def getNextPosition(self):
# Diffusion is one-way down.
positions = Position.getSameHeightNeighborPositions(self.position)
unoccupied_positions = []
for position in positions:
if not Context.getField().isSiteOccupied(position):
unoccupied_positions.append(position)
if len(unoccupied_positions) == 0:
return self.position
rn = random.randint(0, len(unoccupied_positions)-1)
position = unoccupied_positions[rn]
return Context.getField().getLowestUnoccupiedPosition(position.x, position.y)
