Why is Chinese chess (xiangqi) harder for computers than international chess?
When one looks at Chinese chess in comparison to international chess, one cannot help but notice how extremely limited several of the pieces are:
- Kings are confined to 9 positions overall (the palace) and have 4 possible moves at most (only 1 step orthogonally).
- The two advisors are confined to 5 shared positions overall (diagonals of the palace), where except for the central palace position they are confined to 1 move.
- The two elephants are confined to 7 shared positions (two touching diamonds side by side, like an 8 on its side), due to having a move that naturally restricts them to a quarter of the board (only every other square the same color as its starting square (on a checkered board)), but then also being restricted to not move across the central dividing river which cuts it down to an expected 1/8. In fact the 7 positions out of 90 which the elephants can reach is an even worse ratio than 1/8 which is due to the specific board dimensions penalizing it even further.
Is there something about the two horses, two chariots (rooks), two cannons, and five chinese pawns that makes this game harder for computers than international chess? Or is it mostly due to the larger board size, 90 vs 64? Or is there some other consideration that I have overlooked?