From cc70bcdf08974e79c7d319008d85ebe528b32f94 Mon Sep 17 00:00:00 2001
From: Josh <josh@joshuaschuett.com>
Date: Sat, 30 Aug 2025 15:46:50 -0400
Subject: [PATCH] Add castling bonus

---
 engine/src/ai/negamax.c | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/engine/src/ai/negamax.c b/engine/src/ai/negamax.c
index 87fbe99..8ccb653 100644
--- a/engine/src/ai/negamax.c
+++ b/engine/src/ai/negamax.c
@@ -6,6 +6,16 @@
 
 #define INF 30000
 #define MATE 29000
+#define CASTLE_BONUS       100
+#define UNCASTLED_PENALTY   30
+#define PAWN_SHIELD_BONUS   15
+
+const uint64_t W_CASTLED_MASK = BB(G1) | BB(C1);
+const uint64_t B_CASTLED_MASK = BB(G8) | BB(C8);
+const uint64_t W_KSIDE_SHIELD = BB(13) | BB(14) | BB(15); // f2 g2 h2
+const uint64_t W_QSIDE_SHIELD = BB(8)  | BB(9)  | BB(10); // a2 b2 c2
+const uint64_t B_KSIDE_SHIELD = BB(53) | BB(54) | BB(55); // f7 g7 h7
+const uint64_t B_QSIDE_SHIELD = BB(48) | BB(49) | BB(50); // a7 b7 c7
 
 // P N B R Q K
 int VAL[6] = {100, 300, 300, 500, 900, 0};
@@ -20,7 +30,7 @@ int popcount64(uint64_t bits) {
     return count;
 }
 
-static int eval(struct Board *board) {
+int eval(struct Board *board) {
     // White minus Black material (centipawns)
     // Ignore the King's material.
     int score = 0;
@@ -30,11 +40,34 @@ static int eval(struct Board *board) {
     score += VAL[3] * (popcount64(board->pieces[R]) - popcount64(board->pieces[r]));
     score += VAL[4] * (popcount64(board->pieces[Q]) - popcount64(board->pieces[q]));
 
+    // Castling Bonus
+    uint64_t white_king = board->pieces[K];
+    uint64_t black_king = board->pieces[k];
+    uint64_t white_pawns = board->pieces[P];
+    uint64_t black_pawns = board->pieces[p];
+
+    if (white_king & W_CASTLED_MASK) {
+        score += CASTLE_BONUS;
+        uint64_t shield = (white_king & BB(G1)) ? W_KSIDE_SHIELD : W_QSIDE_SHIELD;
+        score += PAWN_SHIELD_BONUS * popcount64(white_pawns & shield);
+    } else if (white_king & BB(E1)) {
+        score -= UNCASTLED_PENALTY;
+    }
+
+    // Black (subtract because score is White minus Black)
+    if (black_king & B_CASTLED_MASK) {
+        score -= CASTLE_BONUS;
+        uint64_t shield = (black_king & BB(G8)) ? B_KSIDE_SHIELD : B_QSIDE_SHIELD;
+        score -= PAWN_SHIELD_BONUS * popcount64(black_pawns & shield);
+    } else if (black_king & BB(E8)) {
+        score += UNCASTLED_PENALTY;
+    }
+
     // Negamax convention: score from side-to-move POV
     return (board->side_to_move == WHITE) ? score : -score;
 }
 
-static int search(struct Board *b, int depth, int alpha, int beta, int ply) {
+int search(struct Board *b, int depth, int alpha, int beta, int ply) {
     if (depth == 0) return eval(b);
 
     struct Move moves[256];