Checkpoint — Refactor Under Guard
Rebuild a smelly pricing method as clean code that provably behaves identically — then extend it, with Legacy standing in for the frozen test suite.
The gate (mini-build)
Legacy.Price (provided, do not modify) is the behavior contract:
| tier | qty ≥ 10 | qty < 10 | |---|---|---| | gold | 20% off | 10% off | | silver | 10% off | 5% off | | anything else | no discount | no discount |
Any tier with qty > 100 gains an extra 2% off, stacked.
Implement Solution.Price(qty, unit, tier) with identical behavior, plus
one new requirement: tier "vip" gets a flat 25% off (the >100 bulk
extra still stacks on top). Structure it the clean way — named constants or a
switch expression over the tier, one rule per line, no magic numbers.
Your safety net is the legacy code itself: the tests compare your Solution
against Legacy across a matrix, then pin the boundaries and the new vip
behavior exactly.