feat: add counterfactual regret minimization node and state

Summary:
- Add CFR game state tree that uses a vec as an allocation arena.

Test Plan:
- Added a test.

Cargo.toml

@@ -20,6 +20,8 @@ arbitrary = { version = "~1.4.1", optional = true, features = ["derive"] }
 tracing = { version = "~0.1.41", optional = true}
 approx = { version = "~0.5.1", optional = true}
 uuid  = {version = "~1.11.0", optional = true, features = ["v7"]}
+little-sorry = { version = "~0.5.0", optional = true}
+ndarray = { version = "~0.16.1", optional = true}
 
 [dev-dependencies]
 criterion = "0.5.1"
@@ -33,7 +35,7 @@ approx = { version = "0.5.1"}
 default = ["arena", "serde"]
 uuid = ["dep:uuid"]
 serde = ["dep:serde", "dep:serde_json", "uuid?/serde"]
-arena = ["dep:tracing", "uuid"]
+arena = ["dep:tracing", "dep:little-sorry", "dep:ndarray", "uuid"]
 arena-test-util = ["arena", "dep:approx"]
 
 [[bench]]

src/arena/cfr/mod.rs

@@ -0,0 +1,43 @@
+mod node;
+mod state;
+
+pub use node::{Node, NodeData, PlayerData, TerminalData};
+pub use state::CFRState;
+
+#[cfg(test)]
+mod tests {
+    use std::vec;
+
+    use crate::arena::game_state::{Round, RoundData};
+
+    use crate::arena::GameState;
+    use crate::core::{Hand, PlayerBitSet};
+
+    #[test]
+    fn test_should_fold_all_in() {
+        let num_agents = 2;
+        let hand_zero = Hand::new_from_str("AsKsKcAcTh4d8d").unwrap();
+        let hand_one = Hand::new_from_str("JdTcKcAcTh4d8d").unwrap();
+
+        // The board is the last 5 cards of the hand
+        let board = hand_zero.iter().skip(2).copied().collect();
+        // Zero is all in.
+        let stacks: Vec<f32> = vec![0.0, 900.0];
+        let player_bet = vec![1000.0, 100.0];
+        // Create a game state where player 0 is all in and player 1 should make a
+        // decision to call or fold
+        let round_data = RoundData::new(num_agents, 100.0, PlayerBitSet::new(num_agents), 1);
+        let _game_state = GameState::new(
+            Round::River,
+            round_data,
+            board,
+            vec![hand_zero, hand_one],
+            stacks,
+            player_bet,
+            5.0,
+            0.0,
+            0.0,
+            0,
+        );
+    }
+}