Cave

Cave example

Rules: Shade some cells so that the unshaded cells form a single, connected region (the titular ‘cave’). Additionally, no shaded cells may be enclosed by the unshaded cells (in other words, every shaded cell must be connected by other shaded cells to the edge of the grid). The clue cells may not be shaded. A clue tells how many unshaded cells in total an observer, standing on the clue, would see in the four cardinal directions (up, down, left, right), counting the clue cell itself. Shaded cells block visibility.

The puzzle was originally presented as a loop puzzle, but I prefer the shading version. Here are the loop-style rules:

Rules: Draw a single, non-intersecting loop running on the grid lines. The clue cells must all be inside the loop. A clue tells how many cells in total an observer, standing on the clue, would see in the four cardinal directions (up, down, left, right), counting the clue cell itself. The loop blocks visibility.

Both rulesets are equivalent: in the shading version, the unshaded cells form the region inside the loop, and the shaded cells are the cells outside the loop.

This puzzle was originally invented by Nikoli, under the name Bag. This puzzle is also widely known as Corral.

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