you battle combinatorial explosion by making a risk analysis on the code. you split your code in pieces and unit test those. with unit-tests usually you can ge

you battle combinatorial explosion by making a risk analysis on the code. you split your code in pieces and unit test those. with unit-tests usually you can get full line + branch coverage, easily and quickly. then you go one level up with the integration tests which these you test highly critical paths where the unittests work together. and finally a black-box e2e test to make sure the pieces work regardless of the implementation details.