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digitalmars.D - Stable Partition3 Redux

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.


* "Assignments" requires the range have assignable elements; 
"Swaps" and "Insertions" works on both swappable and assignable 

* "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