1 · The explosion
3 conditions
At k = 1 and k = 2 all three agree, which is exactly why the shortcut looks correct. Push past 2 and watch path coverage detonate while MC/DC barely moves.
Engineering Method
Coverage, as a game
Every if multiplies the ways code can behave. So how many tests do youactually need? Everything below is built from one thing: a unit, a single branch of a decision. It is a row in a table, a test in the suite, and a pip on this page, all the same object. It wears one of four states, everywhere.
- WonVerified at its level. Counts toward 1.
- OpenOwed a test. Holds the score below 1.
- EscalatedLives above unit: architecture we do not own.
- ParkedUnreachable today. Kept, never deleted.
2 · Find the tests
Required tests found: 0 / 3 · MC/DC complete, this method is a 1
if ( && )
// then: happy path
else
// else: sad pathToggle c1 and c2 to visit every reachable row.
| c1 | c2 | Outcome | Role |
|---|---|---|---|
| T | T | then | Happy path |
| F | T | else | Flip c1 |
| T | F | else | Flip c2 |
| F | F | else | Redundant |
3 · Nothing is lost
A branch a unit test cannot reach does not disappear. It bubbles up to the level thatcan verify it, and stays documented there.
ManualLast resort. A documented human step. Still tracked.
SmokeObserved against the live deployment.
E2EThrough the running app, when a unit cannot isolate it.
UnitFast, isolated, preferred. Most branches live here.
The decisions behind it
- Which criterion?MC/DC, not branches×2 (breaks at 3 conditions) and not 2ᵏ path coverage (explodes).
- Prune unreachable rows?No. Every branch is a permanent row. A future change can make it live.
- A branch a unit cannot reach?It bubbles up the pyramid (E2E → smoke → manual). It is never lost.
- "Cannot be tested"?A finding, not an exemption: fix the missing seam, or document why we do not own it.