I want to thread comments: when a comment quotes another comment it's probably a reply, so attach them together to make the conversation clearer. This means figuring out which comments quote which other comments. I tried a brute force way:

  for each comment A:
    for each later comment B:
      for each word X in A:
        for each word Y in B:
          do A and B match for N words starting at X and Y?

This is psuedocode for an O(n^2*m^2) solution. Not so good. I coded it up in javascript and while I think it's fast enough in Chrome, Firefox, and even my phone, it took IE8 nearly a minute. While I could probably run this on my server and do fine with some combination of caching and C, it seems inefficient. Is there a better algorithm? What is the general version of this problem called?

Update 2012-07-21: Several commenters suggested a better algorithm: to find all quotes of length N, build a dictionary from all sequences of N words to a list of comments in which they appeared. This is O(n*m) and running it it's much faster. I tested it in IE8, and it loaded quickly instead of freezing the browser. I thought briefly about doing something like this earlier, but wrote it off as using insane amounts of memory. After more people suggested it, I realized it only uses N times as much memory as just storing the comments.