Chess 2

Chess2 implemented in Go.

To start a Chess 2 API server and hit it with a request:

make serve &
http -v :8080/new white==c black==k
http -v :8080/move epd="rnbkkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 ck 33" move=d2d4

To test the engine:

make test perft

Interpretation of Chess 2 rules

• There is a duel each time a move other than a king's captures an opponent's piece. The duel can be skipped, meaning that the defender does not issue a challenge. There can be multiple duels for a single move in the case of an Elephant's rampage.
• A duel is "legal" if the defender has enough stones to initiate a challenge and pay for their bid; and the attacker has enough stones to pay for their bid.
• A move is "pseudo-legal" if it:
  • moves one of the player to move's pieces;
  • follows the distance and direction rules for the piece moved;
  • all intermediate squares are empty (except for jump moves) or capturable (for the Elephant's rampage);
  • the target square is empty or contains a capturable piece; and
  • all duels are legal.
  • Additionally, a pass move is pseudo-legal during a king turn.
• A piece is "threatened" if there is a pseudo-legal move which results in the capture of the piece.
• A move is "into check" if it leaves the board in a state where any of the player's kings are threatened.
• A move is "legal" if it is pseudo-legal and not into check.

Bugs

The engine presently incorrectly considers a color in check if an enemy elephant could capture the king, even if that capture is illegal because the rampage would capture the enemy's king.

Performance tests

Here are the timings for chess2_perft at depth 3.

Configuration	Speed	Relative
Python engine	636.73	1.00000
Go engine, brute force	6.62	0.01040
Go engine, fast	2.45	0.00385