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> There are two possible strategies:

> 1. Top-down, bisecting each time, with recursion.
> 2. Bottom-up, doubling each time, with iteration.

> (chunk size 12 for Strategy 1,16 for Strategy 2 — but there
> is no reason why it must be a power of 2, actually).

> There is still more tuning possible, e.g. size-dependent minimum
> chunk length a la Timsort, or implementing the supervising routine
> in C too.

It makes a difference to Strategy 2, but not to Strategy 1. It;s not
hard to understand why: Strategy 1 merges almost equal-sized
chunks all the time, Strategy 2 might have small pieces left over.

chunk_size = n
while chunk_size>too_big do chunk_size=math.ceil(chunk_size/2) end

This way, the number of chunks in Strategy 2 is always a power of 2.