baseline: OpenMU upstream b5a0961 (fresh source)

docs/GameMap.md

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+# GameMap
+
+I want to give a quick description about how the [game map](../src/GameLogic/GameMap.cs),
+especially observing between players/npc works.
+
+## AreaOfInterestManager
+
+Each game map has an area of interest manager. It takes care of the event
+subscriptions between objects, in case they were added, moved or removed at the
+map.
+
+The default implementation ([BucketAreaOfInterestManager](../src/GameLogic/BucketAreaOfInterestManager.cs))
+creates a BucketMap, which is described below.
+Other (simpler) implementations may be possible for the future for maps which
+are more likely to be "empty" or require that all objects of a map need to know
+all other objects (e.g. Duel map).
+
+## BucketMap
+
+The [bucket map](../src/GameLogic/BucketMap{T}.cs) is basically a two-dimensional
+data structure.
+We split the map of 256 x 256 possible coordinates into "[buckets](../src/GameLogic/Bucket{T}.cs)".
+For example, each bucket covers 8 x 8 coordinates.
+
+A player (or other "observers") can observe buckets, so that it gets informed
+when objects (other players, npc, dropped items) enter or leave this bucket.
+When a player moves and comes into the range of a bucket (defined by an
+"info range"), it subscribes this enter/leave events and adds all existing
+objects of this bucket to his view.
+
+You may ask, why I introduced such a structure and not simply use a list of
+objects and compare the distance to them when moving to decide if a object
+should be sent (add/remove) to the game client.
+As far as I know, the original server works like that and has a lot of lag when
+a lot of players (more than thousand at castle siege) are on the same map, all
+moving around.
+This is a O(n²) problem where n is the number of objects on the same map - this
+can get pretty complex very fast.
+
+Now with my concept, this search is only happening when a player enters a new
+bucket - and then it searches only for new buckets in range, not for other
+individual objects.
+So this is not as complex when you have many objects on your map. Of course,
+this needs a bit more memory and is slower on empty maps, but IMHO that's
+really worth it - usually, game maps are packed with monsters, players and a
+lot of dropped items.
+
+## Other ideas
+
+Instead of partitioning a map into buckets (where a lot of them are empty), we
+could try to use some other 2d index structures, like
+[R-trees](https://en.wikipedia.org/wiki/R-tree) or [B-Trees](https://en.wikipedia.org/wiki/B-tree)
+with [Z-Ordering](https://en.wikipedia.org/wiki/Z-order_curve).