This is a port of a Sudoku generator written in Python by Ronald L. Rivest in 2006.
It's not great Swift code, as I was learning Python while porting the original code, but it does work.
You can run it from the command line:
$ swift Sudoku.swift -g -f
Puzzle generated from seed '529800175' :
. . . | . 9 . | . . 7
. . 1 | . . . | . . .
. . 6 | 4 3 . | . . .
------------------------------
2 . . | . 7 . | . . .
8 . . | . . . | . 4 .
7 . . | . 4 9 | . 1 5
------------------------------
. . . | 1 . 7 | 5 2 .
. 4 . | . . . | 8 . 9
. . . | . . . | 1 . .
Puzzle solution:
3 8 2 | 6 9 1 | 4 5 7
4 9 1 | 7 5 8 | 6 3 2
5 7 6 | 4 3 2 | 9 8 1
------------------------------
2 1 4 | 5 7 6 | 3 9 8
8 5 9 | 2 1 3 | 7 4 6
7 6 3 | 8 4 9 | 2 1 5
------------------------------
9 3 8 | 1 6 7 | 5 2 4
1 4 7 | 3 2 5 | 8 6 9
6 2 5 | 9 8 4 | 1 7 3
SOLUTION IS UNIQUE
Puzzle rating: 134
(moderate)
Note that the file is called Sudoku.swift for command-line usage convenience, but if you try to build and run in Xcode, you'll get an error, Expressions are not allowed at the top level. Only a source file named main.swift is allowed to have code that's at the top level so if you want to debug it in Xcode, rename Sudoku.swift to main.swift.