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Gomoku, also written Go-Moku (lit. "five points") is a Japanese board
game traditionally played with Go pieces and board (black and white stones
on a 19x19 line board). The stones are placed on the intersections of the
board, with Black playing first. The winner is the first player to get an
unbroken row of five stones, either orthogonally or diagonally.
A moment's consideration will reveal that having a row of four of your
stones, with blank spaces at each end, is a winning condition; no matter
which end your opponent blocks, you will win by placing a stone at the other
end.
The next step before that, obviously, is having an open row of three
stones; this must be blocked, or it will develop into the open row of four.
The next step is to form two open rows of three, which share a common
stone, which when placed, wins the game: your opponent can't block the
formation of an open row of four stones.
Gomoku is played with many slight variations to the rules. A common
variant is to require a row of exactly five stones for a win, longer rows
(so-called over-lines) not counting. This variant is called standard gomoku,
as opposed to free-style gomoku. The game is also now commonly played on a
15x15 board instead of the traditional 19x19. Another rule often used is the
"rule of three and three", which says that you cannot form two open
three-in-a-rows in the same move, even if such a move is required to block
your opponent from winning. Sometimes, the restriction on over-lines or
"rule of three and three" is only applied to black, who has the first move.
Careful computer analysis by L. Victor Allis has shown that, at least on
a 15x15 board, Black wins with perfect play. This applies whether or not
over-lines are considered as wins, but it assumes that the rule of three and
three is not used.
The game of renju is a variant played on a 15x15 board with a number of
additional rules designed to make the game fairer. Ninuki-renju is a variant
which adds capturing to the game; it was published in the USA in a slightly
simplified form under the name Pente.
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