This is a Melon puzzle. (18 – Pento Coral)
or solve online (using our beta test of Penpa-Edit tools; use tab to shift between shading mode and the linex mode where left click+drag draws lines and right click marks X’s)
Theme: “Easy” Practice (this was a WPC puzzle style)
Rules: Place each of the 11 pentominoes into the grid exactly once, with rotations and reflections allowed, to form a valid “Coral”. In a Coral puzzle, all blackened areas are connected, but no group of 2×2 cells is entirely blackened. Also, there are no unshaded areas enclosed by the coral (meaning each unshaded cell must have a path through its edges with other unshaded cells to a border). Numbers outside the grid indicate the lengths of groups of shaded cells in that row/column, but not necessarily in order.
Or see this example from the 22nd WPC instruction booklet.
Answer String: Enter the identities of the pentominoes (one capital letter for each, even if in multiple cells) appearing in the marked rows from left to right (eg “ILN,WVXT”).
Time Standards (highlight to view): Grandmaster = 4:15, Master = 6:30, Expert = 13:00
Solution: PDF
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.
I got so engrossed in the PBN that I almost forgot about the pentominoes! 96% of the cells can be colored without knowing about the pentomino restriction.
Definitely true. The “Hard” example that will follow will not be so forgiving.
Oddly enough, I didn’t need the pentominoes until the very end, to disambiguate four squares and strictly identify which pentomino each coral square is.
That’s correct. This was the first puzzle I made to practice the type. Due to what you pointed out, I felt it failed at being good practice material. Another attempt was more successful; Thomas has already mentioned that it is coming up.
I think someone is trolling there… “Coral” is nothing like “Cave”, but is often confused with “Corral”, which is another name for the puzzle type (except that its rules are defined as drawing a single non-intersecting loop), which has two Rs. Also, when pronounced, the stress is on the second syllable rather than the first.
Not trolling: the WPC instruction booklet actually referred to this puzzle style as Pento Corral.
I like solving this type in a paint program, because (unlike most polyomino puzzles) you can always flood fill with just three colors: one for the empty squares, and two for the pentominoes to distinguish adjacent pieces. The map of pentominoes is always two-colorable because it’s a tree.
Another way to keep the two puzzle types apart: A coRRal, having 2 R’s, is large enough to allow 2×2’s in the solution, while a coRal, with one R, doesn’t have enough room for two.