Handwritten

Systematic test-fix

A discipline for fixing a function whose behaviour is wrong, when you know what it should do. Optimises for the smallest correct change, not a rewrite.

When to use

A single function returns wrong results for some inputs and you have concrete expected input/output pairs (from a description or a failing check).

Steps

1. Restate the contract. Write down the expected output for 2–3 inputs, including the boundaries and the empty/degenerate case. These are your oracle.
2. Reproduce. Trace the current code by hand on the input that looks most likely to break (a boundary, an empty input, a negative number). Confirm the wrong output before changing anything — do not fix by guessing.
3. Localize. Find the single expression responsible: an off-by-one in a slice or index, a > that should be >=, an accumulator initialised to 0 instead of the first element or negative infinity, a wrong base case.
4. Minimal fix. Change only that expression. Keep the signature. Resist refactoring unrelated code in the same edit.
5. Re-verify against the oracle from step 1, especially the boundary and the degenerate case, since those are where these bugs hide. If any still fails, return to step 3 — the localization was wrong, not the fix.

Common shapes

• Slice/index endpoints (len(x) - n vs len(x) - n + 1).
• Boundary comparisons (inclusive vs exclusive).
• Accumulator seeds (0 breaks all-negative inputs; seed with the first element or float("-inf")).
• Missing empty-input handling.