private string ValidateBasic()
{
var error = string.Empty;
if (_pos.SideToMove != Player.White && _pos.SideToMove != Player.Black)
{
error = AddError(error, $"{nameof(_pos.SideToMove)} is not a valid");
}
if (_board.PieceAt(_pos.GetKingSquare(Player.White)) != Pieces.WhiteKing)
{
error = AddError(error, "white king position is not a white king");
}
if (_board.PieceAt(_pos.GetKingSquare(Player.Black)) != Pieces.BlackKing)
{
error = AddError(error, "black king position is not a black king");
}
if (_pos.EnPassantSquare != Square.None && _pos.EnPassantSquare.RelativeRank(_pos.SideToMove) != Ranks.Rank6)
{
error = AddError(error, $"{nameof(_pos.EnPassantSquare)} square is not on rank 6");
}
return(error);
}
/// <summary>
/// Computes whether the current position contains a pawn fence which makes the game a draw.
/// </summary>
/// <returns>true if the game is a draw position - otherwise false</returns>
public bool IsBlocked()
{
// Quick check if there is only pawns and kings on the board It might be possible to
// have a minor piece and exchange it into a passing pawn
if (_pos.Board.PieceCount(PieceTypes.AllPieces) > _pos.Board.PieceCount(PieceTypes.Pawn) + 2)
{
return(false);
}
var up = _us.PawnPushDistance();
MarkOurPawns(up);
MarkTheirPawns();
var isFenceFormed = IsFenceFormed();
if (!isFenceFormed)
{
return(false);
}
ComputeFenceRanks();
var ourKsq = _pos.GetKingSquare(_us);
var theirKsq = _pos.GetKingSquare(_them);
if (ourKsq.Rank.RelativeRank(_us) > _fenceRank[ourKsq.File.AsInt()].RelativeRank(_us))
{
return(false);
}
_dynamicPawns |= ComputeDynamicFencedPawns(_them);
while (_dynamicPawns)
{
var sq = BitBoards.PopLsb(ref _dynamicPawns);
var(r, f) = sq.RankFile;
var rr = sq.RelativeRank(_us);
if (r > _fenceRank[f.AsInt()])
{
if ((_theirPawns & sq.PassedPawnFrontAttackSpan(_us)).IsEmpty && (theirKsq.File != f || theirKsq.Rank.RelativeRank(_us) < rr))
{
return(false);
}
}
else if (_fence & sq)
{
if (rr >= Ranks.Rank6)
{
return(false);
}
if (_pos.GetPiece(sq + _us.PawnDoublePushDistance()) != _theirPawn)
{
if (theirKsq.File != f || theirKsq.Rank.RelativeRank(_us) < rr)
{
return(false);
}
if (f != Files.FileA &&
_pos.GetPiece(sq + Directions.West) != _ourPawn ||
BitBoards.PopCount(_ourPawns & PreviousFile(f)) > 1 ||
(_fixedPawn & (sq + Directions.West)).IsEmpty ||
(_fence & (sq + Directions.West)).IsEmpty)
{
return(false);
}
if (f != Files.FileH &&
_pos.GetPiece(sq + Directions.East) != _ourPawn ||
BitBoards.PopCount(_ourPawns & NextFile(f)) > 1 ||
(_fixedPawn & (sq + Directions.East)).IsEmpty ||
(_fence & (sq + Directions.East)).IsEmpty)
{
return(false);
}
}
if ((sq + _us.PawnDoublePushDistance()).PawnAttack(_us) & _theirPawns)
{
return(false);
}
if (BitBoards.PopCount(_ourPawns & f) > 1)
{
return(false);
}
}
else if (r < _fenceRank[f.AsInt()])
{
sq += up;
r = sq.Rank;
rr = sq.RelativeRank(_us);
while ((_fence & Square.Make(r, f)).IsEmpty)
{
var pawnAttacks = sq.PawnAttack(_us);
if (_theirPawns & pawnAttacks)
{
return(false);
}
if (_pos.GetPiece(sq) == _ourPawn)
{
break;
}
sq += up;
r = sq.Rank;
}
if ((_fence & Square.Make(r, f)).IsEmpty || _pos.IsOccupied(sq))
{
continue;
}
if (rr >= Ranks.Rank6)
{
return(false);
}
if ((_theirPawns & (sq + _us.PawnDoublePushDistance())).IsEmpty)
{
if (theirKsq.File != f || theirKsq.Rank.RelativeRank(_us) < rr)
{
return(false);
}
if (f != Files.FileA &&
_pos.GetPiece(sq + Directions.West) != _ourPawn ||
BitBoards.PopCount(_ourPawns & (f - 1)) > 1 ||
(_fixedPawn & Square.Make(r, PreviousFile(f))).IsEmpty ||
(_fence & Square.Make(r, PreviousFile(f))).IsEmpty)
{
return(false);
}
if (f != Files.FileH &&
_pos.GetPiece(sq + Directions.East) != _ourPawn ||
BitBoards.PopCount(_ourPawns & (f + 1)) > 1 ||
(_fixedPawn & Square.Make(r, NextFile(f))).IsEmpty ||
(_fence & Square.Make(r, NextFile(f))).IsEmpty)
{
return(false);
}
}
if ((sq + up).PawnAttack(_us) & _theirPawns)
{
return(false);
}
}
}
return(true);
}
