A stable, linearithmic sort in constant space written in Rust

A stable, linearithmic sort in constant space written in Rust. Uses the method described in "Fast Stable Merging And Sorting In Constant Extra Space" by Bing-chao Huang and Michael A. Langston. Heavily inspired by (and named in honor of) Andrey Astrelin's grailsort. Documented in more detail in this blog post.

TODO

This implementation needs some work before it's complete. Here are a few possible improvements (most of them suggested by the folks behind the HolyGrailSort project).

Optimizations:

• Use MᴇʀɢᴇBᴜғRɪɢʜᴛ (working from right to left) for the last step in Phase 1.
• IMPORTANT During Phase 2, alternate between a mirrored version of the algorithm and the current version to avoid needing to rotate the internal buffer back to the start of the array at each step.
  • Another alternative (Kotasort?) is to swap the internal buffer into place, then fix up the swapped block's position during BʟᴏᴄᴋRᴏʟʟ.
• Faster sorting algorithm to prepare the movement imitation buffer:
  • Shell sort
  • Quick sort
  • Linear-time "deinterleave"
    • Relies on the fact that BʟᴏᴄᴋRᴏʟʟ does not change the relative order of A and B blocks; it merely interleves them.
    • This interleaving can be undone in linear time since we know the median element in the movement imitation buffer.
    • Not valid for Phase 2 with a the inline block-taggging scheme, since the movement imitation buffer is repurposed as the internal buffer for MᴇʀɢᴇBᴜғ. Would work for Phase 3, though.
• Adaptive merging.
• Faster MᴇʀɢᴇBᴜғ implementation (especially for Phase 2).
  • Current version is a "tape" merge.
  • Could also use a "simple binary" merge: Do an exponential search at each step and swap in batches.
  • Hwang-Lin, a happy medium between the two.
• Make use small allocations (e.g., big enough for the auxiliary buffer but less than N/2).

Features:

• Visualize with ArrayV (or something else that interoperates with Rust).
• Parametric benchmarking:
  • Element size
  • Comparison cost
  • Number of distinct elements