To play, click on a cell to switch between light, dark, and the default indeterminate state. A left-click turns it black, a right-click turns to white, and a middle-click returns to indeterminate. Once the mouse is pressed down, you can drag the mouse to set multiple squares in one sequence. Alternately, you can use the arrow keys to move around the board, and press the space bar to toggle between the states, or the '1' key to turn black, the '0' key to turn white, or the backspace key to set back to indeterminate. You can not alter the state of a white square with a digit.

In Assist Mode (0), the default, only the existance of errors or incomplete numbers is indicated. In Assist Mode (1), the number of errors and incomplete numbers is indicated, see game rules. In Assist Mode (2), errors and completed rules are highlighted with digit colors. In some cases the solve difficulty will greatly decrease in Assist Mode (2), so select depending upon the level of challenge desired. The game border will turn green if the grid is completed without error. You can click the "undo" button on the bottom of the page to undo previous moves.

Below, enter puzzle code (0-0) for one of a list of pre-defined puzzles, or a puzzle descriptor (see "Puzzles" tab). The following puzzle codes have demos, which will walk you through a solve to help you on the path to mastery: []).

Nurikabe (ぬりコかべ or Invisible Wall), is a classic Japanese logic puzzle published by Nikoli. In Nurikabe, the board begins with a collection of numbers within white squares. These numbers indicate a "room size" that must be satisfied in order to complete the puzzle. The room must consist of combined white squares, connected horizontally or vertically. The rules below relate to those numbers and how to complete the puzzle.

1. A number indicates the number of white cells in the area containing the value. All rooms with numbers must be white, and the rooms must be bounded horizontally and vertically by black squares.
2. All squares must be set to black or white, and white squares must be part of a room with a value.
3. Each room with a number can only contain one number.
4. The black cells must all form one contiguous "river" of black cells, connected to each other in the vertical or horizontal direction. A river is not considered connected if it only flows through diagonally adjacent black cells.
5. No "pools" of black cells of size 2x2 or larger are allowd.

Puzzles can be invoked by either a numerical entry from 0-0, or through a character description of the puzzle itself, entered into the Display form on the 'Game Play' tab. The numbered puzzles are hand-crafted of varying difficulty, roughly easy to hard. Puzzle 0 shows a completed correct puzzle.

To describe a puzzle, the character string must be of the form: "WxH:<digit-descriptor>", where W is a number indicating the width of the grid, and H the height. The digit descriptor is a list of the pre-defined digit contents, from left to right, then top to bottom. Blank cells are represented with a '-' character. To assist in the definition of a sparse board, a combination of b consecutive blanks can be represented with a lowercase letter from 'a' to 'z', corresponding to 1 to 26 consecutive blanks. Digit values greater than '9' should be represented with capital letters 'A' to 'Z' representing 10 to 35. A dot '.' can be used as a visual separator, for example between rows. An example descriptor is as follows for entry 1: 'descr'.

For Nurikabe there are 1 demo puzzles that can be helpful to walk through solving methods. They are puzzle codes []. Enter them in the Game Play tab with the code number and there will be added a demo panel to help with the solve.

This playground was written using javascript and is available open source. All code is checked into the github repo at ohmec/puzzles. Feel free to file bugs there!