We propagate the reward for both sides

The AI now properly choses the optimal path for the active player

Cargo.lock

@@ -2,6 +2,12 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "bitflags"
+version = "2.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1b8e56985ec62d17e9c1001dc89c88ecd7dc08e47eba5ec7c29c7b5eeecde967"
+
 [[package]]
 name = "cfg-if"
 version = "1.0.1"
@@ -10,12 +16,13 @@ checksum = "9555578bc9e57714c812a1f84e4fc5b4d21fcb063490c624de019f7464c91268"
 
 [[package]]
 name = "getrandom"
-version = "0.2.16"
+version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "335ff9f135e4384c8150d6f27c6daed433577f86b4750418338c01a1a2528592"
+checksum = "26145e563e54f2cadc477553f1ec5ee650b00862f0a58bcd12cbdc5f0ea2d2f4"
 dependencies = [
  "cfg-if",
  "libc",
+ "r-efi",
  "wasi",
 ]
 
@@ -53,21 +60,26 @@ dependencies = [
 ]
 
 [[package]]
-name = "rand"
-version = "0.8.5"
+name = "r-efi"
+version = "5.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "34af8d1a0e25924bc5b7c43c079c942339d8f0a8b57c39049bef581b46327404"
+checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f"
+
+[[package]]
+name = "rand"
+version = "0.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9fbfd9d094a40bf3ae768db9361049ace4c0e04a4fd6b359518bd7b73a73dd97"
 dependencies = [
- "libc",
  "rand_chacha",
  "rand_core",
 ]
 
 [[package]]
 name = "rand_chacha"
-version = "0.3.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e6c10a63a0fa32252be49d21e7709d4d4baf8d231c2dbce1eaa8141b9b127d88"
+checksum = "d3022b5f1df60f26e1ffddd6c66e8aa15de382ae63b3a0c1bfc0e4d3e3f325cb"
 dependencies = [
  "ppv-lite86",
  "rand_core",
@@ -75,9 +87,9 @@ dependencies = [
 
 [[package]]
 name = "rand_core"
-version = "0.6.4"
+version = "0.9.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
+checksum = "99d9a13982dcf210057a8a78572b2217b667c3beacbf3a0d8b454f6f82837d38"
 dependencies = [
  "getrandom",
 ]
@@ -129,9 +141,21 @@ checksum = "5a5f39404a5da50712a4c1eecf25e90dd62b613502b7e925fd4e4d19b5c96512"
 
 [[package]]
 name = "wasi"
-version = "0.11.1+wasi-snapshot-preview1"
+version = "0.14.2+wasi-0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ccf3ec651a847eb01de73ccad15eb7d99f80485de043efb2f370cd654f4ea44b"
+checksum = "9683f9a5a998d873c0d21fcbe3c083009670149a8fab228644b8bd36b2c48cb3"
+dependencies = [
+ "wit-bindgen-rt",
+]
+
+[[package]]
+name = "wit-bindgen-rt"
+version = "0.39.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6f42320e61fe2cfd34354ecb597f86f413484a798ba44a8ca1165c58d42da6c1"
+dependencies = [
+ "bitflags",
+]
 
 [[package]]
 name = "zerocopy"

Cargo.toml

@@ -12,5 +12,5 @@ categories = ["algorithms", "data-structures"]
 
 
 [dependencies]
-rand = "~0.8"
+rand = "~0.9"
 thiserror = "~2.0"

examples/auto_tic_tac_toe.rs

@@ -40,6 +40,7 @@ fn main() {
         // Find the best move
         match mcts.search() {
             Ok(action) => {
+                mcts.print_tree();
                 println!(
                     "AI chooses: {} (row {}, col {})",
                     action.index,

src/mcts.rs

@@ -6,7 +6,8 @@ use crate::policy::simulation::simulate_reward;
 use crate::state::GameState;
 use crate::tree::arena::Arena;
 use crate::tree::node::{Node, RewardVal};
-use rand::prelude::SliceRandom;
+use crate::tree::print_tree;
+use rand::prelude::IndexedRandom;
 use std::collections::HashMap;
 use std::time::Instant;
 
@@ -74,7 +75,7 @@ impl<'conf, S: GameState + std::fmt::Debug> MCTS<'conf, S> {
         if !selected_node.state.is_terminal() {
             self.expand(selected_id);
             let children: &Vec<usize> = &self.arena.get_node(selected_id).children;
-            let random_child: usize = *children.choose(&mut rand::thread_rng()).unwrap();
+            let random_child: usize = *children.choose(&mut rand::rng()).unwrap();
             selected_id = random_child;
         }
         let rewards = self.simulate(selected_id);
@@ -130,6 +131,10 @@ impl<'conf, S: GameState + std::fmt::Debug> MCTS<'conf, S> {
             None => Err(MCTSError::NoBestAction),
         }
     }
+
+    pub fn print_tree(&self) {
+        print_tree(self.root_id, &self.arena)
+    }
 }
 
