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    <p>If I was writing logic that dealt with a single ship, or single missile, asteroid, etc Then that logic should clearly belong in the respective class.</p> <p>If I'm writing logic that deals with multiple objects or even worse, multiple objects of different types, I would tend to put that logic in a class that represents a higher level concept.</p> <p>For example, you might have a Universe class that contains a collection of SpaceEntity's. And there is a Universe.FindCollisions() that returns a collection of Collision objects.</p> <pre><code>public class Universe { private IEnumerable&lt;SpaceEntity&gt; _entities; public IEnumerable&lt;Collision&gt; FindCollisions() { foreach(var e1 in _entities) { foreach(var e2 in _entities) { if (e1.CollidesWith(e2)) { yield return new Collision(e1, e2); } } } } } </code></pre> <p>Another way to tackle it might be to have a CollisionCalculator class that accepts a collection of SpaceEntity's in its constructor and is the place where that specific logic lives.</p> <pre><code>public class CollisionCalculator { public IEnumerable&lt;Collision&gt; Collisions {get; set;} public CollisionCalculator(IEnumerable&lt;SpaceEntity&gt; entities) { // logic to build the Collisions collection goes here } } </code></pre>
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    1. PO'Self-aware' classes?
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    1. USDylan Smith
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    1. COThat nested foreach loop will be return two collisions for each overlapping pair of entities. One for the missile colliding with the rock and one for the rock colliding with the missle. You may need to filter these out, depending on how your collision resolution is handled.
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      1. UStcarvin
    2. COIt may be worth noting that the double nested for loop will become impractical as the number of objects gets large (over a few hundred). If there's some distance between objects which will guarantee that they can't possibly collide, it may be helpful to subdivide the universe into blocks which are larger than that, and keep a list of the objects which are at least partially in each block. Using simple partitioning in 3d space, some objects might have to appear in up to eight lists, but testing 100 groups of ten objects for collisions will be faster than testing one group of 1,000.
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      1. USsupercat
    3. COIncidentally, it's possible and not overly difficult to subdivide 3-dimensional space into regions such that no object would fall into more than four regions (rather than eight). If rounding Y to the nearest integer would yield an odd number, add 0.5 to X. If rounding Z to the nearest integer would yield an odd number, add 0.5 to both X and Y. Then round all three coordinates to integers. A mapping between spacial coordinates and XY values would show that space was divided into unit cubes placed so that no more than four would be within a quarter unit of any point.
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