/// <summary>
/// Return available castling options for <paramref name="king"/>, given
/// a <paramref name="position"/> and <paramref name="moveEntries"/>.
/// </summary>
/// <param name="king">Must be <see cref="King"/>.</param>
/// <param name="position">A given <see cref="Board.Position"/>.</param>
/// <param name="moveEntries">A read-only <see cref="MoveEntry"/> collection of
/// previously proccessed moves.</param>
/// <returns></returns>
public static IReadOnlyCollection <Move> Castles(
this IPiece king,
IReadOnlyDictionary <Square, IPiece> position,
IReadOnlyCollection <MoveEntry> moveEntries)
{
var kingSquare = ((Piece)king).GetSquareFrom(position);
if (
king is not King ||
kingSquare is null ||
kingSquare.Rank != (king.Color ? Ranks.one : Ranks.eight) ||
king.HasMoved(position, moveEntries))
{
return(Enumerable.Empty <Move>().ToList());
}
var rookSquares = position
.Where(kv => kv.Value.Color == king.Color && kv.Value is Rook)
.Where(kv => !kv.Value.HasMoved(position, moveEntries))
.Where(kv => kv.Key.IsSameRankAs(kingSquare))
.Select(kv => kv.Key)
.ToList();
return(rookSquares
.Where(
rs => kingSquare
.InBetweenSquares(rs)
.All(s => !position.TryGetValue(s, out IPiece piece)))
.Select(rs =>
(rs.File < kingSquare.File) ?
new Move(
new Square(kingSquare),
new Square(Files.c, kingSquare.Rank),
MoveType.Castle) :
new Move(
new Square(kingSquare),
new Square(Files.g, kingSquare.Rank),
MoveType.Castle))
.ToList());
}
