shell sort is using a very suboptimal gap sequence

[cq4]

"Divide by 2" is far from optimal

For practical and easy use I'd recommend Tokuda: divide by 2.25 (or multiply - depending where you start)
see https://en.wikipedia.org/wiki/Shellsort#Gap_sequences

[Myphz]

Thank you for your contribution! I appreciate it. This is an old project of mine, but I'll show it some love when I have time in the near future and I'll work on it!