One of the most accessible ways to be involved in the development of the software is to use the online software interface. There is an AMPL/Pyomo version of APOPT or an APMonitor version of APOPT and BPOPT.
Two popular approaches for constraint handling are active-set and interior point methods. The APOPT solver uses an active-set method while the BPOPT solver uses an interior point method.
For mixed-integer problems, an NLP solver that can quickly solve from a nearby solution or eliminate infeasible points is critical for overall convergence. A focus area in APOPT solver development is in detection of sub-optimal or infeasible solutions.
The best solvers are designed to exploit the abilities of multi-core computer architectures. With increasingly parallelized architectures, a solver must exploit these resources.
APOPT maintains a development environment for mixed integer nonlinear programming (MINLP) solvers in MATLAB. The solvers are written in both a higher level programming language (MATLAB) and as an efficient compiled version (C++/Fortran). The development platform aids the testing of exploratory algorithms and techniques.