Mostly Python and Fortran. See for example https://github.com/oorb/oorb https://github.com/oorb/oorb. The hardest problems are always the social ones. How do y

Mostly Python and Fortran. See for example https://github.com/oorb/oorb https://github.com/oorb/oorb. The hardest problems are always the social ones. How do you get uptake of a new method, how do you get funding, how do you politely tell a collaboration they are doing the wrong thing, etc. But if you mean pure technical stuff - the hardest problem I had to solve was rethinking some of the inner loops of the THOR algorithm. The problem was essentially to speed up a Hough transform in 6D space. Lots of time spent profiling CPU cache timings to get that fast.