I'm not an expert (sigh...) but the hardest work I had to do was implementing a computational geometry library. For the first time in my career I had to buy tex

I'm not an expert (sigh...) but the hardest work I had to do was implementing a computational geometry library. For the first time in my career I had to buy text books. Looks like the closer you are to the hardware, harder it is. If I could go back in time and pick a field I'd choose from OS/runtime development, HPC, CAD, graphics, compilers, scientific computing (this feels a bit dry and too hard, so, perhaps not \0/), network security/protocols.