[lazyant]

The problem with this question is that outside the junior people who won't understand the hints, many good programmers would pass or fail just based on whether they have seen a similar problem or know what a Bloom filter is. CS is so broad that you may have been working for many years and never come across one topic that the interviewer deems standard.

[dekhn]

I don't fail people who don't suggest a bloom filter. What I meant was: knowledge of probabilistic data structures and knowing this is a situation where they could be used is a sign of a more experienced programmer (likely one who's worked in clickstream processing).

If all you do in my interview is know that a hash table or other associative data structure is good for maintaining counts, that using a full ASCII byte to store symbols from { A G T C } is wasteful and compress the letters to 2 bits each, you can pack multiple 2 bits into bytes, and can write a function that codes and decodes such data, I'm happy and you get a pass. I'm even happy to sit there and help people through nearly all the steps of the codec. Not trying to trick anybody or select for obscure CS knowledxge.

To me the real question is, how much hinting is reasonable for the coding and decoding function? Many programmers (including senior ones) struggle to implement:

x <<= 2 # left shift the int to make room for the next item x |= pattern # or the new 2-bit item into the accumulator.

Since I never use bit munging in my day job (it's 90% python data science) should I really ding somebody for not knowing about left shift or or-assignment?

What I really don't get is why people immediately jump to trying to implement huffman coding, RLE, or lookbacks as a solution to reduce the size of the DNA string. I still wonder if how I present the question is giving everybody a fair chance to shine.

[mbrodersen]

I am 100% certain that your questions are not giving everybody a fair chance to shine. You have to start with the area of expertise they have already worked in and not the area that you happen to have experience in or be interested in. I could (for example) ask “simple” dynamic optimisation questions that are “easy” for me and I know that 95% of great experienced developers would fail. So obviously I don’t do that. I want to hire smart developers who can learn. Not developers who happens to have the same experience/interest that I do.