I downloaded the code and had a look. For context, I've ramped up on quite a few code bases in my day in a bunch of different languages. Here was what I thought

I downloaded the code and had a look. For context, I've ramped up on quite a few code bases in my day in a bunch of different languages. Here was what I thought would happen in order of likelyhood: 1. The code would not build at all on a standard linux distro unless you knew exactly what packages to install. (I had an interview like this once) 2. Locking the key was going to be inexplicably linked to the operation (got very worried when I saw `add_delta` and `do_add_delta`). Turns out add_delta just calls do_add_delta wrapping it in a lock. The lock API is very simple. 3. There would be a massive amount of code duplication that would make it difficult to add this feature without doing serious refactoring. Turns out you basically just change `bool incr` for `math_op_t` or something and provide a way to map from binary/string input to math_op_t. This would be very easy. Depending on how comfortable I was with the editing environment they talk about elsewhere in the hn comments it wouldn't be too bad.