WikiSort

WikiSort is an implementation of “block merge sort”, which is a stable merge sort based on the work described in “Ratio based stable in-place merging”, by Pok-Son Kim and Arne Kutzner [PDF]. It’s generally as fast as a standard merge sort while using O(1) memory, and can be modified to use additional memory optionally provided to it which can further improve its speed.

C, C++, and Java versions are currently available, and you have permission from me and the authors of the paper (Dr. Kim and Dr. Kutzner) to do whatever you want with this code.

Related: Check out the GrailSort project for a similar algorithm based on a paper by Huang and Langston, or the Rewritten Grailsort project which continues its work.

If you want to learn how it works, check out the documentation:

  • Chapter 1: Tools

  • Chapter 2: Merging

  • Chapter 3: In-Place

  • Chapter 4: Faster!

Or you can check out the Wikipedia page for block merge sort. WikiSort was actually (poorly) named out of the hope that you’ll update and improve its various components almost like a Wikipedia article, since this is still very much an open area of research that could use your expertise!

WikiSort vs. std::stable_sort()
^{(clang++ version 3.2, sorting 0 to 1.5 million items)}

Using a 512-item fixed-size cache for O(1) memory:

Test             Fast comparisons   Slow comparisons   150,000,000 items    0-32 items
Random               6% faster        95% as fast         35% faster        45% faster
RandomFew            5% faster        16% faster          20% faster        45% faster
MostlyDescending    97% as fast       13% faster          99% as fast       53% faster
MostlyAscending    149% faster       117% faster         286% faster        47% faster
Ascending         1280% faster       518% faster        1101% faster       242% faster
Descending          23% faster       121% faster          12% faster       164% faster
Equal             1202% faster       418% faster        1031% faster       227% faster
Jittered           526% faster       298% faster         733% faster        70% faster
MostlyEqual         15% faster        57% faster          10% faster        42% faster
Append             153% faster        90% faster         348% faster       112% faster

Using a dynamically allocated half-size cache:

Test             Fast comparisons   Slow comparisons
Random              11% faster         3% faster
RandomFew           10% faster         5% faster
MostlyDescending    19% faster        26% faster
MostlyAscending     98% faster        79% faster
Ascending          861% faster       463% faster
Descending          39% faster       142% faster
Equal              837% faster       460% faster
Jittered           326% faster       243% faster
MostlyEqual         15% faster         2% faster
Append             159% faster        94% faster