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"I can perform a simple AND operation on the 3 monthly bit arrays, and then count the number of “1” bits."

I think an old-timer would tell you that he'd be using an "OR" operation to count the number of hosts seen in the last three months.

As a long-time embedded systems engineer, you learn to guard every CPU cycle and memory location jealously. That behavior, along with avoiding needless abstractions that complicate the system AND cost CPU and memory, have served me well as I've switched to higher-level software.

As an aside, memory resident databases are now practical even for problems of this size. I've used the CERN Colt library to provide sparse arrays stored in memory in place of databases several times now. My current pet project is processing a mouse genome for my daughter's lab. With the amount of data being read, I'd always be I/O bound if I wasn't packing a lot of working data into memory. This is the programmers equivalent of the CPU's cache, but one we can easily control. Give it a try!

It may be because I haven't had my coffee yet, but how would using OR help here? 1 OR 0 OR 0 returns 1, but the host was offline in 2 of the past 3 months, which means that it should be 0...

You might be right ... but it's entirely dependent upon what the author meant. The problem statement was "How Many Total Hosts Were Seen as “Up” in the Past 3 Months" which I read as "How Many Total Hosts Were Seen as “Up” in any of the Past 3 Months". You've correctly pointed out that the author might have meant "How Many Total Hosts Were Seen as “Up” in all of the Past 3 Months".

This sort of ambiguity is why BDUF is so hard to pull off. Only lawyers are trained to write completely unambiguous prose (and they fall back to having very precise meanings for words to help them succeed).

Given that the modern app developer did a de-dupe operation for his solution, and that the most plain reading of the English text is "in any of the scans", I think the OR is correct.

Should the last operation by the old-timer system developer be "(this_month XOR last_month) AND this_month" instead of "(this_month XOR last_month) XOR this_month" ? Or am I missing something there?

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