int i = 0; i < States; i++)
{
for (int j = 0; j < States; j++)
{
double temp = tempInstancts[t].Alpha[i] * Transitions.getValue(i, j) * Emissions.getValue(j, nextObservation) * tempInstancts[t + 1].Beta[j];
s += temp;
if (temp != 0)
{
currentXi.setValue(i, j, temp);
}
}
}
if (s != 0) // Scaling
{
for (int i = 0; i < States; i++)
{
for (int j = 0; j < States; j++)
{
if (currentXi.getValue(i, j) != 0)
{
currentXi.setValue(i, j, currentXi.getValue(i, j) / s);
}
}
}
}
return(currentXi);
}
/// <summary>
/// Calculates the most likely sequence of hidden states
/// that produced the given observation sequence.
/// </summary>
/// <remarks>
/// Decoding problem. Given the HMM M = (A, B, pi) and the observation sequence
/// O = {o1,o2, ..., oK}, calculate the most likely sequence of hidden states Si
/// that produced this observation sequence O. This can be computed efficiently
/// using the Viterbi algorithm.
/// </remarks>
/// <param name="observations">A sequence of observations.</param>
/// <param name="probability">The state optimized probability.</param>
/// <returns>The sequence of states that most likely produced the sequence.</returns>
private int[] viterbi(int[] observations, out double probability)
{
if (observations == null)
{
throw new ArgumentNullException("observations");
}
if (tempInstancts == null)
{
throw new ArgumentNullException("tempInstancts");
}
if (observations.Length == 0)
{
probability = 0.0;
return(new int[0]);
}
int T = observations.Length;
double maxWeight;
int maxState;
// 1. Base
for (int i = 0; i < States; i++)
{
tempInstancts[0].Delta[i] = Probabilities[i] * Emissions.getValue(i, observations[0]);
}
// 2. Induction
for (int t = 1; t < T; t++)
{
for (int i = 0; i < States; i++)
{
maxWeight = 0.0;
maxState = 0;
for (int k = 0; k < States; k++)
{
double weight = tempInstancts[t - 1].Delta[k] * Transitions.getValue(k, i);
if (weight > maxWeight)
{
maxWeight = weight;
maxState = k;
}
}
tempInstancts[t].Delta[i] = maxWeight * Emissions.getValue(i, observations[t]);
tempInstancts[t].State[i] = maxState;
}
}
// Find maximum value for time T-1
maxWeight = tempInstancts[T - 1].Delta[0];
maxState = 0;
for (int k = 1; k < States; k++)
{
if (tempInstancts[T - 1].Delta[k] > maxWeight)
{
maxWeight = tempInstancts[T - 1].Delta[k];
maxState = k;
}
}
// Trackback
int[] path = new int[T];
path[T - 1] = maxState;
for (int t = T - 2; t >= 0; t--)
{
path[t] = tempInstancts[t + 1].State[path[t + 1]];
}
// Returns the sequence probability as an out parameter
probability = maxWeight;
// Returns the most likely (Viterbi path) for the given sequence
return(path);
}
/// <summary>
/// Forward algorithm (with scaling)
/// </summary>
/// <param name="observations">A sequence of observations.</param>
/// <returns></returns>
private double forward(int[] observations)
{
if (observations == null)
{
throw new ArgumentNullException("observations");
}
if (tempInstancts == null)
{
throw new ArgumentNullException("tempInstancts");
}
int T = observations.Length;
// 1. Initialization
for (int i = 0; i < States; i++)
{
tempInstancts[0].c += tempInstancts[0].Alpha[i] = Probabilities[i] * Emissions.getValue(i, observations[0]);
}
if (tempInstancts[0].c != 0) // Scaling CHECK
{
for (int i = 0; i < States; i++)
{
tempInstancts[0].Alpha[i] = tempInstancts[0].Alpha[i] / tempInstancts[0].c;
}
}
// 2. Induction
for (int t = 1; t < T; t++)
{
for (int i = 0; i < States; i++)
{
double sum = 0.0;
for (int k = 0; k < States; k++)
{
sum += (tempInstancts[t - 1].Alpha[k] * Transitions.getValue(k, i));
}
tempInstancts[t].Alpha[i] = sum * Emissions.getValue(i, observations[t]);
tempInstancts[t].c += tempInstancts[t].Alpha[i]; // Scaling coefficient
}
if (tempInstancts[t].c != 0) // Scaling
{
for (int i = 0; i < States; i++)
{
tempInstancts[t].Alpha[i] = tempInstancts[t].Alpha[i] / tempInstancts[t].c;
}
}
}
// 3. Termination
double POGivenLambdaScaled = 0.0;
for (int i = 0; i < Probabilities.Length; i++)
{
POGivenLambdaScaled += tempInstancts[T - 1].Alpha[i];
}
double scaling = 1.0;
for (int t = 0; t < T; t++)
{
scaling *= tempInstancts[t].c;
}
var POGivenLambda = POGivenLambdaScaled * scaling;
return(POGivenLambda);
}
