  We present flatspin, a novel simulator for systems of interacting mesoscopic
spins on a lattice, also known as artificial spin ice (ASI). Our magnetic
switching criteria enables ASI dynamics to be captured in a dipole model.
Through GPU acceleration, flatspin can simulate realistic dynamics of millions
of magnets within practical time frames. We demonstrate flatspin's versatility
through the reproduction of a diverse set of established experimental results
from the literature. In particular, magnetization details of "pinwheel" ASI
during field-driven reversal have been reproduced, for the first time, by a
dipole model. The simulation framework enables quick exploration and
investigation of new ASI geometries and properties at unprecedented speeds.
