[OT] shuffling lines in a stream

[Andrei Alexandrescu]

Sean Kelly wrote:
> It would be slower than the seeking option, but something like a 
> randomized mergesort would work as well.  If the program can buffer k 
> lines in a file containing n lines, then read the first k lines into 
> memory, shuffle them, and write them out to a temporary file.  Repeat 
> until the input is exhausted.  Now randomly pick two of the temporary 
> files and randomly merge them.  Repeat until two temporary files remain, 
> then output the result of the final random merge to the screen.
> 
> For small files (ie. where n<k) the file would be read, shuffled in 
> memory, and printed to the screen, assuming the proper checks were in 
> place.

I think I found a problem with your solution. Consider you break the 
file in three chunks: a, b, c. Then you pick at random b and c and 
randomly merge them. The problem is, you make early the decision that 
nothing in a will appear in the first two thirds of the result. So the 
quality of randomness suffers. How would you address that?

Andrei