[OT] shuffling lines in a stream

[Sergey Gromov]

Fri, 10 Oct 2008 16:10:29 -0500,
Andrei Alexandrescu wrote:
> Sergey Gromov wrote:
> > Fri, 10 Oct 2008 15:54:18 -0500,
> > 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?
> > 
> > After merging b and c you end up with a and bc.  Then you randomly merge 
> > these two files and lines from a have all the chances to appear anywhere 
> > within result.
> > 
> > When randomly merging, the probability of picking a line from a file 
> > should be proportional to a number of lines left in that file.
> 
> How do you "randomly merge"? Describe the process.

a[] and b[] are files to merge.  ab[] is the result.  a.length and 
b.length are number of lines left in either file.

while (a.length || b.length)
{
  if (uniform(gen, 0, a.length + b.length) < a.length)
  {
    ab.put(a.head);
    a.next();
  }
  else
  {
    ab.put(b.head);
    b.next();
  }
}