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The Answer Guy

By James T. Dennis, tag@lists.linuxgazette.net
Starshine Technical Services, http://www.starshine.org/

(?)Shuffling Lines in a File

From David Stanaway on the Linux Programmers Support Team mailing list on 20 Sep 1998

Now I'm trying to shuffle the order of the lines in a text file without reading in the whole file... Does anyone have any advice, code, etc on this? If I can read in the whole file, this is simple, but I might want to shuffle a file several megs long.

What do you mean by shuffle?

(!)I think he means something like: randomly or arbtrarily reorder the lines of the file without reading the whole thing into RAM/core.
I think the approach I'd take is to lock the file from access by whatever programs and/or processes are intended to read the data out of it.
Then I'd "index" the file --- search through it finding all of the line boundary offsets and their lengths. I'd then use an standard shuffling techniques on that index file. The problem with "shuffling" a normal text file on line boundaries is the variable record lengths. So we create a table of offsets and lengths to those --- and all of the offset/length pairs are of a fixed size.
So I could use the index file and "shuffle" it with the following psuedo code:
open index file
while read index file entry (readbuf)
pick a random place to put it
load the "place to put it" entry (writebuf)
swap these entries in read and write buf.
write both buffers
If the intent is to shuffle the files by some other criteria (arbitrary vs. random) when you'd modify the above algorithm accordingly. If the criteria for resequencing has to do with the data in the files (i.e. your "sorting" the file) you'd have a bit more work ahead of you.
... actually I'd optimize this a bit by read x entries into a buffer, for looping through that, and maintain a few write bufs into random locations into the file. For example I might load 100 entries in the read buffer and up to ten unique randomly selected write buffers. For each of the 100 read buffer entries I'd randomly select among the open write buffers (1 to 10) and randomly select a place in that buffer to put it). At the end of the for loop I'd write everything back out, read the next read buff, select more write buffs, and so on until the end of the file.
Every entry in the index file will have been exchanged with some random entry at least once --- and the average will be two. There is a small chance that a given entry would be swapped out of and back into the same location (which is usually a good feature of a shuffling algorithm).
Then I'd open the original text file and the shuffled index file and I'd walk through the shuffle file sequentially reading offset/length pairs and using them to seek into the text file and copy to a new file. After each seek I'd do one sanity check --- it there should be a newline there, and as I was copying I'd do another, there should be no newlines between my offset and the end of my length. I'd abend with an error message if either sanity check failed, or if any seek failed (the original file was shortened while I was shuffling).
Finally I'd mv the new file back into place.
This algorithm assumes that you have files with variable length records delimited by newlines. It also assumes that you are not disk space constrained (that you have at least enough room to make one full copy of the file to be shuffled + enough for an index file. Oddly enough the index file could, in some degenerate circumstances be several times the size of the original file. (that happens if all of the lines in the old file were only zero or one characters long and that your offsets and lengths are 32 bits each.
Note that I chose to use a file for the index rather than RAM. If I'm guaranteed that the file will have a "reasonable" number of lines I can build that in memory --- thus simplifying the code somewhat. I chose the method that I describe so that you could as easily shuffle multi-gigabyte files as multi-megabyte.
The whole program could probably run in less than a 100K and work on any size file that was supported by your OS.
You could also look at the sources for the GNU 'sort' utility. I handles arbitrarily large inputs (using sequences of temp files which then merged together).

(?)If you open a file for reading, the only space it takes up is the read buffer, so if you read a line at a time, the memory usage depends on how you are shuffling.

If you wanted to reverse the file, you could jsut be writing the lines you read to another file.


Then you may like to read the source file from the tail first. I don't know how to do this in C, or C++, but it is possible in Java.

(!)There is a program called tac ("cat" backwards) which does exactly this. I'm sure it's written in C and the sources can be found at any good GNU or BSD software archive.

(?)You really need to say more about what you mean by <Shuffle>
David Stanaway

(!)I think the term is sufficiently unambiguous.
Shuffle: to resequence. to place a group of objects into some arbitrary or random order.
The problem at hand is a classic CS homework assignment. It has quite a bit to do with the variable length nature of the objects to be sorted. We can't do this with "in place" editing (arbitrary seeks and writes into the orginal file) because the record we're trying to move might overwrite two or more record fragments at its destination.
When you are editing a file (the whole thing being in memory) there are ways that the editor's buffer handling handles the issue --- look at the sources to 'vi' or some other smaller, simpler editor and find out how they "delete a line" in terms of their internal data structures. These don't work well for files since you might end up re-writing from the current offset to the end of the file for each replacement.
If the lines are of a fixed length it is much easier, we can skip the indexing step and we can, if we wish, shuffle the file "in place" --- without the copying. Naturally we'll still want to lock the file (or move it to someplace where other processes and programs won't be giving us concurrency fits).

Copyright © 1998, James T. Dennis
Published in Linux Gazette Issue 33 October 1998

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