. . . . . . . . . . . . . . . . . . . . . . . . . . . k . . . . . . . . . . . . . K F . . . . . . . . . . . . . f . . . . . . .White to move and win by capturing Black's ferz (without losing his own on the next ply). The solution takes no less than 39 ply, starting with Kb4. The principal variation with optimal alternatives is

1 Kb4 Kd6 2 Kc4 Ke6 3 Kd4 Kf6 4 Kd5 Kf7 5 Ke5 (Fd2) Kg7 6 Ke6 (Fd2) Kf8 7 Kd6 (Kf6/Kf5/Fb4/Fd2) Ke8 8 Kc6 (Ke5/Fd2) Kd8 9 Kb6 (Kd5/Fd2) Kc8 (Ke8/Ke7) 10 Kc5 (Fd2) Kd7 11 Kb5 (Fd2) Kc7 (Ke7/Ke6) 12 Kc4 (Fd2) Kd6 13 Kb4 (Fd2) Kc6 (Ke6/Ke5) 14 Fd2 Kd5 15 Kc3 Ke4 16 Kb3 Kd4 (Kd3/Kf3) 17 Kc2 Kc4 (Ke4) 18 Kb1 (Fc1 Fe1) Kd3 19 Fc1 ~ 20 KxfBlack's attempted defense is to maintain the same distance from the White king as the White ferz has from the Black one, which White defeats by driving the Black king to the edge. Indeed, White cannot win if his King has to avoid e6/f5.

I'd like to think that al-Suli arrived at his problem by polishing

. . . F . . . k . . . . . . . . . . . . . . . . K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f . . . . . . .which has the maximum distance-to-win among all such positions, at 53 plies, and the very same first move Kb4 (though no longer unique). Witness the polishing:

1 Kb4 (Ka4/Fc7) Kg7 (Kg8) 2 Kb3 (Fc7) Kf7 (Kf8) 3 Fc7 Ke6 (Ke7) 4 Fb6 Kd5 (Kd6) 5 Fa5 Kc5 6 Fb4 Kc6 6 Fc3 Kd5A C-program computing and navigating the solution space. Compile without optimization to avoid segmentation faults.

In a 2006 posting to newsgroup rec.games.chess.computer, Will Entriken proved an upper bound of 23937533792747905898433845980097921846050276105440 on the number of chess diagrams (roughly 2^164). The following Haskell program improves this to 45193640626062205213735739171550309047984050718 (less than 2^155) for the number of positions.

import Control.Monad import System(getArgs) import Array -- given a maximum number of pawns (normally 8, but 7 with fixed en-passant) -- and maximum number of initial rooks (normally 2, but 1 or 0 with castling) -- to choose, return list of possibly armies, where each army is summarized -- as a tuple of <#pieces, #pawns, #promotions, #factorial_product> armies np ir = do q <- [0..1+np] let prom1 = max (q-1) 0 r <- [0..ir+np-prom1] let prom2 = prom1 + max (r-ir) 0 b <- [0..2+np-prom2] let prom3 = prom2 + max (b-2) 0 n <- [0..2+np-prom3] let proms = prom3 + max (n-2) 0 guard $ proms <= np p <- [0..np-proms] return (q+r+b+n, p, proms, fac q*fac r*fac b*fac n*fac p) -- precompute first 65 factorials into array for efficiency fac :: Int -> Integer fac n = fac64!n where fac64 = listArray (0,64) (scanl (*) 1 [1..64]) -- given a number of pawns to fix for each color (1 for en passant) -- and amount of space available for pawns -- and number of white and black rookks fixed by castling -- return number of possible positions count fixp pspace fixwr fixbr = sum $ do let np = 8-fixp (wpcs,wp,wproms,wprod) <- armies np (2-fixwr) let wpx = np-wp-wproms -- white p captured (bpcs,bp,bproms,bprod) <- armies np (2-fixbr) let bpx = np-bp-bproms -- black p captured let caps = 30-2*fixp-fixwr-fixbr-wp-bp-wpcs-bpcs guard $ wproms <= bpx + caps guard $ bproms <= wpx + caps let space = 62-4*fixp-fixwr-fixbr-wp-bp return $ (fac pspace `div` fac (pspace-wp-bp)) * (fac space `div` fac (space-wpcs-bpcs)) `div` (wprod * bprod) count0 (mul,ek) = mul * count 0 (46+ek) 0 0 -- Diagrams -- there are 212 configurations of kings on ranks 1&8 -- there are 1448 configurations of one king on ranks 1&8, and the other on 2-6 -- there are 1952 configurations of kings on ranks 2-6 main0 = print . sum $ map count0 [(212,2),(1448,1),(1952,0)] -- 22124621884617108585387385940828998876019391612 -- given a multiplier, number of edge (first or last rank) kings, -- fixed white and fixed black rooks -- return number of possible positions -- this assumes that side-to-move is known, say white -- each of the squares a5-h5 can have a black pawn en-passent -- capturable by 2 white pawns, except a5/h5, which could only -- be captured by 1 white pawn countep (mul,ek,fwr,fbr) = mul * (count 0 (46+ek) fwr fbr + (8*2-2) * count 1 (42+ek) fwr fbr) -- Positions -- the first seven tuples count configurations -- of kings that can castle in various ways -- and where the other king is either on ranks 1&8 or 2-6 -- The factor (* 2) counts for either side-to-move main = print . (* 2) . sum $ map countep [(1,2,2,2),(4,2,1,2),(4,2,1,1), (2*9,2,0,2),(2*43,1,0,2),(4*11,2,0,1),(4*44,1,0,1), (212,2,0,0),(1448,1,0,0),(1952,0,0,0)] -- 45193640626062205213735739171550309047984050718I have a significantly more complex program that proves an upper bound of 7728772977965919677164873487685453137329736522 (about 10^45.888 or ~ 2^152.437) on the number of positions, but, like the bound of ~10^46.25 published by Shirish Chinchalkar in "An Upper Bound for the Number of Reachable Positions", ICCA Journal, Vol. 19, No. 3, pp. 181-183, 1996, it requires much better documentation to be considered verifiable,

The longest possible chess game.

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