# Viterbi Algorithm in HMM

Viterbi algorithm in the context of a Hidden Markov Model (HMM) aims to find the sequence of hidden states which gives the highest posterior probability. Let $X=X_{1:T}$ (all observations from time 1 to time $T$) be the observations and $Z=Z_{1:T}$ be the latent variables. Viterbi algorithm solves the following problem

\begin{align*} \max_{Z}p(Z|X) & \propto\max_{Z}p(X,Z)\\ & =\max_{Z_{1:T}}p(z_{1})\prod_{i=1}^{T}p(x_{i}|z_{i})\prod_{i=2}^{T}p(z_{i}|z_{i-1}) \end{align*}

If each $Z_{i}$ have $K$ states, then we have totally $K^{T}$ possible sequences to consider. Certainly it is computationally prohibitive to enumerate each sequence, evaluate its probability and take the maximal one. Instead, we want to take advantage of the chain structure and design an efficient algorithm which incrementally finds the best state for the next time point $t+1$ given the solution upto time $t$. This is what the Viterbi algorithm does.

Consider the case where the chain has length 1 i.e., $T=1$. The problem becomes

$\max_{Z_{1}}p(z_{1})p(x_{1}|z_{1})$

which can be solved straightforwardly by enumerating just $K$ possible values of $Z_{1}$.

With $T=2$, we have \begin{align*} \max_{Z}p(X,Z) & =\max_{Z_{1:2}}p(z_{1})p(x_{1}|z_{1})p(z_{2}|z_{1})p(x_{2}|z_{2})\\ & =\max_{Z_{2}}p(x_{2}|z_{2})\overbrace{\max_{Z_{1}}p(z_{1})p(x_{1}|z_{1})p(z_{2}|z_{1})}^{\text{function of }Z_{2}}. \end{align*}

For $T=3$, we have \begin{align*} \max_{Z}p(X,Z) & =\max_{Z_{1:3}}p(z_{1})p(x_{1}|z_{1})p(z_{2}|z_{1})p(x_{2}|z_{2})p(z_{3}|z_{2})p(x_{3}|z_{3})\\ & =\max_{Z_{3}}p(x_{3}|z_{3})\underbrace{\max_{Z_{2}}p(x_{2}|z_{2})p(z_{3}|z_{2})\overbrace{\max_{Z_{1}}p(z_{1})p(x_{1}|z_{1})p(z_{2}|z_{1})}^{\text{function of }Z_{2}}}_{\text{function of }Z_{3}}. \end{align*} It can be seen that the solution to the case $T=3$ contains some part which is exactly identical to the term in the solution for the case $T=2$, namely $\overbrace{\max_{Z_{1}}p(z_{1})p(x_{1}|z_{1})p(z_{2}|z_{1})}^{\text{function of }Z_{2}}$. This suggests that if we have the solution up to time $t$, solving the problem up to time $t+1$ can be carried out by updating the solution. Updating the solution is done through a function (actually a vector of $K$ values) $m_{t}$.

Define $m_{1}(k):=p(x_{1}|Z_{1}=k)p(Z_{1}=k)$. The solution to the case $T=1$ is \begin{align*} \max_{Z_{1}}m_{1}(Z_{1}) & . \end{align*} If we define $m_{2}(k)$ as \begin{align*} m_{2}(k) & :=p(x_{2}|Z_{2}=k)\max_{Z_{1}}m_{1}(Z_{1})p(Z_{2}=k|Z_{1}), \end{align*} then the solution to the case $T=2$ is $\max_{Z_{2}}m_{2}(Z_{2}).$ Following the same pattern, for $T=3$, we define $m_{3}$ as \begin{align*} m_{3}(k) & :=p(x_{3}|Z_{3}=k)\max_{Z_{2}}m_{2}(Z_{2})p(Z_{3}=k|Z_{2}) \end{align*} and we have the solution to the case $T=3$ as $\max_{Z_{3}}m_{3}(Z_{3})$. The problems written in terms of $m_{1},m_{2}$ and $m_{3}$ are exactly the same as previously given in terms of conditional probabilities alone.

In general, define $m_{t}(k)$ as $\begin{eqnarray*} m_{t}(k) & := & p(x_{t}|Z_{t}=k)\max_{z_{t-1}}m_{t-1}(z_{t-1})p(Z_{t}=k|z_{t-1})\\ m_{1}(k) & := & p(x_{1}|Z_{1}=k)p(Z_{1}=k). \end{eqnarray*}$ So the solution to our original problem is $\max_{Z_{T}}m_{T}(Z_{T})$. Note that $m_{t}(k)$ can be interpreted as the maximum probability among all possible sequences which end with $Z_{t}=k$. Obviously the best sequence can be found by updating $m_{t}$ from $t=1$ to $T$ and keep track of

$Z_{t}=\arg\max_{k}m_{t}(k).$