  We extend the random-walk model of Vitvitska et al. for predicting the spins
of dark matter halos from their merger histories. Using updated merger rates,
orbital parameter distributions, and N-body constraints we show that this model
can accurately reproduce the distribution of spin parameters measured in N-body
simulations when we include a weak correlation between the spins of halos and
the angular momenta of infalling subhalos. We further show that this model is
in approximate agreement with the correlation of the spin magnitude over time
as determined from N-body simulations, while it slightly underpredicts the
correlation in the direction of the spin vector measured from the same
simulations. This model is useful for predicting spins from merger histories
derived from non-N-body sources, thereby circumventing the need for very high
resolution simulations to permit accurate measurements of spins. It may be
particularly relevant to modeling systems which accumulate angular momentum
from halos over time (such as galactic discs) - we show that this model makes
small but significant changes in the distribution of galactic disc sizes
computed using the Galacticus semi-analytic galaxy formation model.
