  Non-Gaussian quantum states, described by negative valued Wigner functions,
are important both for fundamental tests of quantum physics and for emerging
quantum information technologies. One of the promising ways of generation of
the non-Gaussian states is the use of the cubic (Kerr) optical non-linearity,
which produces the characteristic banana-like shape of the resulting quantum
states. However, the Kerr effect in highly transparent optical materials is
weak. Therefore, big number of the photons in the optical mode ($n\gtrsim10^6$)
is necessary to generate an observable non-Gaussianity. In this case, the
direct approach to calculation of the Wigner function becomes extremely
computationally expensive.
  In this work, we develop quick algorithms for computing the Husimi and Wigner
quasiprobability functions of these non-Gaussin states by means of the Kerr
nonlinearity. This algorithm can be used for any realistic values of the
photons number and the non-linearity.
