“Elastique” time stretch algorithm by zplane (0) seems to be a common choice by plug-in companies. Their C++ library is pretty straightforward to use, and most importantly it sounds good.
Generally speaking, phase vocoder (1) algorithms are what you’d want for developing your own pitch shifting algorithm.
Generally speaking, phase vocoder (1) algorithms are what you’d want for developing your own pitch shifting algorithm.
[0]https://licensing.zplane.de/
[1]https://cmtext.indiana.edu/synthesis/chapter4_pv.php