Chess Engine

1import java.util.*;
2/** Immutable legal-move engine. Includes castling, en passant, promotion, mate and stalemate.
3 * Excludes repetition, fifty/seventy-five-move and insufficient-material draw adjudication. */
4public final class Solution {
5    private Solution() {}
6    public enum Status { PLAYING, CHECK, CHECKMATE, STALEMATE }
7    public record Move(int from,int to,char promotion){}
8    private static boolean white(char p){return Character.isUpperCase(p);}
9    private static boolean own(char p,boolean side){return p!='.'&&white(p)==side;}
10    private static boolean inside(int r,int c){return r>=0&&r<8&&c>=0&&c<8;}
11    public static int square(String algebraic){if(algebraic==null||!algebraic.matches("[a-h][1-8]"))throw new IllegalArgumentException("square");return (8-(algebraic.charAt(1)-'0'))*8+algebraic.charAt(0)-'a';}
12    public static String algebraic(int square){if(square<0||square>=64)throw new IllegalArgumentException("square");return ""+(char)('a'+square%8)+(8-square/8);}
13    public static Move move(String uci){if(uci==null||!uci.matches("[a-h][1-8][a-h][1-8][qrbn]?"))throw new IllegalArgumentException("move");return new Move(square(uci.substring(0,2)),square(uci.substring(2,4)),uci.length()==5?uci.charAt(4):'\0');}
14    public static final class Position {
15        private final char[] board;
16        private final boolean whiteTurn;
17        private final int rights,ep;
18        private Position(char[] board,boolean side,int rights,int ep){this.board=board.clone();whiteTurn=side;this.rights=rights;this.ep=ep;}
19        public static Position initial(){return fromFen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq -");}
20        /** Four-field fixture FEN. Checks representation, not historical reachability. */
21        public static Position fromFen(String fen){
22            String[] fields=fen.split(" ");if(fields.length!=4)throw new IllegalArgumentException("four FEN fields required");String[] ranks=fields[0].split("/");if(ranks.length!=8)throw new IllegalArgumentException("ranks");
23            char[] board=new char[64];Arrays.fill(board,'.');int wk=0,bk=0;
24            for(int r=0;r<8;r++){int c=0;for(char p:ranks[r].toCharArray()){if(p>='1'&&p<='8')c+=p-'0';else{if("prnbqkPRNBQK".indexOf(p)<0||c>=8)throw new IllegalArgumentException("piece");board[r*8+c++]=p;if(p=='K')wk++;if(p=='k')bk++;}}if(c!=8)throw new IllegalArgumentException("rank width");}
25            if(wk!=1||bk!=1||!Set.of("w","b").contains(fields[1])||!fields[2].matches("-|K?Q?k?q?"))throw new IllegalArgumentException("position");
26            int rights=0;for(char p:fields[2].toCharArray())rights|=switch(p){case 'K'->1;case 'Q'->2;case 'k'->4;case 'q'->8;default->0;};
27            int ep=fields[3].equals("-")?-1:square(fields[3]);if(ep>=0&&ep/8!=(fields[1].equals("w")?2:5))throw new IllegalArgumentException("en passant rank");
28            return new Position(board,fields[1].equals("w"),rights,ep);
29        }
30        public String fen(){
31            var s=new StringBuilder();for(int r=0;r<8;r++){int empty=0;for(int c=0;c<8;c++){char p=board[r*8+c];if(p=='.')empty++;else{if(empty>0){s.append(empty);empty=0;}s.append(p);}}if(empty>0)s.append(empty);if(r<7)s.append('/');}
32            s.append(whiteTurn?" w ":" b ");String flags="";if((rights&1)!=0)flags+="K";if((rights&2)!=0)flags+="Q";if((rights&4)!=0)flags+="k";if((rights&8)!=0)flags+="q";return s+(flags.isEmpty()?"-":flags)+" "+(ep<0?"-":algebraic(ep));
33        }
34        private int king(boolean side){char k=side?'K':'k';for(int i=0;i<64;i++)if(board[i]==k)return i;throw new IllegalStateException("king missing");}
35        public boolean inCheck(){return attacked(king(whiteTurn),!whiteTurn);}
36        private boolean attacked(int target,boolean byWhite){
37            int r=target/8,c=target%8,pawnRow=r+(byWhite?1:-1);
38            for(int dc:new int[]{-1,1})if(inside(pawnRow,c+dc)&&board[pawnRow*8+c+dc]==(byWhite?'P':'p'))return true;
39            int[][] jumps={{1,2},{2,1},{-1,2},{-2,1},{1,-2},{2,-1},{-1,-2},{-2,-1}};
40            for(var d:jumps)if(inside(r+d[0],c+d[1])&&board[(r+d[0])*8+c+d[1]]==(byWhite?'N':'n'))return true;
41            for(int dr=-1;dr<=1;dr++)for(int dc=-1;dc<=1;dc++){
42                if(dr==0&&dc==0)continue;
43                for(int step=1;inside(r+dr*step,c+dc*step);step++){
44                    char p=board[(r+dr*step)*8+c+dc*step];if(p=='.')continue;
45                    if(own(p,byWhite)){
46                        char kind=Character.toLowerCase(p);
47                        if(step==1&&kind=='k')return true;
48                        if(kind=='q'||(kind=='b'&&dr!=0&&dc!=0)||(kind=='r'&&(dr==0||dc==0)))return true;
