Play a valid chess move, given a board on stdin

Question

The program plays white.

Example stdin:

8 ║♜ ♞ ♝ ♛ ♚ ♝ ♞ ♜
7 ║♟ ♟ ♟ ♟ … ♟ ♟ ♟
6 ║… … … … … … … …
5 ║… … … … ♟ … … …
4 ║… … … … … … … …
3 ║… … ♘ … … … … …
2 ║♙ ♙ ♙ ♙ ♙ ♙ ♙ ♙
1 ║♖ … ♗ ♕ ♔ ♗ ♘ ♖
——╚═══════════════
—— a b c d e f g h

Example stdout:

8 ║♜ ♞ ♝ ♛ ♚ ♝ ♞ ♜
7 ║♟ ♟ ♟ ♟ … ♟ ♟ ♟
6 ║… … … … … … … …
5 ║… … … … ♟ … … …
4 ║… … … … ♙ … … …
3 ║… … ♘ … … … … …
2 ║♙ ♙ ♙ ♙ … ♙ ♙ ♙
1 ║♖ … ♗ ♕ ♔ ♗ ♘ ♖
——╚═══════════════
—— a b c d e f g h

Any valid move is ok. "En passant" and castling are ignored. It is ok to show error messages or print nothing if there is no valid move.

The answer with the most votes wins.

Answer 1

I'm not complaining about upvotes, but to be fair... my solution here isn't actually all that great. Ugoren's is better, apart from lacking unicode support. Be sure to look at all the answers before voting, if you've come across this question only now!
Anyway.

Haskell, 1074 bytes (without castling)

χ=w⋈b;w="♙♢♤♔♕♖♗♘";b="♟♦♠♚♛♜♝♞"
μ=t⤀ζ++((\(x,y)->(x,-y))⤀)⤀μ;q c|((_,m):_)<-((==c).fst)☂(χ⋎μ)=m
t(x:y:l)=(d x,d y):t l;t _=[];d c=fromEnum c-78
ζ=["NM","NL","MMOM","MMMNMONMNOOMONOO",σ⋈δ,σ,δ,"MLOLPMPOOPMPLOLM"]
σ=l>>=(\c->'N':c:c:"N");δ=[l⋎l,reverse l⋎l]>>=(>>=(\(l,r)->[l,r]))
l="GHIJKLMOPQRSTU"
α c|c∊"♢♤"='♙'|c∊"♦♠"='♟'|c∊χ=c;π('♙':_)=6;π _=1
(⋎)=zip;(⤀)=map;(∊)=elem;(✄)=splitAt;(☂)=filter;(⋈)=(++)
φ r@(x,y)p a
 |x>7=φ(0,y+1)p a
 |y>7=[]
 |c<-a✠r=(c⌥r)p a y⋈φ(x+1,y)p a
(c⌥r)p a y
 |c==p!!0=(a☈r)c χ++const(y==π p)☂(a☈r)(p!!1)χ++(a☈r)(p!!2)('…':w)
 |c∊p=(a☈r)c χ
 |True=[]
a✠(x,y)=a!!y!!(x*2);o(x,y)=x>=0&&x<8&&y>=0&&y<8
(n➴a)(x,y)|(u,m:d)<-y✄a,(l,_:r)<-(x*2)✄m=u⋈(l⋈(n:r):d)
(a☈r@(x,y))c b=(α c➴('…'➴a)r)⤀((\r->o r&&not((a✠r)∊b))☂((\(ξ,υ)->(x+ξ,y+υ))⤀q c))
main=interact$unlines.uncurry((⋈).zipWith((⋈).(:" ║"))['8','7'..]
 .head.((all(any('♔'∊)).φ(0,0)b)☂).φ(0,0)w.(drop 3⤀)).(8✄).lines

Try it online!

