维特比算法实际是用动态规划解隐马尔科夫模型的预测问题,首先导入两个变量 δ \delta δ和 ψ \psi ψ。
定义在时刻 t t t状态为 q i q_i qi的所有单个路径 ( y 1 , y 2 , ⋯   , y t ) ({y_1},{y_2}, \cdots ,{y_t}) (y1,y2,⋯,yt)中概率最大值为:
δ t ( q i ) = max y 1 , y 2 , ⋯   , y t − 1 P ( y t = q i , y t − 1 , ⋯   , y 1 , x t , ⋯   , x 1 ∣ λ ) , i = 1 , 2 , ⋯   , N {\delta _t}\left( {{q_i}} \right) = \mathop {\max }\limits_{{y_1},{y_2}, \cdots ,{y_{t - 1}}} P\left( {{y_t} = {q_i},{y_{t - 1}}, \cdots ,{y_1},{x_t}, \cdots ,{x_1}\left| \lambda \right.} \right),i = 1,2, \cdots ,N δt(qi)=y1,y2,⋯,yt−1maxP(yt=qi,yt−1,⋯,y1,xt,⋯,x1∣λ),i=1,2,⋯,N
则变量 δ \delta δ的递推公式为:
δ t + 1 ( q i ) = max y 1 , y 2 , ⋯   , y t P ( y t + 1 = q i , y t , ⋯   , y 1 , x t + 1 , ⋯   , x 1 ∣ λ ) = max 1 ≤ j ≤ N [ δ t ( j ) a j i ] b i ( x t + 1 ) , i = 1 , 2 , ⋯   , N ; t = 1 , 2 , ⋯   , n − 1 {\delta _{t + 1}}\left( {{q_i}} \right) = \mathop {\max }\limits_{{y_1},{y_2}, \cdots ,{y_t}} P\left( {{y_{t + 1}} = {q_i},{y_t}, \cdots ,{y_1},{x_{t + 1}}, \cdots ,{x_1}\left| \lambda \right.} \right) = \mathop {\max }\limits_{1 \le j \le N} \left[ {{\delta _t}\left( j \right){a_{ji}}} \right]{b_i}({x_{t + 1}}),i = 1,2, \cdots ,N;t = 1,2, \cdots ,n - 1 δt+1(qi)=y1,y2,⋯,ytmaxP(yt+1=qi,yt,⋯,y1,xt+1,⋯,x1∣λ)=1≤j≤Nmax[δt(j)aji]bi(xt+1),i=1,2,⋯,N;t=1,2,⋯,n−1
定义在时刻 t t t状态为 q i q_i qi的所有单个路径 ( y 1 , y 2 , ⋯   , y t − 1 , q i ) ({y_1},{y_2}, \cdots ,{y_{t - 1}},q_i) (y1,y2,⋯,yt−1,qi)中概率最大的路径的第 t − 1 t-1 t−1个节点为:
ψ t ( q i ) = arg max 1 ≤ j ≤ N [ δ t − 1 ( j ) a j i ] , i = 1 , 2 , ⋯   , N {\psi _t}(q_i) = \arg \mathop {\max }\limits_{1 \le j \le N} \left[ {{\delta _{t - 1}}\left( j \right){a_{ji}}} \right],i = 1,2, \cdots ,N ψt(qi)=arg1≤j≤Nmax[δt−1(j)aji],i=1,2,⋯,N
维特比算法步骤:
输入:模型 λ = [ A , B , ∏ ] \lambda = [A,B,\prod ] λ=[A,B,∏]和观测序列 X = { x 1 , x 2 , ⋯   , x n } X = \left\{ {{x_1},{x_2}, \cdots ,{x_n}} \right\} X={x1,x2,⋯,xn}
输出:最优状态序列 Y = { y 1 , y 2 , ⋯   , y n } Y = \left\{ {{y_1},{y_2}, \cdots ,{y_n}} \right\} Y={y1,y2,⋯,yn}
(1)初始化:
δ 1 ( q i ) = π i b i ( x 1 ) , i = 1 , 2 , ⋯   , N ψ 1 ( q i ) = 0 , i = 1 , 2 , ⋯   , N \begin{array}{l} {\delta _1}({q_i}) = {\pi _{_i}}{b_i}\left( {{x_1}} \right),i = 1,2, \cdots ,N\\ {\psi _1}\left( {{q_i}} \right) = 0,i = 1,2, \cdots ,N \end{array} δ1(qi)=πibi(x1),i=1,2,⋯,Nψ1(qi)=0,i=1,2,⋯,N
