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# Loop — Per-Iteration Procedure
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Repeat this cycle up to `max_iterations` times, or until a stop condition in
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`stop-conditions.md` is hit.
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## 1. Select the next unit of behavior
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Within the current target module, pick one specific, nameable behavior that isn't
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covered: a function, a branch (if/else path), an error case, or a boundary condition
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(empty input, zero, max value, duplicate). Prefer, in order:
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1. Untested error paths and edge cases in code that *is* partially tested (these hide
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   the most real bugs).
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2. Fully untested public functions/methods.
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3. Untested branches within partially-tested functions.
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Avoid picking multiple unrelated behaviors in one iteration — one behavior per cycle
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keeps the red/green signal clean.
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## 2. Write the failing test (red)
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- Name the test after the behavior, not the implementation:
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  `"throws when quantity is negative"`, not `"test2"`.
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- Assert on outcome (return value, thrown error, emitted event, persisted state), not
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  on internal implementation details that would make the test brittle to refactors.
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- Run just this test (or the smallest relevant subset) and confirm it **fails**. If it
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  passes immediately, the test isn't exercising new behavior — revise it before
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  proceeding; a test that never fails is worse than no test.
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- Confirm the failure reason matches expectations (e.g. "expected error, got
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  undefined" — not a typo or import error unrelated to the behavior under test).
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## 3. Implement (green)
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- Write the minimum code needed to make the test pass. Resist adding unrelated
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  improvements in the same step — that's a separate change.
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- Run the same test again and confirm it now passes.
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## 4. Run the full suite
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- Run the complete test suite (not just the new test) to catch regressions.
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- If something else broke, fix it before moving on — do not proceed to the next
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  iteration with a red suite.
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## 5. Record the delta
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- Re-run the coverage tool (or estimate if per-iteration coverage runs are too slow —
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  in that case, run coverage every N iterations and note the cadence).
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- Note: test added, coverage before → after for the targeted file, iteration count.
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## 6. Decide: continue or stop
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Check `stop-conditions.md`. If none apply and `max_iterations` isn't reached, return to
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step 1 and pick the next behavior.
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