Add razoring (#19)

README.md

@@ -87,9 +87,10 @@ Note that the config file is compiled with the engine itself, so if you are usin
     * Countermove heuristic.
 * Transposition table.
 * Pruning:
+    * Reverse futility pruning.
     * Null-move pruning.
+    * Razoring.
     * Futility pruning.
-    * Reverse futility pruning.
     * Delta pruning.
 * Late move reductions.
 * Search extensions:

catto.config.js

@@ -1,6 +1,6 @@
 module.exports = {
     // Current version to show in UCI
-    version: "v0.11.0",
+    version: "v0.12.0",
     // Late move reduction config
     lmrFullDepth: 4, // Number of moves to be searched in full depth
     lmrMaxReduction: 3, // Only apply LMR above this depth

package.json

@@ -1,6 +1,6 @@
 {
   "name": "catto",
-  "version": "0.11.0",
+  "version": "0.12.0",
   "description": "The Catto chess engine",
   "main": "index.js",
   "scripts": {

src/core.ts

@@ -303,8 +303,9 @@ export class Engine {
         if (depth === 0) return this.quiescence(alpha, beta);
 
         // Reverse futility pruning
+        const isPv = beta - alpha > 1;
         const currentEval = evaluateBoard(this.chess);
-        if (depth < 3 && !inCheck && !(beta - alpha > 1) && Math.abs(beta - 1) > -48900) {
+        if (depth < 3 && !inCheck && !isPv && Math.abs(beta - 1) > -48900) {
             let rfpMargin = mgMaterial[0] * depth; // Scaled for each depth by a pawn
 
             if (currentEval - rfpMargin >= beta) return currentEval - rfpMargin;
@@ -363,6 +364,23 @@ export class Engine {
         possibleMoves = this.sortMoves(possibleMoves);
         let searchedMoves = 0, bestMoveSoFar: Move;
 
+        // Razoring, skipping an entire subtree
+        if (!isPv && !inCheck && depth <= 3) {
+            // Prepare score for first phase
+            let scaledScore = currentEval + mgMaterial[0];
+
+            if (scaledScore < beta) {
+                const qScore = this.quiescence(alpha, beta);
+
+                if (depth === 1) return Math.max(qScore, scaledScore);
+
+                // Second phase for depth 2 and depth 3
+                scaledScore += mgMaterial[0];
+
+                if (scaledScore < beta && qScore < beta) return Math.max(qScore, scaledScore);
+            }
+        }
+
         // Futility pruning
         let fpEnabled = false;
         if (depth < 4 && Math.abs(alpha) < 48000) {