# Plinko Name Picker

> Drop a ball through pegs to pick a name — fair, replayable and provable. Pick one or more names with a physics plinko board. The organiser publishes a SHA-256 fingerprint of a secret before the drop, the audience adds a word, and afterwards anyone can replay the exact drop here and check that every name had the same chance.

Part of usefulweb.site (https://usefulweb.site/). Part of [Fieldday](https://fieldday.usefulweb.site/) (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://fielddayplinkopicker.usefulweb.site/ (interactive tool at https://fielddayplinkopicker.usefulweb.site/tool)
- JSON API: POST https://fielddayplinkopicker.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://fielddayplinkopicker.usefulweb.site/api/v1/tool)
- OpenAPI: https://fielddayplinkopicker.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://fielddayplinkopicker.usefulweb.site/mcp - tool `fieldday_plinko_picker`

## Inputs
- names (list, required): Names (one per line)
- picks (integer): How many to pick (one ball each, winners leave the board)
- rows (integer): Peg rows
- 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.
- checkDrops (integer): Fairness check: extra test drops (0 = off) - Counts where test balls land and which names they pick, to show the board’s bias and that names are still equally likely.

## Method
- Fairness proof (commit-reveal): fingerprint = SHA-256 of “fieldday-party-v1|plinko|<entries and settings as sorted JSON>|<secret>”. It is shown before the drop, so the secret and the list cannot change unnoticed. The random numbers come from SHA-256 of the secret plus the audience word, which is chosen after the fingerprint is public.
- Physics: a ball (radius 9.6) falls under gravity (900 units/s², step 1/240 s) through rows of pegs 40 units apart, bouncing off pegs and walls with restitution 0.45 and a small random spin on every peg hit. It starts above the centre with a random offset. Only +, −, ×, ÷ and square roots are used, so every JavaScript engine replays the identical path.
- Why names are equally likely: before each drop the names are shuffled onto the slots (Fisher–Yates; one slot per name, small lists repeat to at least 8 slots so each name holds equally many). Real boards favour some slots, but because every name is equally likely to sit on every slot, each name wins with probability exactly 1 / number of names (up to the 32-bit resolution of the generator).
- Several picks: the winner leaves, a new board is laid out for the remaining names and a new ball drops. The optional fairness check repeats the drop with test seeds and reports slot counts, name counts and a chi-square statistic against equal chances.

## Limits
- For games, chores and small prizes. Without a secret the draw is predictable. For legally regulated lotteries follow the rules that apply to you.