Adding a basic print_tree function to visualize the MCTS search tree

This probably is not a good thing to run on a very large tree.

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

@@ -6,6 +6,7 @@ use crate::policy::simulation::simulate_reward;
 use crate::state::GameState;
 use crate::tree::arena::Arena;
 use crate::tree::node::{Node, RewardVal};
+use crate::tree::print_tree;
 use rand::prelude::SliceRandom;
 use std::collections::HashMap;
 use std::time::Instant;
@@ -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/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,
@@ -85,7 +84,7 @@ impl<S: GameState> Node<S> {
         let pv = self
             .player_view
             .entry(player)
-            .or_insert(PlayerNodeView::default());
+            .or_insert(PlayerRewardView::default());
         pv.rewards.push(reward);
         pv.reward_sum += reward;
         pv.reward_average = pv.reward_sum / pv.rewards.len() as f64;
@@ -95,8 +94,7 @@ 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 +105,19 @@ 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(())
+    }
+}