How would you implement tail efficiently?

I don't think there are solutions different than "keep the latest N lines while reading forward the data" or "start from the end and go backwards until you read the Nth line".

The point is that you'd use one or the another based on the context.

The "go to the end and go backwards" is better when tail accesses a random access file, or when the data is small enough to be put on memory. In this case the runtime is minimized, since you scan the data that has to be outputted (so, it's "optimal")

Your solution (keep the N latest lines) is better when tail is fed with a pipeline or when the data is huge. In this case, the other solution wastes too much memory, so it is not practical and, in the case the source is slower than tail (which is probable) scanning all the file doesn't matter that much.

Read backwards from the end of the file until N linebreaks are read or the beginning of the file is reached.

Then print what was just read.

I dont think any fancy datastructures are needed here.

Here is the source code of tail if you're interested.

First use fseek to find the end-of-file then subtract 512 and fseek to that offset, then read forward from there to end. Count the number of line-breaks because if there are too few you will have to do the same with a subtracted offset of 1024 ... but in 99% of cases 512 will be enough.

This (1) avoids reading the whole file forward and (2) the reason why this is probably more efficient than reading backwards from the end is that reading forward is typically faster.