fn update_flow(&mut self, particle: &Particle) {
        self.flow_updates.x += particle.velocity.x;
        self.flow_updates.y += particle.velocity.y;
        self.particle_count += 1;
    }
// cache an update to this cell's flow according to a particle in it
// call by each particle in this cell
cache an update to this cell's flow according to a particle in it
call by each particle in this cell
cache an update to this cell ' s flow according to a particle in it call by each particle in this cell
fn update_flow(&mut self, particle: &Particle) {
        self.flow_updates.x += particle.velocity.x;
        self.flow_updates.y += particle.velocity.y;
        self.particle_count += 1;
    }
fn update_flow ( & mut self , particle : & Particle ) { self . flow_updates . x += particle . velocity . x ; self . flow_updates . y += particle . velocity . y ; self . particle_count += 1 ; }
cache an update to this cell's flow according to a particle in it
call by each particle in this cell
cache an update to this cell ' s flow according to a particle in it call by each particle in this cell
&mut self
particle
&Particle
&mut self
particle
&Particle