Policy Softmax Temperature at depth=2 (#64)

Applies a small amount of PST to the moves in children of the root node.

Passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,4.00>
Total: 19552 W: 4594 L: 4380 D: 10578
Ptnml(0-2): 237, 2229, 4663, 2377, 270
https://tests.montychess.org/tests/view/670799dcb12c9e78f1354ad3

Passed LTC:
LLR: 2.95 (-2.94,2.94) <1.00,5.00>
Total: 32646 W: 6533 L: 6258 D: 19855
Ptnml(0-2): 128, 3593, 8634, 3812, 156
https://tests.montychess.org/tests/view/6707bc17b12c9e78f1354b00

Bench: 1358297

src/mcts.rs

@@ -234,9 +234,19 @@ impl<'a> Searcher<'a> {
 
         // relabel root policies with root PST value
         if self.tree[node].has_children() {
-            self.tree[node].relabel_policy(&self.root_position, self.params, self.policy);
+            self.tree[node].relabel_policy(&self.root_position, self.params, self.policy, 1);
+
+            for action in &*self.tree[node].actions() {
+                if action.ptr().is_null() || !self.tree[action.ptr()].has_children() {
+                    continue;
+                }
+
+                let mut position = self.root_position.clone();
+                position.make_move(Move::from(action.mov()));
+                self.tree[action.ptr()].relabel_policy(&position, self.params, self.policy, 2);
+            }
         } else {
-            self.tree[node].expand::<true>(&self.root_position, self.params, self.policy);
+            self.tree[node].expand(&self.root_position, self.params, self.policy, 1);
         }
 
         let search_stats = SearchStats::default();
@@ -310,7 +320,7 @@ impl<'a> Searcher<'a> {
         } else {
             // expand node on the second visit
             if self.tree[ptr].is_not_expanded() {
-                self.tree[ptr].expand::<false>(pos, self.params, self.policy);
+                self.tree[ptr].expand(pos, self.params, self.policy, *depth);
             }
 
             // select action to take via PUCT

src/mcts/params.rs

@@ -131,6 +131,7 @@ macro_rules! make_mcts_params {
 
 make_mcts_params! {
     root_pst: f32 = 3.64, 1.0, 10.0, 0.4, 0.002;
+    depth_2_pst: f32 = 1.2, 1.0, 10.0, 0.4, 0.002;
     root_cpuct: f32 = 0.314, 0.1, 5.0, 0.065, 0.002;
     cpuct: f32 = 0.314, 0.1, 5.0, 0.065, 0.002;
     cpuct_var_weight: f32 = 0.851, 0.0, 2.0, 0.085, 0.002;

src/tree/node.rs

@@ -94,11 +94,12 @@ impl Node {
         self.set_gini_impurity(0.0);
     }
 
-    pub fn expand<const ROOT: bool>(
+    pub fn expand(
         &self,
         pos: &ChessState,
         params: &MctsParams,
         policy: &PolicyNetwork,
+        depth: usize,
     ) {
         let mut actions = self.actions_mut();
 
@@ -121,16 +122,19 @@ impl Node {
             max = max.max(policy);
         });
 
+        let pst = match depth {
+            0 => unreachable!(),
+            1 => params.root_pst(),
+            2 => params.depth_2_pst(),
+            3.. => 1.0,
+        };
+
         let mut total = 0.0;
 
         for action in actions.iter_mut() {
             let mut policy = f32::from_bits(action.ptr().inner());
 
-            policy = if ROOT {
-                ((policy - max) / params.root_pst()).exp()
-            } else {
-                (policy - max).exp()
-            };
+            policy = ((policy - max) / pst).exp();
 
             action.set_ptr(NodePtr::from_raw(f32::to_bits(policy)));
 
@@ -150,7 +154,13 @@ impl Node {
         self.set_gini_impurity(gini_impurity);
     }
 
-    pub fn relabel_policy(&self, pos: &ChessState, params: &MctsParams, policy: &PolicyNetwork) {
+    pub fn relabel_policy(
+        &self,
+        pos: &ChessState,
+        params: &MctsParams,
+        policy: &PolicyNetwork,
+        depth: u8,
+    ) {
         let feats = pos.get_policy_feats();
         let mut max = f32::NEG_INFINITY;
 
@@ -163,10 +173,17 @@ impl Node {
             max = max.max(policy);
         }
 
+        let pst = match depth {
+            0 => unreachable!(),
+            1 => params.root_pst(),
+            2 => params.depth_2_pst(),
+            3.. => unreachable!(),
+        };
+
         let mut total = 0.0;
 
         for policy in &mut policies {
-            *policy = ((*policy - max) / params.root_pst()).exp();
+            *policy = ((*policy - max) / pst).exp();
             total += *policy;
         }