From d2fdbb23bb56d9e06b1f11623c6475e6928f98e1 Mon Sep 17 00:00:00 2001
From: Adam <adammegarules1@gmail.com>
Date: Tue, 18 Aug 2026 12:08:20 +0200
Subject: fix(book): fixing critical castling bug

---
 src/book.cpp | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/book.cpp b/src/book.cpp
index abf9693..2d499ca 100644
--- a/src/book.cpp
+++ b/src/book.cpp
@@ -81,6 +81,18 @@ static uint16_t PolyglotToMono(uint16_t polyglotMove) {
   uint8_t from = (polyglotMove >> 6) & 63;
   uint8_t promo = (polyglotMove >> 12) & 7;
 
+  // Polyglot encodes castling as king moving to the rook's square.
+  // Convert to king-moves-two-squares form that the engine expects.
+  if (from == 4 && to == 7) {
+    to = 6; // white kingside: e1h1 -> e1g1
+  } else if (from == 4 && to == 0) {
+    to = 2; // white queenside: e1a1 -> e1c1
+  } else if (from == 60 && to == 63) {
+    to = 62; // black kingside: e8h8 -> e8g8
+  } else if (from == 60 && to == 56) {
+    to = 58; // black queenside: e8a8 -> e8c8
+  }
+
   PieceType promotion = NONEPIECE;
   switch (promo) {
   case 0:
-- 
cgit v1.2.3