All-In Price Comparison Engine

Why `Math.min` is wrong

Two listings: **$80** and **$95**. The $80 quotes pre-fee and charges 30% at checkout, so it is really **$104**. Ranking the raw numbers doesn't just misprice them — it puts them in the **wrong order**, in the direction that costs the buyer money. That is the bug this exists for, and it is the default behaviour of almost every price-comparison feature.

**Unknown fees mean unrankable, not zero.** A source that quotes pre-fee but publishes no fee model gets excluded with a stated reason. Treating it as all-in ranks it first and is wrong by exactly the fees — a confident, wrong winner. The comparison reports itself as *partial* so the UI can say "cheapest of the 3 we could price in full" rather than "cheapest". That distinction is the whole difference between informing someone and misleading them.

**Fees are not one percentage.** Percentage (often capped) + per-ticket + per-order + delivery, then tax on the total. A flat "add 25%" is wrong at both ends of the range, and a cap can flip which marketplace wins.

**Per-ticket and per-order are different numbers.** A big per-order fee makes a source lose at quantity 1 and win at quantity 5. The cheapest source genuinely depends on how many you're buying, so quantity is an input, not an afterthought.

**A listing you can't buy is not a price.** Split rules (sellers refusing to strand a single seat), minimum quantities, and insufficient inventory are excluded *before* ranking — not ranked and then failed at checkout. Notably, these exclusions do **not** mark the comparison partial: that listing genuinely can't be bought, so nothing cheaper is being hidden.

**Never convert currencies without an explicit, timestamped rate.** Missing or stale rate is a refusal, never a conversion at 1.0.

**A stale price is not a price.** A feed that goes quiet otherwise keeps winning the cheapest slot forever with a number that no longer exists.

Verified by mutation

Every rule had its guard removed one at a time and the suite was confirmed to fail — **11 of 11**, including the headline case (ranking raw prices across mixed fee semantics) and both directions of the partiality flag: claiming completeness when something was hidden, and over-warning when nothing was.

**35 tests passing · typecheck clean · 11/11 mutations caught.**

Shape

Pure functions over plain objects — no network, no database, no clock. Time is always passed in, so results are reproducible. Money is integer minor units throughout; floats do not belong in a checkout total. Built for ticket resale, but nothing in it is ticket-specific: it applies to any market where sources quote fees differently.

Delivery

Private repository invite within 24 hours. Single-product commercial license: use and modify in one shipped product; no redistribution or resale of the source.