About the contest

• This is an online only puzzle contest with 16 Yajilin puzzles.
• A new Yajilin puzzle will be live in every 30 hours between July 27th until August 15th.
• Cumulative points from each day's solving will determine the final scores.
• There will be a leaderboard for early-birds, apart from the official one. Details here.

About the Author

• All puzzles for this contest are authored by Jeffrey Bardon (12831)

Yajilin Rules

• Draw a loop through some empty cells and shade the rest.
• The loop may not cross or self-intersect.
• Shaded cells may not share an edge.
• Clue cells may not be passed over by the loop or be shaded.
• Clue cells give the number of shaded cells in the indicated direction.
• It is possible that a clue cell will have no value on it - this will confer no information beyond that it is not shaded and not a part of the loop.
  

Example Puzzle

Points to remember

• The loop must pass through all unshaded and unclued cells.
• The presence of a clue cell will influence the loop, regardless of its value. In the example puzzle, the 1 up clue does not need its value given for the puzzle to work, but the clue cell does need to be there.

Penpa Interface

• Only the loop will be checked as part of solution checking.
• Any other modes in the grid can be used for auxiliary marking.
• All puzzles will have two modes enabled: Composite -> Yajilin, and Line -> Normal.
• Yajilin input mode has the following features:
  • Line drawing - left click and drag (PC) or tap and drag (mobile)
  • Shading - left click a cell (PC) or tap a cell (mobile)
  • Auxiliary dots - right click a cell (PC, supports dragging) or tap a shaded cell (mobile)
  • Cross marks - right click a cell boundary (PC) or tap a cell boundary (mobile)
• Line input mode only has the first input from Yajilin, and also has 5 colors available to switch between. Any of the colors can be used for the loop.
• If Yajilin mode is hard to use, switching to Line to avoid accidentally placing auxiliary marks is recommended.

Solving Tips

• Marking cells that must be part of the loop (and therefore, not shaded) is often useful. There can be cases where shading a cell would create a dangling endpoint or force the loop to self-intersect; in these cases, the cell must be unshaded.
• Noting places where there must be a shaded cell, even without knowing exactly which cell it belongs in, is often useful. A cross mark is recommended to note a place where a shaded cell is in one of two cells.
• There cannot be any dangling loop ends. Even without knowing how the ends connect, it can be determined if the last possible entrance is used or not. In this example, the blue lines are deducible and the red line is a contradiction.
  

  

Walkthrough of Example

1. The 0 right clue sees no shaded cells, so let's use dots to mark those as unshaded.
2. The clue cells and grid corners have 12 places where shading a cell would create a dangling loop end, so let's mark those unshaded.
   
3. Since shaded cells cannot be adjacent, we can mark the 2 down as having 1 shaded cell in each pair.
4. The 3 left can be resolved. This could have been done immediately.
5. Drawing in the loop segments that follow directly.
6. There are 2 loop entrances to the top right: since there must be an even amount, the last option is not used, and must be shaded.
7. The bottom right needs at least 2 exits, so both possibilities are used.
8. The cell in row 6, column 3 must not be shaded for two reasons. If it were shaded, it would create multiple loops. Additionally, it would force the loop to self-intersect.
   
9. Finished!

History Of Yajilin

• See this page on GM Puzzles for a brief history on Yajilin.