Oust is an elimination game invented by Mark Steere in 2007. It is a unique and highly organic mechanism, finite, suitable for any size grid, whether square or hexagonal, starting on an empty board and ending with the elimination of all of one player's stones.
Oust cannot end in a draw.
|The diagram shows a base-11 board. There are two players, Black and White. Both have a sufficient number of stones in their color. At the start of the game the board is empty. Black moves first.|
Definition: a group consists of one stone or two or more like colored and orthogonally connected stones.
Note that in these rules, reference to a 'group' will include a single stone.
The object of Oust is to capture all opponent's stones on the board.
There are two kinds of legal moves a player can make:
- Non-capturing move
A player may put his stone on any empty intersection if that stone does not enlarge one of his own groups.
- Capturing move
A player may put his stone on an empty intersection if that stone does enlarge one or more of his own groups, if and only if by doing so the player creates a group (implicitly including the placed stone) that meets the following conditions:
When making such a move all opponent's groups touching the player's group are removed from the board and the player must make another move in the same turn, and proceed in that manner till he has made a non-capturing move!
- The player's new group of stones touches at least one opponent's stone.
- All opponent's groups touching the moving player's group must be smaller than that group.
|The marked point F9 shows a capturing move. By placing there Black creates a group of three black stones that touches one white group containing two stones and another white group containing a single stone. Both white groups are smaller than the formed black group so this is a legal capturing move.|
Five rows down the result is depicted: Black has a group of 3 and the three capured white stones have been removed.
|If stones of different color are adjacent to begin with, the capturing stone does not have to be the 'contacting' stone: top-left white moves B8 (but he could use C7 or C9 to the same effect). By placing here, White creates a group of two white stones that touches one black group containing a single stone. This black group is smaller than the formed white group so this is a legal capture.|
Five rows down the result is depicted: White has a group of 2 and the capured black stone has been removed.
Top-right G9 and H10 are illegal for white because they would enlarge a white group. H8 and I9 are illegal because the resulting white group of 2 is not be bigger than the black group.
The bottom-right situation shows four illegal extensions for white, and two legal moves to capture the black stone.
|Since every capture results in the moving player having to make another move in the same turn, sequential captures are possible.|
With (1) black captures the group of two, after which (2) captures the group of three. The result is depicted five rows down.
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