[update] Improve heuristic.

connect_four.js

@@ -1,6 +1,6 @@
 /* ============================================================
  *  Connect Four — Browser Edition
- *  A single-file game: AI (minimax + alpha-beta), demo mode,
+ *  A single-file game: AI (minimax + alpha-beta + heuristic), demo mode,
  *  game log (localStorage), blunder mode, idle timeout.
  *
  *  Include this script in an HTML page that has:
@@ -168,6 +168,7 @@ function scanBoard(b) {
 
 function evaluateBoard(b, aiP, huP) {
     let score = 0;
+    let aiThreats = 0, huThreats = 0;
 
     // Center column bonus
     for (let r = 0; r < ROWS; r++) {
@@ -177,16 +178,27 @@ function evaluateBoard(b, aiP, huP) {
 
     // Score a window of 4 cells by piece counts
     function scoreWindow(c, r, dc, dr) {
-        let ai = 0, hu = 0;
+        let ai = 0, hu = 0, emptyC = -1, emptyR = -1;
         for (let i = 0; i < 4; i++) {
-            const v = b[c + i * dc][r + i * dr];
+            const cc = c + i * dc;
+            const rr = r + i * dr;
+            const v = b[cc][rr];
             if (v === aiP) ai++;
             else if (v === huP) hu++;
+            else { emptyC = cc; emptyR = rr; }
         }
         if (ai > 0 && hu > 0) return 0;
-        if (ai === 3) return 50;
+        if (ai === 3) {
+            aiThreats++;
+            const playable = emptyR === 0 || b[emptyC][emptyR - 1] !== 0;
+            return playable ? 100 : 40;
+        }
         if (ai === 2) return 5;
-        if (hu === 3) return -50;
+        if (hu === 3) {
+            huThreats++;
+            const playable = emptyR === 0 || b[emptyC][emptyR - 1] !== 0;
+            return playable ? -100 : -40;
+        }
         if (hu === 2) return -5;
         return 0;
     }
@@ -208,6 +220,10 @@ function evaluateBoard(b, aiP, huP) {
         for (let c = 0; c <= COLS - 4; c++)
             score += scoreWindow(c, r, 1, -1);
 
+    // Fork bonus: multiple threats are disproportionately dangerous
+    if (aiThreats >= 2) score += 200;
+    if (huThreats >= 2) score -= 200;
+
     return score;
 }
 
@@ -324,6 +340,7 @@ function checkGameEnd() {
 
     if (gameState !== State.DEMO) {
         games = logGame(games, gameMenuMode, gameLevel, won ? w : 0, currentMoves);
+        console.log(`Game: ${currentMoves} → ${won ? playerName(w) + " wins" : "Draw"}`);
     }
     gameState = won ? State.FINISHED_WIN : State.FINISHED_DRAW;
     demoResetTimer = performance.now() / 1000;