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# For positive direction:
if self.directions[id]:
    if last_desired[0] + x_stride < self.arena_limits[0][1]:
        new_desired = (last_desired[0] + x_stride, last_desired[1])
    else:
        # Instead of keeping last_desired[0], reset x to arena_limits[0][0]
        new_desired = (self.arena_limits[0][0], last_desired[1] + y_stride)
        self.directions[id] = not self.directions[id]
else:
    if last_desired[0] - x_stride > self.arena_limits[0][0]:
        new_desired = (last_desired[0] - x_stride, last_desired[1])
    else:
        # Reset x here as well
        new_desired = (self.arena_limits[0][0], last_desired[1] + y_stride)
        self.directions[id] = not self.directions[id]