refactor for testability; add tests #1419

All 118 tests pass in 14s

118 tests +5 118 ✔️ +5 14s ⏱️ -6s
  14 suites ±0     0 💤 ±0
    1 files ±0     0 ❌ ±0

Results for commit 41cbed1. ± Comparison against earlier commit 3f3ba89.