2D sketch of the spherical cloud collapse model with the center of mass (CM) in theorigin. Pebble swarms (blue circles) are initiated over a Hill sphere with spherical velocities and positions. During evolution pebble swarms feel both gravity f_g and gas drag f_d. The dashed shells indicate the zones for collision evolution. As the pebbles settle, zones are being rebuilt such that the number of swarms per zone never falls below a desired value N_z.