Built this on top of a side project (quizzy.earth).

The interesting engineering problem was the best-thirds seating logic – FIFA pre-publishes a 495-row lookup table (Annexe C) covering every possible combination of third-place qualifiers. AI kept hallucinating bracket rules so I ended up reading the actual regulations PDF. Each of the 495 scenarios has up to 24,000 valid bracket arrangements; FIFA pre-commits to one per scenario so there's no ambiguity on the night.

For a half-finished bracket I fall back to bipartite matching (seat the most-constrained team first). The moment all 8 thirds are chosen it switches to the official lookup.

No sign-up. Anonymous picks. Curious what HN thinks the brackets looks like.

I ended up going deeper on the Annexe C rabbit hole than I expected — wrote it up here:

https://dev.to/apostopher/eight-kids-eight-chairs-one-rule-e...

Explaining it to my 8-year-old with dining chairs turned out to be the only way I actually understood it myself. Feynman was right.