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Yajilin, also called 'Arrow Ring', is a Nikoli puzzle in which the aim of the game is to draw a single, continuous loop around a grid. The loop must pass all cells except for those that are either a hint or a black cell. These black cells have to be discovered first however.
Yajilin can be played on any grid, but the bigger the grid the more difficult the puzzle becomes. The difficulty doesn't just depend on the grid size though, it also depends on the given clues and the amount of black cells on the grid.
The rules are as follows:
- Draw a single, continuous loop on the grid using the indicative cells as indicators of both what cells to avoid and as clues to where the black cells are.
- The loop cannot branch off or cross itself.
- Indicative cells indicate how many black cells there are between that cell and the edge of the map in the direction the cell is pointing.
- There may be more black cells other than those hinted at by the indicative cells.
- Black cells cannot touch each other vertically or horizontally, but they can touch corners (diagonally).
A Yajilin puzzle can be a difficult one to solve. While deduction work can help you make a decent start, it sometimes requires trial and error to get further. But even figuring out where to start can be a hassle to some.
The best places to start are those locked in by either the edges of the map or by clusters of indicative cells. For example, in my example below the bottom left corner will definitely have lines going around the upward pointing 1 and the leftward pointing 2.
Then there are clusters like the two 2's in the middle. The one pointing to the right shows two of the three cells have to be black. Since the first cannot possibly be white (it would prevent a continuous loop) it therefore definitely has to be black. Then, since 2 black cells cannot be next to each other according to the rules, the second cell must be white and third must be black.
This also reveals where the other black cell is below, near the upward pointing 1.
Yajilin offers a lot of fun in terms of adaptations. The loop could be a path you have to follow in a cave, a looping automatic band that passes various devices (black cells), the route a security droid takes in a warehouse, the mine shafts and chutes inside a mountain wall, and so on.
For my example I went with a path of safety. My character finds herself trapped in a room, all she's told is that there's a safe path she can follow, which will not hurt her. She's also given the clues and the instructions on how to solve a Yajilin puzzle. If she ever wanders from the correct path she'll either be shocked or fall into a hidden pit (the black cells).
Optionally, you could add chests or other elements in various places around the loop, this way the party will have an incentive to explore the entire loop, rather than leaving at whichever end is their way out.
As long as my character figures out the puzzle she'll be fine and able to exit at a specific point in the loop. Should she fail to either stick to the path or figure out the correct path she'll face the consequences. These could be incredibly varied as well. Besides the shocks and pitfalls I mentioned earlier she could also miss out on loot (falling through a pitfall could prevent her from reaching chests for example), it might unleash monsters in the puzzle room, change the loop to a different one (requires multiple puzzles, obviously), and so on.
So in short, there's a huge amount of variety to be played with in terms of adapting Yajilin. The biggest challenge will be creating the puzzle itself, but that shouldn't be too difficult either. Simply draw a path across a grid, then add hints and black cells in the gaps. Do make sure you give enough hints to make the puzzle possible of course.