---

When you have GHC installed (for instance as part of the Haskell platform) you can do just

$ runhaskell def0.hs < examplechessboard.txt
8 ║♜ ♞ ♝ ♛ ♚ ♝ ♞ ♜
7 ║♟ ♟ ♟ ♟ … ♟ ♟ ♟
6 ║… … … … … … … …
5 ║… ♘ … … ♟ … … …
4 ║… … … … … … … …
3 ║… … … … … … … …
2 ║♙ ♙ ♙ ♙ ♙ ♙ ♙ ♙
1 ║♖ … ♗ ♕ ♔ ♗ ♘ ♖
——╚═══════════════
—— a b c d e f g h

Answer 2

C, 734 672 640 characters

Characters counted without removable whitespace.
The file format I used is not as requested, but simplified ASCII.
I need to add Unicode character support, it would cost some characets.

char*r=" kpnbrq  KPNBRQ $ ,&)$wxy()879()8(6:GI(",B[256],*b=B,i;
e(x,d,m,V,c,r,n,p){
    for(r=0,p=b[x];m/++r;){
        n=x+d*r;
        if(p==2+8*(d<0)||n&136||!(b[n]?r=8,8^p^b[n]^8&&c&65^64:c&65^65)
            ? r=m,0
            : V?v(n,x):b[n]==1)
            return b[x]=0,b[n]=p%8-2||n/16%7?p:p+4;
    }
    return d>0&&e(x,-d,m,V,c);
}
d(x,v,m,i)char*m;{
    return(i=*m-40)?e(x,i%64,b[x]%8-2?b[x]&4?7:1:(x/16-1)%5|i%2?1:2,v,i)||d(x,v,m+1):0;
}
v(t,f){
    bcopy(B,b+=128,128);
    b[t]=b[f];b[f]=0;
    i=a(1,63);
    b=B;
    return!i;
}
a(c,n){
    return b[i=n*2-n%8]&&b[i]/8==c&&d(i,!c,r+r[b[i]%8+15]-10)||n--&&a(c,n);
}
main(){
    for(;gets(b);b+=8)for(;*b;b++)*b=strchr(r,*b)-r;b=B;
    for(i=64*!a(0,63);i<64;i++%8-7||puts(""))putchar(r[b[i*2-i%8]]);
}

Input/output file format:
Must be exactly 8 lines of exactly 8 characters. pnbrqk are used for white pieces, PNBRQK for black pieces, spaces for spaces:

RNBQKBNR
PPPP PPP

 n  P


pppppppp
r bqkbnr

The logic is quite simple:
For each possible move of each white piece, try each possible move of each black piece.
If no black move captures the white king, the white move is valid.

The board is maintained as char[256], treated as a 16x16 matrix, where only the top-left 8x8 is used. Positions and movement vectors are kept in 8-bit integers (x:4,y:4). The extra bit allows using simple arithmetic (new_pos = old_pos + steps*direction), with easy detection of the board edge (&0x88 does the magic). r[] encodes three things:

1. The first 15 bytes map internal piece codes (K=1,P=2,N=3,B=4,R=5,Q=6) to letters.
2. The next 6 bytes map internal piece codes to offsets in the last part (K and Q are the same, B is their tail).
3. The last 16 bytes encode the movement of all pieces, as '('+vector.

Functions:

1. main reads the board, converts letters to internal code, calls a to find white moves, prints the board.
2. arecursively loops over the 64 squares. For each piece of the right color (parameter c), it finds the movement rule for the piece and calls d.
3. d recursively loops over the encoded movement rule, which is a list of vectors, calling e for each one. It gives e the original position, the vector and the range limit (7 for pieces above B, 2 for second rank pawns, 1 otherwise).
4. e tests all movements along a vector. If the move is possible (i.e. pawns move forward, within board, not blocked, pawn capture diagonally), checks one of two things. For white moves, runs v to validate the move. For black moves, checks if the white king is captured. If true, the move is played on the board.
5. v validates a white move. It copies the board aside, executes the move to test, and calls a again, to look for black moves.

