# Random Generator

> Random numbers, dice, coins, list picks, shuffles, fair teams, dates, strings and UUIDs. Generates random whole or decimal numbers (optionally unique), dice rolls such as 3d6+2, coin flips, picks from a list, shuffles, balanced teams (by optional skill), random dates, strings and UUID v4/v7. Uses cryptographic randomness by default; with a seed the output is reproducible so others can verify a draw.

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

## Inputs
- mode (select): Generate
- count (integer): How many - Numbers, rolls, flips, picks, dates, strings or UUIDs.
- min (number): Minimum
- max (number): Maximum
- unique (boolean): No repeats (numbers and picks)
- decimals (integer): Decimal places
- dice (text): Dice notation - NdS±M, e.g. 2d20, 4d6+2, d100.
- items (list): List (one per line; "name, skill" for teams)
- teams (integer): Number of teams
- dateFrom (date): Earliest date
- dateTo (date): Latest date
- length (integer): String length
- charset (select): Characters
- seed (text): Seed (optional, for reproducible results) - Empty = cryptographic randomness, different every time. With a seed, the same settings always give the same result — never use seeded output for passwords or keys.
- uuidTime (datetime): UUID v7 timestamp (optional) - Default: now (UTC). Set it together with a seed for reproducible v7 UUIDs.

## Method
- Without a seed, bytes come from the server's cryptographically secure generator (Node.js crypto.randomBytes); nothing is stored. With a seed, bytes come from SHA-256 in counter mode over the seed, so anyone can reproduce the draw with the same seed and settings.
- Whole numbers in a range use rejection sampling (no modulo bias). Decimals use 53 random bits scaled into [min, max) and rounded to the chosen places. Unique numbers use a partial Fisher–Yates shuffle or Floyd's algorithm for huge ranges.
- Shuffles and picks use Fisher–Yates. Teams: without skills, a random round-robin; with skills, a snake draft over a random order followed by swaps that shrink the gap between the strongest and weakest team.
- UUID v4 has 122 random bits; UUID v7 has a 48-bit Unix millisecond timestamp and 74 random bits (RFC 9562).

## Limits
- Seeded output is predictable by design. For passwords or keys, leave the seed empty (or use a dedicated password generator on your own device).