These programs use ZDDs (zero-suppressed binary decision diagrams) to solve logic puzzles. See Knuth, "The Art of Computer Programming", Volume 4, Fascicle 1, which I partly summarized:

Build with make.

All programs read a puzzle from standard input then print the solution on standard output.

The file spi.txt is a Slither Link puzzle I used loop.c to construct. Try solving it by hand first.

Standard Sudokus are too big to be efficiently solvable by ZDDs. I found out the hard way: see sud.c.

I began a Nurikabe solver but I have since been distracted.