In Nurikabe you start with a grid of squares with a scattering of numbers seemingly at random.
The idea is to paint the blank squares to make walls, leaving patches of white behind. The rules are simple:
Each number defines a specific area of adjacent white cells which can't be walls. You have to find
which cells are white starting with the numbered square. A number 1 for example, is one white cell and therefore
there are four black 'walls' above, below, to the left and to the right.
You must not join up groups of white cells with two or more numbers.
No group of painted cells can form a block of four.
All painted cells must join up in the final solution; there can't be any isolated walls.
Key selling points
This is excellent for children as well as adults since the only activity on the board consists of colouring in black
squares. The game requires logical thought and can be tricky according to the design. No arthimetic is involved apart from keeping track
of the number of white cells.
Grid Sizes
A 'normal' Nurikabe board is around 10 x 15 cells, or 15 x 10. The smallest we produce is 10 x 10 and the largest
is 20 x 20.
Strategies
Look for 1s. If there are any 1s on the board the adjacent squares can be filled in as walls.
Numbers one cell apart. Any two numbers that are one cell apart must be divided by a wall
and you can colour in that square.
Forced cells. When expanding a white group from a starting number sometimes there is only one
way to go. If so, mark that cell as white. The most informative way to do this is to use a number one
less than the starting cell. Spotting forcing cells means you can look for new walls.
Reach of Number 2. If a cell has a number 2 (or it is the second to last cell in a white group) then
we can say something about the cells that are diagonally one square away. There must be one white more in
the group. This means the cell after must be a wall. You have to check carefully and make sure that the last
cell can only go in two directions.
Unreachable cells. If you are stuck try look for blank cells which are too far away for a numbered
cell to reach. Be careful, as some numbers might be quite large, but as long as you count upwards and downwards
or horizontally, you may find squares that must be walls because no white group can reach them.
Orphaned Walls. (from Rule 3) An orphaned wall is a group of painted cells that can only join on to the rest
of the black cells via one cell. This cell must therefore be a wall.
Fourth Corner. (from Rule 2) If you've created a block of three black cells in a 2 x 2 area then your
know the forth cell is white. Is there a white number/group nearby that can reach it? Careful, the straightest
path might not be correct, but in a tight spot it should be clear enough.
Number Bubble. Sometimes a white area gets surrounded by black walls and the remainder of the
space can only be white cells.