Methodology

Mizukagami is a hindsight simulation site focused on buying on a past JST date and holding until now. The UI may be playful, but price handling, FX attribution, rankings, and share outputs are frozen on the server side so results do not drift later.

1. What is calculated

The product is limited to "if you had bought on that past JST date and held until now."
It does not provide forward-looking investment advice or public result-specific pages.
"What if you had never sold" remains share-copy framing only, not a separate calculator mode.

2. Price sources and JST dates

Historical prices are handled through Pyth Benchmarks-oriented datasets and daily archives.
Date boundaries are treated in JST, and the current JST date is excluded as not yet final.
Pre-listing dates are blocked, with controlled provider-layer overrides only where needed.

3. Current-price cache

Current prices are read from a server-side cache refreshed every 60 seconds.
Production is expected to use Redis or DB-backed cache so page refreshes do not hammer live providers.
Even without a fresh cron tick, stale checks and refresh behavior remain under server-side control.

4. JPY display and FX attribution

JPY display is derived consistently from asset USD prices plus USD/JPY data.
The detail section separates crypto movement from FX-driven impact.
The English mode uses USD as the display currency and omits the FX impact section.

5. Snapshots, sharing, and rankings

Rankings and share cards are based on persisted simulation snapshots.
Share images and X copy are generated from stored snapshots, not live recomputation.
That keeps shared and ranked values frozen even after current-price caches refresh later.

6. Limits and safeguards

Ranking eligibility is limited to initial investments at or below JPY 50,000,000, one record per person per board, with one-minute rate limiting.
Identity is based on browser_id plus IP hash, and display names pass validation.
Future outbound posting flows can expand, but source-of-truth calculations, rankings, and share values remain server-controlled.