[OT] shuffling lines in a stream

[Sean Kelly]

Andrei Alexandrescu wrote:
> 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?

When a is merged with (b&c) then its lines will be interleaved randomly 
in the result.


Sean