Inner Coral by Palmer Mebane

Inner Coral by Palmer Mebane


or solve online (using our beta test of Penpa-Edit tools).

Theme: Clue Symmetry and Logic

Author/Opus: This is the 47th puzzle from our contributing puzzlemaster Palmer Mebane.

Rules: Shade some empty cells black to create a single connected wall (the “Coral”). The shaded cells cannot form a 2×2 square anywhere in the grid, and all unshaded cells including clue cells must be connected to an edge of the grid. Clues in the gray cells indicate the lengths of the first shaded segments visible from this cell in all four directions (clues are given in ascending order). [For clarity, when considering the “first shaded segment” in a direction, gray cells see through all empty and gray cells to the edge of the grid when finding this segment, if any.]

Also see this example:

Inner Coral Example by Thomas Snyder

Answer String: Enter the length in cells of each of the shaded segments from left to right for the marked rows, starting at the top. Separate each row’s entry from the next with a comma.

Time Standards (highlight to view): Grandmaster = 4:30, Master = 8:00, Expert = 16:00

Solution: PDF; a solution video is also available here.

Editorial Note: Many solvers can confuse Coral with another puzzle type with similar spelling. An easy way to remember the difference is that Cave has no R’s and also has no 2×2 Region constraint. Coral puzzles, with an R, do have this region constraint. The other general rules (about not enclosing white spaces and having a single shaded group) are shared between Cave and Coral puzzles.

  • JuffoWup says:

    The rules should clarify that the grey clue cells can “see through” each other – otherwise it looks like multiple solutions are possible.

  • edderiofer says:

    If the clue cells behave like unshaded cells for the purposes of connectivity and being seen through by other clue cells, is there actually any merit to shading them grey?

    • Avatar photo drsudoku says:

      Not really (maybe to not have it look like a Tapa but the clues really make that impossible). These puzzles were made for practice on the WPC and we have kept to the exact art style used there to keep true to what we were practicing for.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.