Please stop the coding challenges

I once ended a job interview at a company because of their coding challenges at the end of a 2-3 hour interview. It was a job for a PHP developer working in a DDD environment. They were asking people to do algorithms, the last one just took the cake because it was completely unnecessary.

The question made sense: how do you add to very large numbers together? I assumed the key knowledge they wanted was that the numbers had to be represented as strings and you should use bc_math to compute them. So I answered that, and they said, "Yeah, great. Now write a function to do that." I sat there for a few moments thinking about it and realized it was just nuts. So I asked if I could just end the interview process. They were clearly shocked, I doubt many people end an interview process on the last question. I said that this wasn't realistic and that if I did that for real it would get code-reviewed to hell. One of them kept saying but it's to see how you would do it if bc_math wasn't installed. I simply stated "I would have them install it" to which one of the interviewers instinctively nodded since that's the real-world answer.

It seemed like it would have been a good place to work up to that point. I didn't like my current job at the time, it paid considerably more, had more public holidays due to weird German reasons, etc.

Ugh, I got that question in an interview. Only they wanted me to write up a basic arbitrary precision math library, in C, on paper. (I may have chosen to do it on paper.) It was harder than I expected. I didn't do that well and didn't manage to finish in the time allotted -- halfway through, I switched from encoding things as ASCII numerals to base-256 binary (or the reverse?) -- but it was enough that the interviewer said it was clear I understood what was required. I got the job.

I thought it was a hard but fair question. I still feel bad about not doing well. It wasn't all that close to what I'd be doing in the position, but it wasn't unrelated. Perhaps if it were a PHP role I'd be more bothered? But even there, it seems like one way to check if someone understands what's going on under the hood, so as to not naively do expensive stuff without realizing it. The fact that one should not write their own library, and that if you tried you should get code reviewed to hell, isn't relevant if that's the purpose.

I think the reason that bothered me was, that this was after a take-home tech test, a long interview, and 2 other questions that were similar.

But really, the fact you should never write one should be very basic and fundamental knowledge. Quite simply it should be an automatic no if someone didn't know they shouldn't write that themselves. They were literally testing people on things they don't do. Which means everyone I would be working with wasn't tested on their ability to do their job.