import numpy as np
class RunningMeanStd(object):
    # https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm
    def __init__(self, epsilon=1e-4, shape=()):
        self.mean = np.zeros(shape, 'float64')
        self.var = np.ones(shape, 'float64')
        self.count = epsilon
        self.variables_name_save = [ 'mean','var','count', ]


    # TODO: refer to the code of soft actor critic for more elegent writing.
    def save(self ):
        variables = []
        for v in self.variables_name_save:
            variables.append( getattr(self,v) )
        return variables

    def load(self, variables):
        for i, v in enumerate(  self.variables_name_save ):
            setattr(self,v, variables[i] )

    def update(self, x):
        batch_mean = np.mean(x, axis=0)
        batch_var = np.var(x, axis=0)
        batch_count = x.shape[0]
        self.update_from_moments(batch_mean, batch_var, batch_count)

    def update_from_moments(self, batch_mean, batch_var, batch_count):
        delta = batch_mean - self.mean
        tot_count = self.count + batch_count

        new_mean = self.mean + delta * batch_count / tot_count        
        m_a = self.var * (self.count)
        m_b = batch_var * (batch_count)
        M2 = m_a + m_b + np.square(delta) * self.count * batch_count / (self.count + batch_count)
        new_var = M2 / (self.count + batch_count)

        new_count = batch_count + self.count

        self.mean = new_mean
        self.var = new_var
        self.count = new_count    


# def test_loadsave():
#     a = RunningMeanStd()
#     vs = a.save()
#     a.load(vs)

def test_runningmeanstd():
    for (x1, x2, x3) in [
        (np.random.randn(3), np.random.randn(4), np.random.randn(5)),
        (np.random.randn(3,2), np.random.randn(4,2), np.random.randn(5,2)),
        ]:

        rms = RunningMeanStd(epsilon=0.0, shape=x1.shape[1:])

        x = np.concatenate([x1, x2, x3], axis=0)
        ms1 = [x.mean(axis=0), x.var(axis=0)]
        rms.update(x1)
        rms.update(x2)
        rms.update(x3)
        ms2 = [rms.mean, rms.var]

        assert np.allclose(ms1, ms2)

        rms_new = RunningMeanStd(epsilon=0.0, shape=x1.shape[1:])
        rms_new.load( rms.save() )
        print('-'*10)
        print( rms_new.save() , '\n', rms.save() )
        assert rms_new.save() == rms.save()