(2)递推:对于 t = 2 , 3 , ⋯   , n t = 2,3, \cdots ,n t=2,3,⋯,n
δ t ( q i ) = max 1 ≤ j ≤ N [ δ t − 1 ( j ) a j i ] b i ( x t ) , i = 1 , 2 , ⋯   , N ψ t ( q i ) = arg max 1 ≤ j ≤ N [ δ t − 1 ( j ) a j i ] , i = 1 , 2 , ⋯   , N \begin{array}{l} {\delta _t}\left( {{q_i}} \right) = \mathop {\max }\limits_{1 \le j \le N} \left[ {{\delta _{t - 1}}\left( j \right){a_{ji}}} \right]{b_i}({x_t}),i = 1,2, \cdots ,N\\ {\psi _t}\left( {{q_i}} \right) = \arg \mathop {\max }\limits_{1 \le j \le N} \left[ {{\delta _{t - 1}}\left( j \right){a_{ji}}} \right],i = 1,2, \cdots ,N \end{array} δt(qi)=1≤j≤Nmax[δt−1(j)aji]bi(xt),i=1,2,⋯,Nψt(qi)=arg1≤j≤Nmax[δt−1(j)aji],i=1,2,⋯,N
(3)终止:
y n = arg max 1 ≤ i ≤ N [ δ n ( q i ) ] {y_n} = \arg \mathop {\max }\limits_{1 \le i \le N} \left[ {{\delta _n}\left( {{q_i}} \right)} \right] yn=arg1≤i≤Nmax[δn(qi)]
(4)最优路径回朔:对于 t = n − 1 , n − 2 , ⋯   , 1 t = n - 1,n - 2, \cdots ,1 t=n−1,n−2,⋯,1
y t = ψ t + 1 ( y t + 1 ) {y_t} = {\psi _{t + 1}}\left( {{y_{t + 1}}} \right) yt=ψt+1(yt+1)
代码参考资料2
import numpy as np
class HMM:
def __init__(self, A, B, Pi, O):
self.A = np.array(A) # 状态转移概率矩阵
self.B = np.array(B) # 观测概率矩阵
self.Pi = np.array(Pi) # 初始状态概率矩阵
self.O = np.array(O) # 观测序列
self.N = self.A.shape[0] # 状态取值个数
self.M = self.B.shape[1] # 观测值取值个数
def viterbi(self):
n = len(self.O) #观测序列和状态序列的长度
Y = np.zeros(n) #初始化状态序列
delta = np.zeros((n,self.N))
psi = np.zeros((n,self.N))
#初始化
for i in range(self.N):
delta[0,i] = self.Pi[i] * self.B[i,self.O[0]]
psi[0,i] = 0
#递推
for t in range(1,n):
for i in range(self.N):
delta[t,i] = self.B[i,self.O[t]] * np.array([delta[t-1,j] * self.A[j,i] for j in range(self.N)]).max()
psi[t,i] = np.array([delta[t-1,j] * self.A[j,i] for j in range(self.N)]).argmax()
print(psi)
#终止
Y[n-1] = delta[n-1,:].argmax()
print(Y[n-1])
#回朔
for t in range(n-2,-1,-1):
Y[t] = psi[int(t+1),int(Y[t+1])]
return Y
if __name__ == '__main__':
print('----------------1.初始化HMM模型参数------------------')
A = [[0,1,0,0],[0.4,0,0.6,0],[0,0.4,0,0.6],[0,0,0.5,0.5]]
B = [[0.5,0.5],[0.3,0.7],[0.6,0.4],[0.8,0.2]]
Pi = [0.25,0.25,0.25,0.25]
print('----------------2.观测序列---------------------------')
O = [0,1,0]
print('----------------3.viterbi 算法-----------------------')
hmm = HMM(A,B,Pi,O)
Y = hmm.viterbi()
1、《统计学习方法》李航
2、https://www.cnblogs.com/d-roger/articles/5719979.html