By providing a comparer that could change over time (as estimated costs on the graph were updated), this meant the priority queue could have its heap property invalidated and thus not maintain a correct ordering. Instead we store elements into the queue with their estimations at the time. This preserves the heap property and thus ensures the queue returns properly ordered results, although it may contain out of date estimations.
This also improves performance. The fixed comparer need not perform expensive lookups into the graph, but can instead use the readily available value. This speeds up adds and removes on the queue significantly.
The path caching works on the assumption that the time saved from reusing a cached path outweights the cost of caching it in the first place.
For unit paths, this assumption holds. For path searchs, we spend more time caching them then we save when we get to reuse these cached paths. This is because they are reused less often, and calculating their key is more expensive in comparison.
This method gets called often via Contains calls. We can significantly speed up the method for flat maps since we know the projection and it is trivial to perform. This avoids an expensive projection lookup.
The caller can enumerate the list more efficiently without the IEnumerable indirection, and the reduced memory allocation is marginally faster than allocating a linked list and several nodes.
As the world tileset is fixed, the pathfinder can look up the terrain information for that tileset on creation. This is implemented by the WorldMovementInfo struct. When calculating node costs, this allows the pathfinder to avoid having to repeat this expensive dictionary lookup on every node.
This method performs an expensive calculation and is called often during pathfinding. We create a cache of the terrain indicies for the map to vastly reduce the cost.
