implemented king legal moves

cpp/src/pieces/king.cpp

@@ -0,0 +1,76 @@
+#include "../board.hpp"
+#include "../coords.hpp"
+#include "piece.hpp"
+
+static bool is_clear_king_side(const Board& b, const Coords xy) {
+    for (int dx = 1; dx < 3; dx++) {
+        Coords c{xy.x + dx, xy.y};
+        if (b.squares[c.to_index()] != Piece::None)
+            return false;
+
+        std::optional<Move> move = move_for_position(b, xy, c);
+        Board board_after_move = b.make_move(move.value());
+        if (board_after_move.is_check_for(b.colour_at(xy)))
+            return false;
+    }
+    return true;
+}
+
+static bool is_clear_queen_side(const Board& b, const Coords xy) {
+    for (int dx = 1; dx < 4; dx++) {
+        Coords c{xy.x - dx, xy.y};
+        if (b.squares[c.to_index()] != Piece::None)
+            return false;
+
+        std::optional<Move> move = move_for_position(b, xy, c);
+        Board board_after_move = b.make_move(move.value());
+        if (dx < 3 && board_after_move.is_check_for(b.colour_at(xy)))
+            return false;
+    }
+    return true;
+}
+
+std::vector<Move> king_moves(const Board& b, const Coords xy) {
+    std::vector<Move> ret;
+
+    // -- Regular moves
+    for (int dx = -1; dx <= 1; dx++) {
+        for (int dy = -1; dy <= 1; dy++) {
+            if (dx == 0 && dy == 0) // skip staying in the same position
+                continue;
+            Coords c{xy.x + dx, xy.y + dy};
+            std::optional<Move> move = move_for_position(b, xy, c);
+            if (move.has_value())
+                ret.push_back(move.value());
+        }
+    }
+
+    if (b.is_check_for(b.colour_at(xy)))
+        return ret;
+
+    // -- Castles
+    CastleSide castling_rights = b.colour_at(xy) == Colour::White
+                                   ? b.w_castle_rights
+                                   : b.b_castle_rights;
+
+    if (castling_rights == CastleSide::Neither)
+        return ret;
+
+    if (castling_rights == CastleSide::King && is_clear_king_side(b, xy)) {
+        ret.push_back(Move{
+            xy.to_index(),
+            Coords{6, xy.y}.to_index(),
+            .castle_side = CastleSide::King
+        });
+    }
+
+    if (castling_rights == CastleSide::Queen && is_clear_queen_side(b, xy)) {
+        ret.push_back(Move{
+            xy.to_index(),
+            Coords{2, xy.y}.to_index(),
+            .castle_side = CastleSide::Queen
+        });
+    }
+
+    return ret;
+}