I run case-sim.com, a free CS2 case opening simulator. As of this week the global counter ticked past 123 million openings. That's a weirdly humbling number — it's also enough rolls that any subtle bug in my probability code would have been screaming at me for years. I want to share what I actually learned building this thing, because most "how to model case openings" tutorials I've seen are wrong in at least one quiet, important way. If you're building anything with weighted RNG — loot tables, gacha, slot mechanics, A/B traffic splitting — some of this will probably save you a bug. This is going to be code-heavy. Sorry/not sorry. The drop rate model (and the off-by-one I shipped for two weeks) Valve disclosed CS2's case tiers back in 2017 and they haven't changed: Tier Color Drop chance Mil-Spec Blue 79.92% Restricted Purple 15.98% Classified Pink 3.20% Covert Red 0.64% Knife/Glove Gold 0.26% Add those up: exactly 100%.…