Here are the official rules for Nurikabe, in so much as they are written by the people who invented the puzzle (Nikoli):
- You cannot fill in cells containing numbers.
- A number tells the number of continuous white cells. Each area of white cells contains only one number in it and they are separated by black cells.
- The black cells are linked to be a continuous wall.
- Black cells cannot be linked to be 2x2 square or larger.
(see
http://nikoli.co.jp/en/puzzles/nurikabe/index_text.htm).
I've
never seen a Nurikabe puzzle that has a rule that you can't have 2x2 white areas, so anyone who thought otherwise presumably had got the wrong end of the stick and simply guessed at the rules (or didn't read them very carefully). The puzzle has been around under various names in the UK for years - e.g. Tough Puzzles published it as Canals (I think 'Canals', anyway, and I have a couple of of Carlton puzzle books from the early 1990s with it in), and the book you mention dates only from 2007.
In fact I just looked on Sam Griffiths-Jones's website and he gives the rules of Nurikabe correctly. Without checking the book myself, I can only assume that after he wrote it he realised he'd got it wrong and at least fixed his instructions online. Looking through the puzzles he has on his site (all dated 2007) I can't find any 2x2 white areas (not that I checked them all), so I can only assume he realised at some point he'd got it wrong. Presumably his puzzles won't have unique solutions unless you use this rule, possibly including the ones on his site (but I'm just guessing there, so that might not be the case).
I'm fascinated to hear you've seen this 2x2 white rule elsewhere. Of course there's absolutely nothing wrong with creating a puzzle which can't have 2x2 white areas - it's just not what you would expect in a book claiming to contain standard Nurikabe puzzles.
Final word from Nikoli, in their oddly charming half-English:
Quote
Nurikabe has more rules than other puzzles, but popular from beginners to experts. Black cells stretch like mollusc and make a linked wall on the grid. As the cells stretch and be linked, you will feel the sense of achievement. The starting point is 1 (you can fill four cells of its sides; up, down, right, and left cells). Enjoy!
(Japanese molluscs must be super-scary!)
Edited 6 time(s). Last edit at 02/21/2010 02:11AM by gareth.