  We compare two calculations of the particle density in the superfluid phase
of the classical XY model with a chemical potential $\mu$ in 1+1 dimensions.The
first relies on exact blocking formulas from the Tensor Renormalization Group
(TRG) formulation of the transfer matrix. The second is a worm algorithm. We
show that the particle number distributions obtained with the two methods agree
well. We use the TRG method to calculate the thermal entropy and the
entanglement entropy. We describe the particle density, the two entropies and
the topology of the world lines as we increase $\mu$ to go across the
superfluid phase between the first two Mott insulating phases. For a
sufficiently large temporal size, this process reveals an interesting fine
structure: the average particle number and the winding number of most of the
world lines in the Euclidean time direction increase by one unit at a time. At
each step, the thermal entropy develops a peak and the entanglement entropy
increases until we reach half-filling and then decreases in a way that
approximately mirror the ascent. This suggests an approximate fermionic
picture.
