Two hard things in twenty three years
In 1990, one of my first roles after leaving university (Mathematics with Computer Science), was to design and implement a set of algorithms to simulate aircraft flying around as part of an air traffic controller trainer.
This involved going back to the university library, reading up on applied mathematics algorithms, and then implementing some approximation methods. I had about 5 pages of equations which then needed to be implemented.
The simulator ran on Apple Macintosh machines, and was written in Think Pascal.
I had about 200 milliseconds in each second to provide the x, y, z, and speed, for 200 aircraft; the rest of time each second was for others to do things like handling the UI and fluffy stuff like that.
It was a really hard, but really rewarding exercise.
After that, the next job involving algorithms was in 1999, implementing a set of algorithms to aid the world's largest semiconductor manufacturer plan their supply line - again, a really satisfying job.
Other than that, in 23 years, that is it!