/// <summary>
/// Worker method to do FEN parsing.
/// NOTE: performance could be improved by passing in the Piece[] preallocated
/// </summary>
/// <param name="fen"></param>
/// <returns></returns>
static FENParseResult DoParseFEN(string fen)
{
int charIndex = 0;
void SkipAnySpaces()
{
while (charIndex < fen.Length && char.IsWhiteSpace(fen[charIndex]))
{
charIndex++;
}
}
void SkipAnySpacesOrDash()
{
while (charIndex < fen.Length && (fen[charIndex] == '-' || char.IsWhiteSpace(fen[charIndex])))
{
charIndex++;
}
}
List <PieceOnSquare> pieces = new List <PieceOnSquare>(32);
int curRank = 0;
int curFile = 0;
// Parse pieces
while (true)
{
char thisChar = fen[charIndex++];
if (thisChar == ' ')
{
break;
}
if (thisChar == '/')
{
if (curRank >= 7)
{
throw new Exception("Illegal FEN, too many squares");
}
curRank++;
curFile = 0;
}
else if (char.IsDigit(thisChar))
{
curFile += (thisChar - '0');
if (curFile > 8)
{
throw new Exception("Illegal FEN, too many files");
}
}
else
{
Piece thisPiece = byteToPieces[thisChar];
if (thisPiece.Type == PieceType.None)
{
throw new Exception("Illegal FEN, found piece character " + thisChar);
}
pieces.Add(new PieceOnSquare(Square.FromFileAndRank(curFile, 7 - curRank), thisPiece));
curFile++;
}
}
SkipAnySpaces();
// 2. Side to move character
char sideMoveChar = char.ToLower(fen[charIndex++]);
SideType sideToMove;
if (sideMoveChar == 'b')
{
sideToMove = SideType.Black;
}
else if (sideMoveChar == 'w')
{
sideToMove = SideType.White;
}
else
{
throw new Exception($"Illegal FEN, side to move character {sideMoveChar}");
}
SkipAnySpaces();
// 3. Castling availability
bool whiteCanOO = false;
bool whiteCanOOO = false;
bool blackCanOO = false;
bool blackCanOOO = false;
while (charIndex < fen.Length)
{
char thisChar = fen[charIndex++];
if (thisChar == 'K' || thisChar == 'H')
{
whiteCanOO = true;
}
else if (thisChar == 'Q' || thisChar == 'A')
{
whiteCanOOO = true;
}
else if (thisChar == 'k' || thisChar == 'h')
{
blackCanOO = true;
}
else if (thisChar == 'q' || thisChar == 'a')
{
blackCanOOO = true;
}
else if (thisChar == ' ')
{
break;
}
}
SkipAnySpaces();
// 4. En passant target square
int numEPChars = 0;
PositionMiscInfo.EnPassantFileIndexEnum epColIndex = PositionMiscInfo.EnPassantFileIndexEnum.FileNone;
int epFile = 0;
int epRank = 0;
while (charIndex < fen.Length)
{
// Try to work around error in FEN whereby dash is missing, and move number immediately follows
if (numEPChars == 0 && char.IsDigit(fen[charIndex]))
{
break;
}
char thisChar = fen[charIndex++];
if (thisChar == '-' || thisChar == ' ')
{
break;
}
else
{
if (numEPChars == 0)
{
epFile = char.ToLower(thisChar) - 'a';
}
else if (numEPChars == 1)
{
epRank = char.ToLower(thisChar) - '0';
}
else
{
throw new Exception("too many en passant characters");
}
numEPChars++;
}
}
SkipAnySpaces();
if (numEPChars > 0)
{
if (numEPChars == 2)
{
if (epFile > (byte)PositionMiscInfo.EnPassantFileIndexEnum.FileH)
{
throw new Exception("Invalid en passant file in FEN");
}
else
{
epColIndex = (PositionMiscInfo.EnPassantFileIndexEnum)epFile;
}
}
else
{
throw new Exception("Invalid en passant in FEN");
}
}
SkipAnySpacesOrDash(); // Sometimes we see ill-formed FENs that end with extaneous dash, e.g. "w q - -"
// 5. Halfmove clock: This is the number of halfmoves since the last capture or pawn advance.
int move50Count = 0;
while (charIndex < fen.Length && charIndex < fen.Length)
{
char thisChar = fen[charIndex++];
if (thisChar == ' ')
{
break;
}
else
{
move50Count = move50Count * 10 + (thisChar - '0');
}
}
SkipAnySpaces();
// Fullmove number
int fullmoveCount = 0;
while (charIndex < fen.Length && charIndex < fen.Length)
{
char thisChar = fen[charIndex++];
if (!char.IsDigit(thisChar))
{
break;
}
else
{
fullmoveCount = fullmoveCount * 10 + (thisChar - '0');
}
}
// Fill in move count with 1 if none found
if (fullmoveCount == 0)
{
fullmoveCount = 1;
}
if (charIndex < fen.Length - 1)
{
string remainder = fen.Substring(charIndex);
if (StringUtils.WhitespaceRemoved(remainder) != "")
{
throw new Exception($"Unexpected characters after FEN {remainder} ");
}
}
PositionMiscInfo miscInfo = new PositionMiscInfo(whiteCanOO, whiteCanOOO, blackCanOO, blackCanOOO, sideToMove, move50Count, 0, fullmoveCount, epColIndex);
return(new FENParseResult(pieces, miscInfo));
}
/// <summary>
/// Constructor (from set of pieces and miscellaneous information).
/// </summary>
/// <param name="pieces"></param>
/// <param name="miscInfo"></param>
public FENParseResult(List <PieceOnSquare> pieces, PositionMiscInfo miscInfo)
{
Pieces = pieces;
MiscInfo = miscInfo;
}