 /// Errors returned by the MCTS algorithm

src/policy/backprop/mod.rs

@@ -69,18 +69,14 @@ fn standard_backprop<S: GameState>(
     let mut current_id: usize = node_id;
     loop {
         let node = arena.get_node_mut(current_id);
-        let player = node.state.get_current_player().clone();
-        match rewards.get(&player) {
-            Some(reward) => {
-                node.increment_visits();
-                node.record_player_reward(player, *reward);
-                if let Some(parent_id) = node.parent {
-                    current_id = parent_id;
-                } else {
-                    break;
-                }
-            }
-            None => (),
+        node.increment_visits();
+        for (player, reward) in rewards.iter() {
+            node.record_player_reward(player.clone(), *reward);
+        }
+        if let Some(parent_id) = node.parent {
+            current_id = parent_id;
+        } else {
+            break;
         }
     }
 }
@@ -94,19 +90,14 @@ fn weighted_backprop<S: GameState>(
     let mut current_id: usize = node_id;
     loop {
         let node = arena.get_node_mut(current_id);
-        let player = node.state.get_current_player().clone();
         let weight = weight_for_depth(depth_factor, node.depth);
-        match rewards.get(&player) {
-            Some(reward) => {
-                node.increment_visits();
-                node.record_player_reward(player, (*reward) * weight);
-                if let Some(parent_id) = node.parent {
-                    current_id = parent_id;
-                } else {
-                    break;
-                }
-            }
-            None => (),
+        for (player, reward) in rewards.iter() {
+            node.record_player_reward(player.clone(), (*reward) * weight);
+        }
+        if let Some(parent_id) = node.parent {
+            current_id = parent_id;
+        } else {
+            break;
         }
     }
 }

src/policy/simulation/random.rs

@@ -4,14 +4,14 @@
 
 use crate::state::GameState;
 use crate::tree::node::{Node, RewardVal};
-use rand::prelude::SliceRandom;
+use rand::prelude::IndexedRandom;
 use std::collections::HashMap;
 
 pub fn simulate<S: GameState>(node: &Node<S>) -> HashMap<S::Player, RewardVal> {
     let mut state: S = node.state.clone();
     while !state.is_terminal() {
         let legal_actions = state.get_legal_actions();
-        let action = legal_actions.choose(&mut rand::thread_rng()).unwrap();
+        let action = legal_actions.choose(&mut rand::rng()).unwrap();
         state = state.state_after_action(&action);
     }
     state.rewards_for_players()

src/state.rs

@@ -7,7 +7,7 @@ use std::hash::Hash;
 ///
 /// When leveraging MCTS for your game, you must implement this trait to provide
 /// the specifics for your game.
-pub trait GameState: Clone {
+pub trait GameState: Clone + Debug {
     /// The type of actions that can be taken in the game
     type Action: Action;
 

src/tree/mod.rs

@@ -1,2 +1,27 @@
 pub mod arena;
 pub mod node;
+
+use crate::state::GameState;
+use crate::tree::arena::Arena;
+use crate::tree::node::Node;
+
+pub fn print_tree<S: GameState>(node_id: usize, arena: &Arena<S>) {
+    let mut to_print: Vec<usize> = Vec::new();
+    to_print.push(node_id);
+    while let Some(node_id) = to_print.pop() {
+        let node: &Node<S> = arena.get_node(node_id);
+        if node.depth > 0 {
+            for _ in 0..node.depth - 1 {
+                print!("|  ")
+            }
+            print!("|- ");
+        }
+        println!(
+            "{:?} a:{:?} v:{}  {:?}",
+            node_id, node.action, node.visits, node.player_view
+        );
+        for child_id in node.children.iter().rev() {
+            to_print.push(*child_id);
+        }
+    }
+}

src/tree/node.rs

@@ -1,5 +1,4 @@
 use std::collections::HashMap;
-use std::fmt::Debug;
 
 use crate::state::GameState;
 
@@ -15,7 +14,7 @@ pub type RewardVal = f64;
 ///
 /// This class is not thread safe, as the library does not provide for parallel
 /// search.
-#[derive(Debug)]
+#[derive(std::fmt::Debug)]
 pub struct Node<S: GameState> {
     /// The game state at the given node, after `action`
     pub state: S,
@@ -30,7 +29,7 @@ pub struct Node<S: GameState> {
     pub visits: u64,
 
     /// The player's evaluation of the node
-    pub player_view: HashMap<S::Player, PlayerNodeView>,
+    pub player_view: HashMap<S::Player, PlayerRewardView>,
 
     /// The depth of the node in the tree, this is 0 for the root node
     pub depth: usize,
@@ -82,10 +81,7 @@ impl<S: GameState> Node<S> {
     }
 
     pub fn record_player_reward(&mut self, player: S::Player, reward: RewardVal) {
-        let pv = self
-            .player_view
-            .entry(player)
-            .or_insert(PlayerNodeView::default());
+        let pv = self.player_view.entry(player).or_default();
         pv.rewards.push(reward);
         pv.reward_sum += reward;
         pv.reward_average = pv.reward_sum / pv.rewards.len() as f64;
@@ -95,8 +91,7 @@ impl<S: GameState> Node<S> {
 /// A player's specific perspective of a node's value
 ///
 /// Each player has their own idea of the value of a node.
-#[derive(Debug)]
-pub struct PlayerNodeView {
+pub struct PlayerRewardView {
     /// The total reward from simulations through this node
     pub reward_sum: RewardVal,
 
@@ -107,12 +102,19 @@ pub struct PlayerNodeView {
     pub rewards: Vec<RewardVal>,
 }
 
-impl Default for PlayerNodeView {
+impl Default for PlayerRewardView {
     fn default() -> Self {
-        PlayerNodeView {
+        PlayerRewardView {
             reward_sum: 0.0,
             reward_average: 0.0,
             rewards: Vec::new(),
         }
     }
 }
+
+impl std::fmt::Debug for PlayerRewardView {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{{sum={} avg={}}}", self.reward_sum, self.reward_average)?;
+        Ok(())
+    }
+}