import operator
MIN_FLOAT=-3.14e100

def get_top_states(t_state_v,K=4):
    items = t_state_v.items()
    topK= sorted(items,key=operator.itemgetter(1),reverse=True)[:K]
    return [x[0] for x in topK]

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 range(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 = states.get(obs[t],all_states)
        obs_states = set(obs_states) &  set(prev_states_expect_next)

        if len(obs_states)==0: obs_states = prev_states_expect_next
        if len(obs_states)==0: obs_states = all_states

        for y in obs_states:
            (prob,state ) = max([(V[t-1][y0] + trans_p[y0].get(y,MIN_FLOAT) + 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)