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Subject: It is impossible to represent board position by 100 bits

Author: blass uri

Date: 17:49:02 05/21/99


I think that even representing positions when
there are 32 pieces on the board is impossible by 100 bits.

we have 15^8 options for the pawns.
15 options  in every file(choosing the pawns in a file is chossing 2 squares out
of 6).

31 bits are not enough to represent 15^8.

17 bits are not enough to represent the bishops on the board(after the choice of
the squares for the pawns)
(in the worst case we have 32*31*16*15 options to choose squares for the bishops
if the squares for the pawns were with the same colour and it is more than 2^17)

10 bits are not enough to represent the kings on the board.
10 bits are not enough to represent the queens on the board.
9 bits are not enough to represent the white rooks on the board.
9 bits are not enough to represent the black rooks on the board.
9 bits are not enough to represent the white knights on the board.
9 bits are not enough to represent the black knights on the board.

31+17+10+10+9+9+9+9=104 bits

I remember that I checked random positions with 32 pieces(generated by a
computer program) some years ago and I found that most of them are not legal but
a small part of them(more than 1/10 of the cases that I checked) are legal and I
could generate a game that prove that they are legal.

We probably need more than 100 bits only to represent positions with all the
pieces on the board and I believe that these is only a small part of the legal
chess positions.

Uri



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