## digitalmars.D - Stable Partition3 Redux

- Xinok (41/41) Feb 01 2016 I'm not sure what the convention is for resurrecting old threads

I'm not sure what the convention is for resurrecting old threads but I felt it would be best to start fresh. For reference, the older thread is here: http://forum.dlang.org/thread/pyduqwmskkkoicsxifzb forum.dlang.org tl;dr - I studied a paper with the goal of implementing a stable 3-way partitioning algorithm which runs in linear time without allocating any memory on the heap. I spent over a month studying that paper and only managed to understand about 80% of the content. However, that was enough for me to derive a partitioning algorithm which runs in O(n) time using O(log n) space. The good news is that the O(log n) space can easily be allocated on the stack so this achieves the original goal of avoiding memory allocations. The bad news is that, despite running in linear time, it comes at the cost of a huge constant factor which makes it too slow for any practical purposes. It's a complex algorithm which I never fully implemented but preliminary benchmarks were discouraging. With that said, I've been sitting on this for several months and Phobos is still lacking a stable partition3 implementation so it's about time I did something about that. I implemented a few "potential candidates" with various pros and cons, two of which can accept a variable-length buffer of a specific type. DPaste isn't the best platform for running benchmarks so I suggest compiling and running on your own machine. http://dpaste.dzfl.pl/2a22af60baf9 Notes: * "Assignments" requires the range have assignable elements; "Swaps" and "Insertions" works on both swappable and assignable elements. * "Assignments Large Buffer" is how you would classically implement a stable partitioning algorithm using O(n) space. Clearly, it's by far the fastest. * "Swaps Large Buffer" vigorously thrashes the cache so it gets much slower on larger inputs. * The recursive code is identical in all three implementations; all that changes is the method for partitioning small sublists. So it's possible to merge all these techniques into a single function or even make them external functions separate from the recursive code. Any feedback is appreciated. I'll make a pull request once we have consensus on what is the best solution.

Feb 01 2016