Swordfish
The X-Wing with three rows and three columns: if a digit's places in three rows fall in only three columns, those columns are covered.
The idea
Pick a digit and look for three rows where it has two or three places, and all those places together lie in only three columns.
The three rows must put the digit in three different columns, and they have only these three to choose from. So the three columns get their copy of the digit from these rows, and every other cell in those columns can't be that digit.
Each row doesn't need three places: two in some rows is enough, as long as the three rows together use just three columns. The same works with rows and columns swapped. With four rows and columns it is a Jellyfish.
How to spot it
- With notes, list for one digit the rows where it has two or three places and write down their columns.
- Find three such rows whose columns add up to only three. That is the hard part, and why Swordfish is a master technique.
Worked example
Swordfish on a real puzzle
From our master puzzle #51 (27 clues), after 29 earlier steps of our solver. Gold cells make the pattern; red notes are the ones it rules out.
How often our puzzles need it
Our solver works like a careful player: it always uses the easiest technique that still makes progress. Counting the puzzles where it had to use swordfish at least once:
- 26 of our 1000 master puzzles
The easiest level that can need it is master Sudoku.