improve evenness of FutureMoves tree

Cargo.toml

@@ -42,3 +42,7 @@ criterion = { version = "0.5", features = [ "html_reports" ] }
 [[bench]]
 name = "future_children"
 harness = false
+
+[lints.rust]
+# fix weird warnings about `test` not being expected
+unexpected_cfgs = { level = "allow", check-cfg = ['cfg(test)'] }

src/logic/future_moves.rs

@@ -59,10 +59,38 @@ impl FutureMoves {
         self.arena.len()
     }
 
+    fn leaf_moves(&self) -> Vec<usize> {
+        let mut indexes = (0..self.arena.len())
+            // we want to select all nodes that don't have children, or are lazy (need to maybe be regenerated)
+            .filter(|&idx| {
+                let got = &self.arena[idx];
+                !got.is_trimmed && !got.tried_children && got.winner == Winner::None
+            })
+            .filter(|&idx| self.is_connected_to_root(idx))
+            .collect::<Vec<usize>>();
+
+        // we want to try and make the tree even in depth
+        // by first generating children for younger moves
+        indexes.sort_by_key(|&x| self.depth_of(x));
+        indexes
+    }
+
+    /// Find the current depth of the arena by
+    /// looking at leaf moves and finding the smallest value
+    fn determine_current_depth(&self) -> Option<usize> {
+        // leaf_moves is sorted from min to max depth
+        self.leaf_moves().first().map(|&i| self.depth_of(i))
+    }
+
     /// Generate children for all children of `nodes`
     /// only `pub` for the sake of benchmarking
     pub fn extend_layers(&mut self) {
-        for _ in (self.current_depth + 1)..=self.config.max_depth {
+        // recover from partial tree extention
+        if let Some(current_depth) = self.determine_current_depth() {
+            self.current_depth = current_depth;
+        }
+
+        for _ in self.current_depth..self.config.max_depth {
             let pstyle_inner = if cfg!(test) {
                 ""
             } else {
@@ -73,13 +101,10 @@ impl FutureMoves {
                 )
             };
 
-            let cf = (0..self.arena.len())
-                // we want to select all nodes that don't have children, or are lazy (need to maybe be regenerated)
-                .filter(|&idx| {
-                    let got = &self.arena[idx];
-                    !got.is_trimmed && !got.tried_children && got.winner == Winner::None
-                })
-                .filter(|&idx| self.is_connected_to_root(idx))
+            let cf = self
+                .leaf_moves()
+                .into_iter()
+                .filter(|&i| self.depth_of(i) == self.current_depth)
                 .collect::<Vec<usize>>()
                 .into_iter()
                 .progress_with_style(ProgressStyle::with_template(pstyle_inner).unwrap())
@@ -198,7 +223,7 @@ impl FutureMoves {
         let by_depth_vec = self.by_depth(indexes);
 
         // reversed so we build up the value of the closest (in time) moves from the future
-        for (depth, nodes) in by_depth_vec.into_iter().rev() {
+        for (_, nodes) in by_depth_vec.into_iter().rev() {
             for idx in nodes {
                 let children_value = self.arena[idx]
                     .children