implement razoring

lib/search/engine.rs

@@ -111,14 +111,25 @@ impl Engine {
         }
     }
 
+    /// An implementation of [razoring].
+    ///
+    /// [razoring]: https://www.chessprogramming.org/Razoring
+    fn razor(&self, deficit: Score, draft: Depth) -> Option<Depth> {
+        match deficit.get() {
+            ..0 => None,
+            s @ 0..600 => Some(draft - (s + 30) / 210),
+            600.. => Some(draft - 3),
+        }
+    }
+
     /// An implementation of [reverse futility pruning].
     ///
     /// [reverse futility pruning]: https://www.chessprogramming.org/Reverse_Futility_Pruning
     fn rfp(&self, surplus: Score, draft: Depth) -> Option<Depth> {
         match surplus.get() {
             ..0 => None,
-            s @ 0..440 => Some(draft - (s + 40) / 120),
-            440.. => Some(draft - 4),
+            s @ 0..360 => Some(draft - (s + 60) / 140),
+            360.. => Some(draft - 3),
         }
     }
 
@@ -229,6 +240,14 @@ impl Engine {
                 }
             }
 
+            if let Some(d) = self.razor(alpha - upper, draft) {
+                if !is_pv && t.draft() >= d {
+                    #[cfg(not(test))]
+                    // The razoring heuristic is not exact.
+                    return Ok(transposed.convert());
+                }
+            }
+
             if let Some(d) = self.rfp(lower - beta, draft) {
                 if !is_pv && t.draft() >= d {
                     #[cfg(not(test))]