Make rollouts about 1 percent faster by not evaluating obvious positions

crates/engine/src/evaluator.rs

@@ -31,6 +31,16 @@ pub trait Evaluator {
     where
         F: Fn(&Probabilities) -> f32,
     {
+        // Two optimizations so that we don't have to call eval that often.
+        // The function would also work without the next 6 lines.
+        if positions.len() == 1 {
+            return positions.first().unwrap();
+        }
+        if let Some(end_of_game_position) = positions.iter().find(|p| p.has_lost()) {
+            return end_of_game_position;
+        }
+
+        // No obvious position found, so now we have to call `eval` for all of them.
         positions
             .iter()
             .map(|pos| (pos, value(&self.eval(pos))))

crates/engine/src/position.rs

@@ -121,6 +121,10 @@ impl Position {
         self.pips[pip]
     }
 
+    pub fn has_lost(&self) -> bool {
+        self.o_off == NO_OF_CHECKERS
+    }
+
     pub fn game_state(&self) -> GameState {
         debug_assert!(
             self.x_off < 15 || self.o_off < 15,
@@ -499,10 +503,12 @@ mod tests {
     fn game_state_normal() {
         let given = pos!(x 19:14; o);
         assert_eq!(given.game_state(), GameOver(LoseNormal));
+        assert!(given.has_lost());
         assert_eq!(
             given.switch_sides().game_state(),
             GameOver(LoseNormal.reverse())
         );
+        assert!(!given.switch_sides().has_lost());
     }
 
     #[test]