This is quick-and-dirty code that I don't intend to publish to Crates.io. It gets the right answer, but there is no documentation and it definitely could be prettier.
Licensed under either the MIT license or Apache 2.0, at your choice.
A Sudoku board (the Board struct) is represented as a 9x9 array of squares
(the Square struct).
Each square is either known, a number 1-9, or unknown, in which case the set of numbers it is allowed to be is stored.
The Square struct uses a u16, storing known numbers in the lower 4 bits and
using the upper bits as a bitset to store the possibilites.
It loops through the squares, and any unknown square with only one possibility in its set gets promoted to a known square, and the possibility sets of its neighbors (the squares in its row, column, or box) get updated.
If there are no such squares, it finds the unknown square with the fewest possibilities and tries all of them recursively.
You need a Board – this can be done either with a [[Square; 9]; 9] or the
from_text method. Examples of the text format are in the sudokus directory.
Then, calling .solve() will solve it. .solve(true) will attempt to find all
solutions, while .solve(false) will return the first one it finds.