King legal moves implementation (#75)

* removed castling rights to the empty fen position

* implemented king legal moves

Author: Hardcode3

include/bitbishop/board.hpp

@@ -67,7 +67,7 @@ class Board {
     return {"rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"};
   }
 
-  [[nodiscard]] static Board Empty() noexcept { return {"8/8/8/8/8/8/8/8 w KQkq - 0 1"}; }
+  [[nodiscard]] static Board Empty() noexcept { return {"8/8/8/8/8/8/8/8 b - - 0 1"}; }
 
   /**
    * @brief Retrieves the piece on a given square.

include/bitbishop/movegen/king_moves.hpp

@@ -0,0 +1,23 @@
+#include <bitbishop/board.hpp>
+#include <bitbishop/color.hpp>
+#include <bitbishop/lookups/king_attacks.hpp>
+#include <bitbishop/move.hpp>
+#include <utility>
+#include <vector>
+
+void generate_legal_king_moves(std::vector<Move>& moves, const Board& board, Color us, Square king_sq,
+                               const Bitboard& enemy_attacks, const Bitboard& check_mask) {
+  const Bitboard own = board.friendly(us);
+  const Bitboard enemy = board.enemy(us);
+
+  Bitboard candidates = Lookups::KING_ATTACKS[king_sq.value()];
+
+  candidates &= ~own;
+  candidates &= ~enemy_attacks;
+  candidates &= check_mask;
+
+  for (Square to : candidates) {
+    const bool is_capture = enemy.test(to);
+    moves.emplace_back(king_sq, to, std::nullopt, is_capture, false, false);
+  }
+}