C++ pathfinding with a-star, optimization

Question

Im wondering if I can optimize my pathfinding code a bit, lets look at this map:

+ - wall, . - free, S - start, F - finish
.S.............
...............
..........+++..
..........+F+..
..........+++..
...............

The human will look at it and say its impossible, becouse finish is surrounded... But A-star MUST check all fields to ascertain, that there isnt possible road. Well, its not a problem with small maps. But when I have 256x265 map, it takes a lot of time to check all points. I think that i can stop searching while there are closed nodes arround the finish, i mean:

+ - wall, . - free, S - start, F - finish, X - closed node
.S.............
.........XXXXX.
.........X+++X.
.........X+F+X.
.........X+++X.
.........XXXXX.

And I want to finish in this situation (There is no entrance to "room" with finish). I thought to check h, and while none of open nodes is getting closer, then to finish... But im not sure if its ok, maybe there is any better way?

Thanx for any replies.

Answer 1

First of all this problem is better solved with breadth-first search , but I will assume you have a good reason to use a-star instead. However I still recommend you first check the connectivity between S and F with some kind of search(Breadth-first or depth-first search). This will solve our issue.

Answer 2

Assuming the map doesn't change, you can preprocess it by dividing it to connected components . It can be done with a fast disjoint set data structure . Then before launching A* you check in constant time that the source and destination belong to the same component. If not—no path exists, otherwie you run A* to find the path.

The downside is that you will need additional n-bits per cell where n = ceil(log C) for C being the number of connected components. If you have enough memory and can afford it then it's OK.

Edit: in case you fix n being small (eg one byte) and have more than that number of components (eg more than 256 for 8-bit n) then you can assign the same number to multiple components. To achieve best results make sure each component-id has nearly the same number of cells assigned to it.