Nurikabe

Just a quick query! I've been getting to grips with Nurikabe this month and bought a book full to give me plenty of practice! One thing I noticed was that in this book (The Nurikabe Book by Sam Griffith-Jones) it says that 2x2 blocks of both black and white cells are disallowed. I realised that 2 x 2 blocks of black cells were forbidden, but didn't realise the same was true for white cells. This got me wondering and so I have visited several nurikabe sites and some say both black and white 2 x 2 blocks are forbidden and some only mention black ones are forbidden. Anybody got any thoughts on the matter? I have noticed a couple of Gareth's puzzles contain 2 x 2 white blocks. Does it depend on the individual publisher?

Here are the official rules for Nurikabe, in so much as they are written by the people who invented the puzzle (Nikoli):

1. You cannot fill in cells containing numbers.
2. 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.
3. The black cells are linked to be a continuous wall.
4. 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.

Thanks for taking the time to clear that up Gareth - I guess you can't argue with Nikoli since they invented the puzzle!

I think your theory about Sam Griffith-Jones must be correct. It clearly states in the book:-

"The final rule is a little more complex:

Rule 4: "Cycles are forbidden. A cycle is a group of squares that connect all the way around another group of squares (he then shows some diagrams of white islands completely surrounded by black cells which unfortunately I don't know how to scan in to show you). Then it says "The 'cycles rule' means that 2 x 2 blocks of both white and black squares are forbidden."

I also looked at his site and that fourth rule isn't mentioned there. However, the book was first published in 2005 so I imagine he later corrected this as you say.

In fairness, I also looked at the Clarity Media puzzle rules and think I misinterpreted what was written - "you cannot have a 2 x 2 square of filled cells or ocean" - having looked at the rules again carefully obviously the filled cells ane the ocean are one and the same.

Anyway,, at least I have a clear understanding of the rules now, so thanks!

Hi there

Just to clarify, 2 x 2 blocks of white cells ARE allowed in nurikabe: as you say, the filled cells and 'ocean' are references to the same thing in our rules.

I also was unaware of any puzzles that banned 2 x 2 blocks of white cells and would say that such a puzzle is a nurikabe variant rather than nurikabe itself.

More common is to see nurikabe puzzles printed in newspapers, but without any reference to the fact that 2x2 blocks of filled cells are not allowed: I've seen this happen a few times and has the unfortunate consequence of meaning the puzzles appear to have multiple solutions.

Dan