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Oshizan — blind-box odds and fan-trip budgeting for iOS
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The probability planner — hit probability updates live as you adjust lineup, price, and budget
Inspiration
Inspiration
A friend of mine often travels to fan events and told me they track all their fan-activity spending in a spreadsheet — including guessing how much they would need to spend on blind-box merch to get the one item they wanted. That guess is a probability problem, and spreadsheets are bad at it. Oshizan does that math properly.
What it does
Enter the lineup size, the number of items you want, the unit price, and your budget. Oshizan instantly shows:
- Hit probability — the chance of pulling the one you want, updated live as you move the budget slider
- Complete-set odds and expected duplicates
- Trip budgeting — plan travel, tickets, hotels, and merch for each event, plus a yearly spending overview
Everything runs on-device and works offline.
Monetization
The core calculator is free. A one-time purchase (¥1,500, no subscription) unlocks exact quantity/cost targets ("how many boxes for a 90% chance"), expected complete-set cost, plan saving & comparison, and watermark-free share cards. Purchases are handled with the RevenueCat SDK.
How I built it
Flutter (iOS), RevenueCat for the one-time unlock, local-first storage, 200+ automated tests. Solo developer.
Challenges
Shipping fast without shipping sloppy: the probability engine is covered by automated tests, and the App Store metadata went through real keyword research before launch. Getting a one-time (non-subscription) unlock flowing smoothly through StoreKit and RevenueCat was its own learning curve.
Timeline
Built entirely inside the Shipaton window: started August 1, v1.0 live on the App Store August 14, v1.0.1 (Japanese localization fix + branded launch screen + ASO) August 18.
What's next
Reacting to early user feedback on the probability planner, and expanding the trip-budget side based on how real fans plan their events.
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