Arbitrary Precision Calculator a Qt-based user interface to the Calc library “Calc - C-style arbitrary precision calculator” (2.12.2.2 with fixes up to 2.12.4.3) by Landon Curt Noll et. al. http://isthe.com/chongo/tech/comp/calc/
and also https://sourceforge.net/projects/calc/
This project owes an enormous debt of gratitude to Landon.