Rive is a territory game for two players, Black and White. It is played on the cells of a small hexagonal grid with an odd number of cells, and the goal is to have the most stones on the board when it is full. The board is initially empty. Rive was invented by Mark Steere in 2010.
Definitions:
A cluster is a connected set of stones in any combination of colors (only white, only black or both).
There are three kinds of placement, all of which consist in placing exactly one stone of your color on an empty cell:
You plant when you place adjacent to no clusters.
You grow when you place adjacent to only one cluster.
You merge when you place adjacent to multiple clusters.
The only other kind of action is to shrink a cluster, which is to remove stones from it without splitting it into multiple clusters. This action will sometimes involve removing multiple stones, which may be of any combination of colors.
The basic structure of a turn is this:
You must plant if you can, otherwise you may grow or merge. If you merge, you must also shrink, then move again. This means that you may not end your turn before you have made a non-merging placement.
Restriction on growing and merging:
If you cannot plant (and therefore must grow or merge), you must choose a move such that the largest cluster you place adjacent to is as small as possible. This means that you cannot always choose freely between growing and merging, even though you aren't obligated to plant.
Restriction on shrinking:
The cluster resulting from the merging placement must be shrinked down to a cluster of size one more than the biggest cluster that was merged.