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How would you implement tail efficiently?

A friend of mine was asked how he’d implement tail -n.
To be clear, we are required to print the last n lines of the file specified.

I thought of using an array of n strings and overwriting them in a cyclic manner.
But if we are given, say a 10 GB file, this approach doesn’t scale at all.

Is there a better way to do this?

Answer 1

Memory map the file, iterate from the end looking for end of line n times, write from that point to the end of file to standard out.

You could potentially complicate the solution by not mapping the whole file, but just the last X kb of memory (say a couple of memory pages) and seeking there. If there aren’t enough lines, then memory map a larger region until you get what you want. You can use some heuristic to implement the guess for how much memory you want to map (say 1kb per line as a rough estimate). I would not really do this though.