Reversi Go

 Overview Reversi Go is a finite Go variant for two players (Black and White) in which surrounded groups are flipped to the opposite color instead of removed.

 Objective The objective of Reversi Go to have a higher score than your opponent at the end of the game. Your score depends on the amount of territory you have surrounded and the number of stones you have on the board.

 Material The board is a square grid made of an equal number of horizontal and vertical lines. Stones are placed on their intersections, also called points. The suggested board sizes are between 9 and 19 points per side. Each player must have access to a sufficient number of stones of their own color. There is also a special token called button, which is used to avoid ties.

 Definitions A group is a stone along with all other like-colored stones one could reach from it through a series of steps between orthogonally adjacent stones of that color. A liberty of a group is an empty point orthogonally adjacent to at least one stone in that group. A territory is an empty point along with all other empty points one could reach from it through a series of steps between adjacent empty points. A player owns a territory if all stones orthogonally adjacent to points in that territory are of that player's color. To flip a group is to replace all of its stones with stones of the opposite color.      In the first picture, there are two white groups and two black groups. The marked points are all the liberties of those groups. In the second picture, the set of 6 empty points in the top left corner is a white territory, and the set of 4 empty points on the right side is a black territory. The 7-point territory in the bottom left corner is not owned by either player because it is adjacent to stones of both colors. The unmarked empty points also make up a neutral territory, for the same reason.

 Play The game begins with an empty board. The button is placed next to it. Black plays first, then turns alternate. On their turn, a player must perform exactly one of the following actions: Take the button, provided that neither player has taken it yet. Pass, provided that the button is already taken. Place a stone of their color on an empty point, then flip all enemy groups without liberties. In the resulting position, the newly placed stone must be part of a group with at least one liberty. Otherwise, the move is illegal.      On the left, the black group in the top left corner has only one liberty, marked in blue. A placement by White on that point flips the entire black group because it removes its last liberty, as shown on the right.        On the left, a placement by White on the marked point fills the last liberty of both the two-stone black group on top of it and the one-stone white group below it. However, the move is legal because, after flipping the two black stones, the previously isolated white stone is part of a larger group with liberties, as shown on the right.        In each of the two pictures above, a placement by Black on the marked empty point is illegal. On the left, such a move would fill the last liberty of the five-stone black group without flipping any white stones. On the right, such a move would flip the marked white stone, but the resulting eight-stone black group would have no liberties afterwards.

 Komi, button and handicap The komi is the whole number of points which is added to White's score at the end of the game as a compensation for playing second. The value of komi is agreed upon by both players before starting the game. A komi of 7 should be roughly adequate on 9x9 and bigger boards. The button a special token which is used to avoid ties. At the end of the game, the player who has taken the button gets an extra half point in their score. If there is a large difference in skill between the players, a handicap can be used to compensate for it. In this case, the weaker player plays with the black stones and is allowed to place several stones before their opponent makes their first move. The number of handicap stones is agreed upon by both players before starting the game. The commonly used values lie between 2 and 9.

 End of the game Play stops when both players pass in succession. At this point, the players may agree which groups, if any, can be flipped without changing the scores that would result from continuing the game. If there is such an agreement, the agreed groups are flipped, the game ends and the resulting position is used for scoring. If the players do not agree which groups should be flipped, play is resumed. In this case, if both players immediately pass again, with no further placements, the game ends and the current position, with no further flips, is used for scoring. Otherwise, the process repeats until a conclusion is reached. A player's score is the number of stones of their color on the board, plus the number of points in their territories, plus half a point if they have taken the button, plus the value of komi in the case of White. In handicap games, a number of points one less than the number of handicap stones is subtracted from Black's score. The player with the higher score in the final position wins the game.