I remember implementing some of these https://www.stolaf.edu/people/hansonr/sudoku/12rules.htm https://www.stolaf.edu/people/hansonr/sudoku/12rules.htm With a

I remember implementing some of these https://www.stolaf.edu/people/hansonr/sudoku/12rules.htm https://www.stolaf.edu/people/hansonr/sudoku/12rules.htm With a simple array of unsigned int and bit operations like 20 years ago. It could solve a lot of puzzles within microseconds. Later I realized rules 1, 2, 5, 6 are pretty much the same.