Function separation

Separation (§12 flocking): steer away from crowding neighbours.

repulsion = Σ (position − neighbour.position) / |position − neighbour.position|²
desiredVelocity = normalize(repulsion) · maxSpeed
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Each neighbour contributes a unit vector away from it, weighted by 1 / distance, so close neighbours dominate distant ones — the standard inverse-distance falloff. The neighbour set is the neighbourhood (see the module note); a neighbour exactly on top of the agent has no direction to push along and is skipped rather than producing NaN.

With no neighbours — or with contributions that cancel exactly — the result is the zero vector, not a braking −velocity. "Nothing to avoid" must be an abstention, or a weighted blend of behaviours would silently brake whenever the flock spread out.