Fish

Fish are single-digit patterns. Choose a set of base houses and the same number of cover houses. When the cover houses contain all base candidates for a digit, that digit can be removed from other cells in the cover houses.

Base and cover houses are usually rows and columns, but complex fish can also use blocks.

Basic Fish

The number of base and cover houses gives the pattern its name:

• Size 2: X-Wing
• Size 3: Swordfish
• Size 4: Jellyfish
• Size 5: Squirmbag
• Size 6: Whale
• Size 7: Leviathan

Every selected base and cover house must participate in the pattern. Not every intersection needs to contain the fish digit.

Finned Fish

A Finned Fish has additional base candidates outside the selected cover houses. These candidates are fins. A cover candidate can still be removed when it is not a base candidate and it can see every fin.

Xodoku searches Finned X-Wing, Finned Swordfish, and Finned Jellyfish.

Sashimi Fish

A Sashimi Fish is a finned fish whose clean base-cover pattern would be degenerate if the fins were removed. The elimination rule is unchanged: remove only cover candidates that can see every fin.

Xodoku searches Sashimi X-Wing, Sashimi Swordfish, and Sashimi Jellyfish.

Complex Fish

Xodoku also searches bounded complex fish:

• Franken Fish use blocks as sectors.
• Mutant Fish mix rows, columns, and blocks.
• Endo fins are fins inside the fish sector system.
• Cannibalistic eliminations remove candidates that are part of the cover system itself.

The complex fish search is bounded to keep browser hints responsive.

Reserved

Kraken Fish remains reserved for a later Fish-rooted forcing proof. It is not a separate solver step today.