I’m planning to write an interactive C++ geometry processing plug-in that will frequently sort large amounts of data. Although preliminary indications are that the sort will take only a second or two, I’d prefer to show progress during that time – i.e I’d like to update a progress indicator a few times per second. This would be preferable to turning on a wait cursor and leaving the user with a program that freezes for an indeterminate length of time (even if that’s just a few seconds).
If I were to use something like std::sort, I could use the comparison function to update the progress indicator every now and then, but I’d have no idea of ‘percentage complete’. I could also break the sort down into sub-sorts, updating progress between sub-sorts, and then merging. My best bet may be to write own sort method, although I don’t know how much effort it would require to get performance as good as std::sort (and ensure correctness). In any case, that sort method would occasionally send a "percentage complete" to a callback method.
I was wondering whether other people have encountered and solved this problem – I’m hoping that maybe there’s a sort method in a standard library that does what I want, or some other technique that I haven’t thought of.
Update: Thanks for the great answers so far. There have been a few very good suggestions, and I’m going to hold off on choosing the accepted answer until I’ve had a chance to test out the ideas in my upcoming project.
Update 2: I completed my project, and this turned out to be a non-issue (at least for the customer. Since they will be selling the software, they may still get feedback from their customers that will change their minds about this). Choosing an accepted answer was difficult because there were many good answers, but in the end the one I chose pointed to a wiki article on Merge Sort that had a very evocative animation. So that’s the first strategy I would have pursued had I needed to go ahead with this).
Since std::sort is template based, the source should be available in a header. You can make a copy of it and insert your progress callback. The big problem will be predicting how close you are to completion – most sort functions will be based on Quicksort, which doesn’t always do the same number of comparisons.
Writing your own Merge sort would be a possibility; the algorithm is easy and the number of steps are well defined.