49                    }break;
50                }
51            }return false;
52        }
53        private void add(List<Move> moves,int from,int to){char p=board[from];if(Character.toLowerCase(p)=='p'&&(to/8==0||to/8==7)){for(char promotion:new char[]{'q','r','b','n'})moves.add(new Move(from,to,promotion));}else moves.add(new Move(from,to,'\0'));}
54        private List<Move> pseudo(){
55            var moves=new ArrayList<Move>();
56            for(int from=0;from<64;from++){
57                char piece=board[from];if(!own(piece,whiteTurn))continue;int r=from/8,c=from%8;char kind=Character.toLowerCase(piece);
58                if(kind=='p'){
59                    int dr=whiteTurn?-1:1,nr=r+dr;
60                    if(inside(nr,c)&&board[nr*8+c]=='.'){add(moves,from,nr*8+c);int start=whiteTurn?6:1;if(r==start&&board[(r+2*dr)*8+c]=='.')add(moves,from,(r+2*dr)*8+c);}
61                    for(int dc:new int[]{-1,1})if(inside(nr,c+dc)){
62                        int to=nr*8+c+dc;char target=board[to];
63                        boolean capture=target!='.'&&!own(target,whiteTurn)&&Character.toLowerCase(target)!='k';
64                        boolean passing=to==ep&&target=='.'&&board[r*8+c+dc]==(whiteTurn?'p':'P');
65                        if(capture||passing)add(moves,from,to);
66                    }
67                }else{
68                    int[][] dirs=kind=='n'?new int[][]{{1,2},{2,1},{-1,2},{-2,1},{1,-2},{2,-1},{-1,-2},{-2,-1}}:new int[][]{{1,0},{-1,0},{0,1},{0,-1},{1,1},{1,-1},{-1,1},{-1,-1}};
69                    for(var d:dirs){if(kind=='b'&&(d[0]==0||d[1]==0)||kind=='r'&&d[0]!=0&&d[1]!=0)continue;
70                        for(int step=1;inside(r+d[0]*step,c+d[1]*step);step++){
71                            int to=(r+d[0]*step)*8+c+d[1]*step;char target=board[to];if(own(target,whiteTurn)||Character.toLowerCase(target)=='k')break;
72                            add(moves,from,to);if(target!='.'||kind=='k'||kind=='n')break;
73                        }
74                    }
75                    if(kind=='k')castles(moves,from);
76                }
77            }return moves;
78        }
79        private void castles(List<Move> moves,int from){
80            int base=whiteTurn?56:0,k=base+4;char rook=whiteTurn?'R':'r';if(from!=k||attacked(k,!whiteTurn))return;
81            int shortFlag=whiteTurn?1:4,longFlag=whiteTurn?2:8;
82            if((rights&shortFlag)!=0&&board[base+7]==rook&&board[base+5]=='.'&&board[base+6]=='.'&&!attacked(base+5,!whiteTurn)&&!attacked(base+6,!whiteTurn))add(moves,from,base+6);
83            if((rights&longFlag)!=0&&board[base]==rook&&board[base+1]=='.'&&board[base+2]=='.'&&board[base+3]=='.'&&!attacked(base+3,!whiteTurn)&&!attacked(base+2,!whiteTurn))add(moves,from,base+2);
84        }
85        private Position applyUnchecked(Move m){
86            char[] next=board.clone();char piece=next[m.from()],target=next[m.to()];next[m.from()]='.';next[m.to()]=m.promotion()=='\0'?piece:(whiteTurn?Character.toUpperCase(m.promotion()):m.promotion());
87            if(Character.toLowerCase(piece)=='p'&&m.to()==ep&&target=='.'&&m.from()%8!=m.to()%8)next[m.to()+(whiteTurn?8:-8)]='.';
88            if(Character.toLowerCase(piece)=='k'&&Math.abs(m.to()-m.from())==2){int rookFrom=m.to()>m.from()?m.from()+3:m.from()-4,rookTo=m.to()>m.from()?m.from()+1:m.from()-1;next[rookTo]=next[rookFrom];next[rookFrom]='.';}
89            int flags=rights;if(piece=='K')flags&=~3;if(piece=='k')flags&=~12;
90            if(m.from()==63||m.to()==63)flags&=~1;if(m.from()==56||m.to()==56)flags&=~2;if(m.from()==7||m.to()==7)flags&=~4;if(m.from()==0||m.to()==0)flags&=~8;
91            int enPassant=Character.toLowerCase(piece)=='p'&&Math.abs(m.to()-m.from())==16?(m.from()+m.to())/2:-1;
92            return new Position(next,!whiteTurn,flags,enPassant);
93        }
94        public List<Move> legalMoves(){var result=new ArrayList<Move>();for(var m:pseudo()){var next=applyUnchecked(m);if(!next.attacked(next.king(whiteTurn),!whiteTurn))result.add(m);}return List.copyOf(result);}
95        public Position play(Move move){if(!legalMoves().contains(move))throw new IllegalArgumentException("illegal move");return applyUnchecked(move);}
96        public Status status(){boolean check=inCheck();if(legalMoves().isEmpty())return check?Status.CHECKMATE:Status.STALEMATE;return check?Status.CHECK:Status.PLAYING;}
97        public long perft(int depth){if(depth<0||depth>4)throw new IllegalArgumentException("depth");if(depth==0)return 1;long result=0;for(var m:legalMoves())result=Math.addExact(result,applyUnchecked(m).perft(depth-1));return result;}
98    }
99}
100