From 27e05369784afd21a32888e4983464aec00ae7d3 Mon Sep 17 00:00:00 2001
From: Henri Bourcereau <henri.bourcereau@gmail.com>
Date: Sat, 7 Mar 2026 22:18:59 +0100
Subject: [PATCH] fix(spiel_bot): mcts fix

---
 spiel_bot/src/mcts/mod.rs    |  8 ++++++--
 spiel_bot/src/mcts/search.rs | 16 +++++++++++++++-
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/spiel_bot/src/mcts/mod.rs b/spiel_bot/src/mcts/mod.rs
index e92bd09..a0a690d 100644
--- a/spiel_bot/src/mcts/mod.rs
+++ b/spiel_bot/src/mcts/mod.rs
@@ -401,8 +401,12 @@ mod tests {
         };
 
         let root = run_mcts(&env, &state, &ZeroEval(514), &config, &mut r);
-        assert!(root.n > 0);
+        // root.n = 1 (expansion) + n_simulations (one backup per simulation).
+        assert_eq!(root.n, 1 + config.n_simulations as u32);
+        // Children visit counts may sum to less than n_simulations when some
+        // simulations cross a chance node at depth 1 (turn ends after one move)
+        // and evaluate with the network directly without updating child.n.
         let total: u32 = root.children.iter().map(|(_, c)| c.n).sum();
-        assert_eq!(total, 5);
+        assert!(total <= config.n_simulations as u32);
     }
 }
diff --git a/spiel_bot/src/mcts/search.rs b/spiel_bot/src/mcts/search.rs
index c4960c7..55db701 100644
--- a/spiel_bot/src/mcts/search.rs
+++ b/spiel_bot/src/mcts/search.rs
@@ -138,8 +138,14 @@ pub(super) fn simulate<E: GameEnv>(
     // ── Apply action + advance through any chance nodes ───────────────────
     let mut next_state = state;
     env.apply(&mut next_state, action);
+
+    // Track whether we crossed a chance node (dice roll) on the way down.
+    // If we did, the child's cached legal actions are for a *different* dice
+    // outcome and must not be reused — evaluate with the network directly.
+    let mut crossed_chance = false;
     while env.current_player(&next_state).is_chance() {
         env.apply_chance(&mut next_state, rng);
+        crossed_chance = true;
     }
 
     let next_cp = env.current_player(&next_state);
@@ -153,7 +159,15 @@ pub(super) fn simulate<E: GameEnv>(
         returns[player_idx]
     } else {
         let child_player = next_cp.index().unwrap();
-        let v = if child.expanded {
+        let v = if crossed_chance {
+            // Outcome sampling: after dice, evaluate the resulting position
+            // directly with the network.  Do NOT build the tree across chance
+            // boundaries — the dice change which actions are legal, so any
+            // previously cached children would be for a different outcome.
+            let obs = env.observation(&next_state, child_player);
+            let (_, value) = evaluator.evaluate(&obs);
+            value
+        } else if child.expanded {
             simulate(child, next_state, env, evaluator, config, rng, child_player)
         } else {
             expand::<E>(child, &next_state, env, evaluator, child_player)