Finding good heuristic for A* search

Question

I'm trying to find the optimal solution for a little puzzle game called Twiddle (an applet with the game can be found here). The game has a 3x3 matrix with the number from 1 to 9. The goal is to bring the numbers in the correct order using the minimum amount of moves. In each move you can rotate a 2x2 square either clockwise or counterclockwise.

I.e. if you have this state

6 3 9
8 7 5
1 2 4

and you rotate the upper left 2x2 square clockwise you get

8 6 9
7 3 5
1 2 4

I'm using a A* search to find the optimal solution. My f() is simply the number of rotations needed. My heuristic function already leads to the optimal solution (if I modify it, see the notice a t the end) but I don't think it's the best one you can find. My current heuristic takes each corner, looks at the number at the corner and calculates the manhatten distance to the position this number will have in the solved state (which gives me the number of rotation needed to bring the number to this postion) and sums all these values. I.e. You take the above example:

6 3 9
8 7 5
1 2 4

and this end state

1 2 3
4 5 6
7 8 9 

then the heuristic does the following

6 is currently at index 0 and should by at index 5: 3 rotations needed
9 is currently at index 2 and should by at index 8: 2 rotations needed
1 is currently at index 6 and should by at index 0: 2 rotations needed
4 is currently at index 8 and should by at index 3: 3 rotations needed

h = 3 + 2 + 2 + 3 = 10

Additionally, if h is 0, but the state is not completely ordered, than h = 1.

But there is the problem, that you rotate 4 elements at once. So there a rare cases where you can do two (ore more) of theses estimated rotations in one move. This means theses heuristic overestimates the distance to the solution.

My current workaround is, to simply excluded one of the corners from the calculation which solves this problem at least for my test-cases. I've done no research if really solves the problem or if this heuristic still overestimates in some edge-cases.

So my question is: What is the best heuristic you can come up with?

(Disclaimer: This is for a university project, so this is a bit of homework. But I'm free to use any resource if can come up with, so it's okay to ask you guys. Also I will credit Stackoverflow for helping me ;) )

Answer 1

Simplicity is often most effective. Consider the nine digits (in the rows-first order) as forming a single integer. The solution is represented by the smallest possible integer i(g) = 123456789. Hence I suggest the following heuristic h(s) = i(s) - i(g). For your example, h(s) = 639875124 - 123456789.

Answer 2

You can get an admissible (i.e., not overestimating) heuristic from your approach by taking all numbers into account, and dividing by 4 and rounding up to the next integer.

To improve the heuristic, you could look at pairs of numbers. If e.g. in the top left the numbers 1 and 2 are swapped, you need at least 3 rotations to fix them both up, which is a better value than 1+1 from considering them separately. In the end, you still need to divide by 4. You can pair up numbers arbitrarily, or even try all pairs and find the best division into pairs.