Rekushu

Overview
Rekushu is a two-player board game invented in 2006 by John Herr.

Board

The game Rekushu is played on a square board with orthogonal lines with 20x20 intersections. Boards of other sizes can also be used (e.g. 8x8, 12x12, 14x14, and 18x18).

There are two players having Black and White stones. Stones are placed on the intersections of the grid lines. The squares that the lines make are called tiles.
 

Objective
The objective of Rekushu is to have claimed more tiles than the opponent when one of the players has no legal moves on his turn. 

Play

Players move alternately, starting from the player having Black stones.

Each move consists of one or two steps: placing a stone and claiming a rectangular region of tiles.

The first step (placing a stone) is mandatory. A stone can be placed on any unoccupied intersection meeting the following conditions:

  • There should be at least one adjacent tile that is not claimed yet.
  • Playing on the intersection should not cause the repetition of the same type of mirror move that was made by the player on his previous turn.
 
A stone can be placed on the specified intersections because there is at least one unclaimed tile (not highlighted squares)  adjacent to each of them.   A stone cannot be placed on the specified intersections because there are no unclaimed tiles adjacent to them.

The second step (claiming a rectangular region of tiles) is optional, i.e. if some region can be claimed then a player can decide whether to do it or not. A rectangular region can be claimed if it meets all of the following conditions:

  1. There are four stones of the player's color placed on the four intersections forming the four vertices of the orthogonal rectangular region.
  2. There are no stones of either color in its edges or interior.
  3. There are no claimed tiles of either color in its interior.

Below are several examples of the rectangle regions, which are not eligible to be claimed by a player for some reason:

   
The highlighted rectangle cannot be claimed by the Black player because there are not four of his stones forming the vertices of the rectangle.   The highlighted rectangle cannot be claimed by the Black player because there is a stone on its edge (D13).   The highlighted rectangle cannot be claimed by the Black player because there are claimed tiles (two white tiles) in its interior.

If all the conditions mentioned above are met and the player decides to claim the rectangle, then the player's four stones on the vertices of the rectangle are removed from the board and all the interior tiles of the rectangle are marked with the player's color:

 
The highlighted rectangle can be claimed by the Black player.   The rectangle is claimed by the Black player and the four stones forming the vertices of the rectangle are removed from the board.

Mirror Moves

There is one special rule that eliminates an obvious forced draw by the second player. A player is not allowed to make the same type of mirror move consecutively. 

There are three types of mirror moves: playing horizontally opposite your opponent (h), playing vertically opposite your opponent (v), and playing at a 180 degree rotation from where your opponent just played (r). A player is allowed to make more than one mirror move in a row, but he cannot make the same type of a mirror move twice in a row. 

The White player placed the 1st stone on J10.

The Black player placed a 2nd stone on E10, making a vertical mirror move.

The White player placed the 3rd stone on C6.

Now the Black player is not allowed to place a stone on L6 because this will be a consecutive vertical mirror move. However, the Black player can place a stone on C9 (a horizontal mirror move) because it is not the same type of mirror move.

If a player cannot place a stone because playing on the last intersection would be a mirror violation, then the game immediately ends.

 


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