Answer 3

Python 2.6, 886 - 1425 characters

My initial version (in the revisions) came in at 886 characters but did not satisfy the spec completely (it did not check for avoiding checkmate ; it didn't even consider the possible moves of the black pieces).

Now it does (and I've fixed several bugs in the original). Alas this comes with a cost in characters: 1425 for now, but there should still be little room for improvement. This version should be a lot more solid in handling edge cases then the previous one.

#-*-coding:utf8-*-
import sys;e=enumerate
B,W=["♟","♜","♞","♝","♛","♚"],["♙","♖","♘","♗","♕","♔"]
R={"♙":[11,42],"♖":[28],"♘":[31],"♗":[8],"♕":[8,28],"♔":[1,21]}
def F(w):return sum([[(i,j)for j,p in e(o)if p==w]for i,o in e(Z)],[])
def G(x,y):
 P=Z[x][y];D=P in W;L=[]
 for o in R[P]if D else R[unichr(ord(P.decode('utf8'))-6).encode('utf8')]:
  r,k="%02d"%o        
  for g,h in[[(-1,-1),(1,1),(-1,1),(1,-1)],[[(1,-1),(1,1)],[(-1,-1),(-1,1)]][D],[(-1,0),(1,0),(0,-1),(0,1)],[(-2,-1),(-2,1),(-1,-2),(-1,2),(1,-2),(1,2),(2,-1),(2,1)],[(-1,0)]][int(r)]:
   J=0
   for i in range(int(k)):
    T=x+(i+1)*g;U=y+(i+1)*h
    if T<0 or T>7 or U<0 or U>7:break
    M=Z[T][U]
    if not J:L.append((T,U,P,M))
    else:break
    if r in"02"and(M in W+B):
     J=1
     if not((D and M in B)or(not D and M in W)):L.pop()
    elif(r=="1"and not((D and M in B)or(not D and M in W)))or(r=="4"and((i==1 and x!=6)or M!="…")):L.pop()
 return L  
Z=[[y for y in l[5:].split()]for l in sys.stdin.readlines()[:-2]]
Q=[]
for p in R:
 for i,j in F(p):
  for M,L,c,_ in G(i,j):
   O=Z[M][L];Z[i][j]="…";Z[M][L]=c;E=[];map(E.extend,map(F,B))
   if not any(any(1 for _,_,_,I in G(v,h)if I==["♔","♚"][c in B])for v,h in E):Q.append((i,j,M,L,c))
   Z[i][j]=c;Z[M][L]=O
(x,y,X,Y,p)=Q[0];Z[x][y]="…";Z[X][Y]=p
for i,h in e(Z):print`8-i`+' ║'+' '.join(h)
print"——╚"+"═"*16+"\n—— a b c d e f g h"

Example input and output:

# INPUT

8 ║♜ ♞ ♝ … ♚ ♝ ♞ ♜
7 ║♟ ♟ ♟ ♟ … ♟ ♟ ♟
6 ║… … … … … … … …
5 ║… … … … ♟ … … …
4 ║… … … … … … ♙ ♛
3 ║… … … … … ♙ … …
2 ║♙ ♙ ♙ ♙ ♙ … ♙ …
1 ║♖ ♘ ♗ ♕ ♔ ♗ ♘ ♖
——╚═══════════════
—— a b c d e f g h

# OUTPUT

8 ║♜ ♞ ♝ … ♚ ♝ ♞ ♜
7 ║♟ ♟ ♟ ♟ … ♟ ♟ ♟
6 ║… … … … … … … …
5 ║… … … … ♟ … … …
4 ║… … … … … … ♙ ♛
3 ║… … … … … ♙ ♙ …
2 ║♙ ♙ ♙ ♙ ♙ … … …
1 ║♖ ♘ ♗ ♕ ♔ ♗ ♘ ♖
——╚════════════════
—— a b c d e f g h