Ruby写的向前算法

class FowardAlgorithm
  def initialize(pi, trans_pro, emit_pro)
    @pi = pi
    @trans_pro = trans_pro
    @emit_pro = emit_pro
  end

  def alpha1(state, ob)
    @pi[state] * @emit_pro[state][ob]
  end

  def alpha(t, state)
    # t starts from 0, states starts from 9
    if t.equal? 0
      alpha1(state, @ob[0])
    else
      sum = (0...@trans_pro.size).inject(0.0) { |sum, i| sum += alpha(t-1, i) * @trans_pro[i][state] }
      sum * @emit_pro[state][@ob[t]]
    end
  end

  def p_rerial_n ob
    n = ob.size
    n -= 1
    @ob = ob
    # time & states
    res = 0.0
    0..@trans_pro.size.times { |i|
      #print "time #{n}, state #{i}"
      tmp = alpha n, i
      res += tmp
    }
    res
  end
end

def test
  pi = [0.2, 0.4, 0.4]
  trans_pro = [
      [0.5, 0.2, 0.3],
      [0.3, 0.5, 0.2],
      [0.2, 0.3, 0.5]
  ]

  emit_pro = [
      [0.5, 0.5],
      [0.4, 0.6],
      [0.7, 0.3]
  ]

  f = FowardAlgorithm.new pi, trans_pro, emit_pro
  puts f.p_rerial_n([0, 1, 0, 1])
end

if $PROGRAM_NAME == __FILE__
  test
end