An implementation of POSIX tsort.
See https://pubs.opengroup.org/onlinepubs/9699919799/utilities/tsort.html
This implementation performs in roughly linear time using the classic Knuth algorithm Algorithm T, Topological sort, from The Art of Computer Programming Vol 1, sec. 2.2.3.
If a cycle is detected, this program prints a single cycle to assist in resolving the problem, using the solution to Knuth sec. 2.2.3 Exercise 23.
For example, given the input a b b c c a c d d e, it prints
input contains a loop:
a ->
b ->
c ->
a
It is heavily commented to show where the code corresponds with Knuth's description.
Licensed 0BSD, free for any use.