# Wheel Spinner with Weights

> Spin a wheel with weighted segments — exact odds, provable result. Spin a decision wheel where each segment can have its own weight (3 × pizza, 1 × sushi). The odds are exactly weight ÷ total, the organiser can publish a SHA-256 fingerprint before the spin, and the result replays identically here.

Part of usefulweb.app (https://usefulweb.app/). Part of [Fieldday](https://fieldday.usefulweb.app/) (ownerApp: fieldday). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://fielddaywheelspinner.usefulweb.app/ (interactive tool at https://fielddaywheelspinner.usefulweb.app/tool)
- JSON API: POST https://fielddaywheelspinner.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://fielddaywheelspinner.usefulweb.app/api/v1/tool)
- OpenAPI: https://fielddaywheelspinner.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://fielddaywheelspinner.usefulweb.app/mcp - tool `fieldday_wheel_spinner`

## Inputs
- segments (table, required): Segments
- spins (integer): Spins
- removeWinners (boolean): Remove each winner before the next spin
- secret (text): Secret (revealed after the draw) - The organiser’s random secret. Its fingerprint is shown before the draw; empty = derived from the entries (predictable, only for demos).
- audienceWord (text): Audience word (optional) - A word someone calls out after the fingerprint is shown. It changes the draw, so the organiser cannot steer it.

## Method
- Each segment covers weight ÷ total weight of the circle, clockwise from the pointer at 12 o’clock. A uniform number u in [0, 1) picks the point 360° × u that stops under the pointer, so a segment wins with probability exactly weight ÷ total (up to the 32-bit resolution of the generator).
- The wheel turns 4–6 whole turns plus the offset that brings that point under the pointer; the app animates exactly this rotation.
- Fairness proof (commit-reveal): fingerprint = SHA-256 of “fieldday-party-v1|wheel|<segments and settings as sorted JSON>|<secret>”, shown before the spin. The random numbers come from SHA-256 of the secret plus the audience word chosen afterwards; the generator is sfc32.
- With “remove each winner”, later spins use the remaining segments with their weights re-normalised.

## Limits
- For games and everyday decisions. Without a secret the spin is predictable. Not for regulated gambling or lotteries.