To set the colors of cells, right-click on a cell to turn dark, or middle-click to turn white. Alternately, you can use the arrow keys to move around the board, and press the space bar to toggle between the states, the '1' key to turn black, or the '0' key to turn white. You can not alter the state of a square with a number and arrow value.

To draw a path, left click on the center of a white cell to begin. From there, drag the mouse in the direction to create the path. The path will draw as the dragging continues. Alternately, once on the board, you can press the shift key and hold while moving around the board with the arrow keys. In addition, you can enter a key to indicate the type of path desired for that cell (and into its neighbors). These keys are as follows, and will extend the line into the center of the adjoining cells:

• '-' key to draw a full horizontal line through the cell
• '|' or 'I' key to draw a full vertical line through the cell
• 'L','J','7', or 'F' key to draw a correspondingly shaped turn in the cell

Once created, a path can be removed by pressing the backspace key within a white cell. This deletes it just for this cell, but deletes the "arms" into the adjoining cells. You can also attempt to redraw with the left-click dragging method.

You can press the '.' key to temporarily insert a dot in a cell to indicate that it will eventually need a path. A backspace clears the dot.

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: []).

Yajilin (ヤジリン or roughly "Arrow Link"), is a classic Japanese logic puzzle published by Nikoli. In Yajilin, the board begins with a collection of numbers and arrows within gray squares. These numbers and arrows indicate the number of black squares to be placed in the direction given. satisfied in order to complete the puzzle. In addition, a single continuous path must pass through all white squares. The rules below relate to those numbers and how to complete the puzzle.

1. A digit with an arrow indicates the number of black cells on the board to the direction of the arrow. I.e. a 3-up-arrow means there must be three black cells above the 3 digit before the edge of the board.
2. Digit cells are gray, and indicate that they do not need to be crossed with the path.
3. Black squares may not touch horizontally or vertically, but may touch diagonally. It is OK for a black cell to touch a gray digit cell.
4. A single continuous path must be drawn that goes through all white squares. The path must connect back with itself but can't cross itself or enter the same square twice.
5. There will be some cells that are not under the rules of given directional digits. Those cells may end up black or white but must not violate the above rules.

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-arrow-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 plus an arrow direction, from left to right, then top to bottom. The arrow directions are indicated with ^ for up, v for down, < for left, or > for right. 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 Yajilin 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!