Add Game "Fox and Geese" - #1583
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Adds the initial file structure, header declarations, and build setup for the Fox and Geese game. - Created `fox_and_geese.h` with state space calculations (that I posted on Mathematics Stack Exchange: https://math.stackexchange.com/questions/5145511/fox-and-geese-state-space-calculation) and game parameters - Added `fox_and_geese` library target to `CMakeLists.txt` - Registered new game in Python bindings test suite
Replace the copied tic-tac-toe initialization with Fox and Geese setup: - Add a compile-time mask for the 33-point cross (7x7 grid minus the four 2x2 corners) and use it to mark out-of-bounds cells. - Add the nested traditional geese layout table, read as a prefix of length num_geese, with the fox on the center point. - Wire up the num_foxes/num_geese parameters and reject anything other than one fox and 13, 15, or 17 geese. - Fix the cell state helpers for the fox/goose/out-of-bounds enum. - Drop the tic-tac-toe line-win logic and IsFull(); IsTerminal() now reads outcome_ only. Move generation, win conditions, undo, and the struct-based constructor are still stubbed and do not compile yet.
Replace the placeholder placement logic with the real rules. - Add compile-time adjacency and jump tables. Diagonals exist only on lines incident to the five hub points, and are recorded in both directions; jumps are derived edge by edge rather than assumed, so hub-to-hub lines like (3,1)-(2,2)-(1,3) are included and off-board ones like (3,3)-(2,2)-(1,1) are not. - Encode actions as (from, to) pairs plus a reserved end-turn action, and update NumDistinctActions and the action struct to match. - Generate fox steps and jumps, and geese steps restricted to forward and sideways in the 15- and 17-goose games. - Keep the move with the fox while further jumps are available, since captures are optional and may be chained. - Set the outcome when the geese fall below kMinGeeseToTrapFox or the player to move has no legal action, and draw at the move limit. - Reverse moves, captures, and chain state via an undo record stack. - Rebuild the struct-based constructor around the board shape and the real piece counts. - Add the games.md entry.
- Start with the geese to move, per the majority of rule sets. - Carry continue_jump_from through the state struct so a fox partway through a jump chain survives serialization, and reject values that no reachable state could produce. - Serialize the stored current player rather than CurrentPlayer(), which reports kTerminalPlayerId and so made terminal states fail to round-trip. - Document that a player with no legal move loses and that reaching the move limit is a draw.
Add fox_and_geese_test.cc covering: - game parameters and the num_foxes/num_geese validators - all three starting layouts (13/15/17 geese) against expected board strings, piece counts, and opening move counts - board geometry (diagonal vs non-diagonal points) via legal-move counts from isolated positions - optional captures and multi-jump chains, including kEndTurnAction - goose movement restrictions in the 15/17-goose games - both win conditions and the no-legal-move rule - UndoAction for steps and captures - state/observation/action struct JSON round-trips, including a mid-jump-chain state - RandomSimTest and RandomSimTestWithUndo across all three configs
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Implements Fox and Geese, an asymmetric, zero-sum, perfect-information hunting game played on a 33-point cross-shaped board (7x7 grid with the four 2x2 corners removed). One fox tries to capture geese by jumping over them; the geese try to trap the fox so it has no legal move. The fox wins if it reduces the geese below the number needed to trap it (4); the geese win by immobilizing the fox; a player with no legal move loses; games reaching the move limit are declared a draw.
Here's the starting position (13 geese, the traditional default):
Only the classic one-fox game is implemented, with the three traditional starting configurations (13/15/17 geese, selectable via the
num_geeseparameter). The two-fox Scandinavian variant (Halatafl/Rävspelet) and Asalto are out of scope - they have different win conditions and board rules, and are separate games rather than parameterizations of this one.Captures are optional and may be chained (a jump keeps the turn if a further jump is available); geese in the 15/17-goose configurations are restricted to forward/sideways movement, per the historical rules that offset the extra material.
Closes #1581.
Testing
fox_and_geese_test,pyspiel_test.py, andapi_test.py.fox_and_geese_test.cccovers all three starting layouts, board geometry (diagonal vs. orthogonal points), optional and chained captures, goose movement restrictions, both win conditions, undo, and JSON struct round-trips (including mid-jump-chain states), plusRandomSimTest/RandomSimTestWithUndoacross all configurations.num_foxes/num_geesevalues are rejected at construction with explanatory errors; verified viapyspiel.load_game.exampleandmcts_example, including 1000-simulation MCTS self-play across all three configurations, which exercised multi-jump chains (up to 5 captures in a single turn) end to end.playthrough_test.py.