Record One Nested Decision, Then Extract a Single Predicate

A nested pricing function consolidates multiple rules, including region-specific policies, bulk surcharges, and coupon stacking into a single block of code. A colleague suggests refactoring this module ahead of an upcoming tax change. The initial attempt splits the code into four helper functions, renames exceptions, and modifies a surcharge for carts containing 12 items.

However, during review, more time is spent reconstructing previous behavior than evaluating the actual extraction needed. The issue lies in treating the failure as a process problem rather than focusing on the clean code aspect. The goal is to identify one nested decision on a hot path, record it before any changes, and only allow a single predicate extraction afterwards.

The failure patterns observed repeatedly in reviews include silent branch collapse, exception reshaping, and rounding drift. To address these issues, a record should be created using a triple consisting of canonical input, outcome tuple, and a stable hash of the pair. This hash acts as a review signal, and any mismatch should immediately stop the extraction process.

If two consecutive runs produce different results, the path is considered too noisy to modify. The recorder should be a compact tool focusing on a single hot path, ignoring style changes and mutations in the module. By freezing the nested decision instead of the entire module, the review process becomes more manageable and focused on preserving the desired outcomes.