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import sys
import operator
MIN_FLOAT = -3.14e100
MIN_INF = float("-inf")
if sys.version_info[0] > 2:
xrange = range
def get_top_states(t_state_v, K=4):
return sorted(t_state_v, key=t_state_v.__getitem__, reverse=True)[:K]
def viterbi(obs, states, start_p, trans_p, emit_p):
V = [{}] # tabular
mem_path = [{}]
all_states = trans_p.keys()
for y in states.get(obs[0], all_states): # init
V[0][y] = start_p[y] + emit_p[y].get(obs[0], MIN_FLOAT)
mem_path[0][y] = ''
for t in xrange(1, len(obs)):
V.append({})
mem_path.append({})
#prev_states = get_top_states(V[t-1])
prev_states = [
x for x in mem_path[t - 1].keys() if len(trans_p[x]) > 0]
prev_states_expect_next = set(
(y for x in prev_states for y in trans_p[x].keys()))
obs_states = set(
states.get(obs[t], all_states)) & prev_states_expect_next
if not obs_states:
obs_states = prev_states_expect_next if prev_states_expect_next else all_states
for y in obs_states:
prob, state = max((V[t - 1][y0] + trans_p[y0].get(y, MIN_INF) +
emit_p[y].get(obs[t], MIN_FLOAT), y0) for y0 in prev_states)
V[t][y] = prob
mem_path[t][y] = state
last = [(V[-1][y], y) for y in mem_path[-1].keys()]
# if len(last)==0:
# print obs
prob, state = max(last)
route = [None] * len(obs)
i = len(obs) - 1
while i >= 0:
route[i] = state
state = mem_path[i][state]
i -= 1
return (prob, route)
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