Add futility pruning (#14)

README.md

@@ -16,6 +16,7 @@
 
 Note that you may also want to use Bun which is a much faster JS runtime env than Node.js.
 
+
 ## Setup
 
 1. Clone the repository to your machine.
@@ -74,6 +75,7 @@ Note that the config file is compiled with the engine itself, so if you are usin
     * Countermove heuristic.
 * Transposition table.
 * Null-move pruning.
+* Futility pruning.
 * Late move reductions.
 * Search extensions:
     * Check extensions.

catto.config.js

@@ -1,6 +1,6 @@
 module.exports = {
     // Current version to show in UCI
-    version: "v0.7.0",
+    version: "v0.8.0",
     // 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": "0.7.0",
+  "version": "0.8.0",
   "description": "The Catto chess engine",
   "main": "index.js",
   "scripts": {

src/core.ts

@@ -1,7 +1,8 @@
-import { Chess, Color, Move } from "chess.js";
+import { Chess, Move } from "chess.js";
 import { evaluateBoard } from "./evaluate";
 import { mvv_lva, PIECE_NUM } from "./evaluations";
 import { genZobristKey } from "./zobrist";
+import { mgMaterial } from "./evaluations";
 
 export enum HashFlag {
     exact,
@@ -309,6 +310,15 @@ export class Engine {
         possibleMoves = this.sortMoves(possibleMoves);
         let searchedMoves = 0, bestMoveSoFar: Move;
 
+        // Futility pruning
+        let fpEnabled = false;
+        if (depth < 4 && Math.abs(alpha) < 48000) {
+            const currentEval = evaluateBoard(this.chess);
+            // Margin for each depth, the shallower the depth the more we reduce the margin
+            const futilityMargin = [ 0, mgMaterial[0], mgMaterial[1], mgMaterial[3] ];
+            fpEnabled = currentEval + futilityMargin[depth] <= alpha;
+        }
+
         // Find the best move
         for (const move of possibleMoves) {
             // const tempMove = move;
@@ -319,6 +329,19 @@ export class Engine {
             this.chess.move(move);
             let score = 0;
 
+            // Apply futility pruning
+            if (
+                fpEnabled &&
+                searchedMoves > 0 &&
+                !move.captured &&
+                !move.promotion &&
+                !this.chess.inCheck()
+            ) { 
+                this.chess.undo();
+                this.ply--;
+                continue;
+            }
+
             // Do normal, alpha-beta full search on first (PV) move
             if (searchedMoves === 0) {
                 score = -this.negamax(depth - 1 + extensions, -beta, -alpha, extended + extensions);