[wodenokoto]

How does this handle files that are gigs in size? It looks to me like you load the entire file into memory and pass it along, shuffled.

[sgk284]

See: reservoir sampling [1]

[1] https://en.wikipedia.org/wiki/Reservoir_sampling

[mappu]

If you have an unknown input size and a finite number of requested output lines, you can still do it in O(n) and no additional memory, by having a steadily-decreasing chance of replacing one of the outputs with the next line.