OpenTally/docs/rng.md

# Deterministic random number generator specification

The deterministic random number generator used in OpenTally is based on an algorithm by [Ronald L Rivest](https://people.csail.mit.edu/rivest/sampler.py).

The algorithm takes a *seed* value, which is an arbitrary character string. The algorithm has, in its internal state, a *counter*, whose value is initially 0.

In order to generate a value between 0 (inclusive) and *n* (exclusive), to the seed is appended a comma (",") followed by the value of the counter, and a SHA-256 hash *H* is computed of the resulting string encoded using UTF-8. The hash *H* is represented as an unsigned hexadecimal integer, *k*. The counter is incremented by 1.

In order to avoid modulo bias, if *k* ≥ ⌊*M*/*n*⌋ × *n* (where *M* = 2^256), *k* is discarded and the algorithm is repeated.

Otherwise, the result is *k* modulo *n*.
Implement --ties random 2021-06-13 03:15:15 +10:00			`# Deterministic random number generator specification`

			`The deterministic random number generator used in OpenTally is based on an algorithm by [Ronald L Rivest](https://people.csail.mit.edu/rivest/sampler.py).`

			`The algorithm takes a seed value, which is an arbitrary character string. The algorithm has, in its internal state, a counter, whose value is initially 0.`

Update documentation 2022-03-11 15:44:49 +11:00			`In order to generate a value between 0 (inclusive) and n (exclusive), to the seed is appended a comma (",") followed by the value of the counter, and a SHA-256 hash H is computed of the resulting string encoded using UTF-8. The hash H is represented as an unsigned hexadecimal integer, k. The counter is incremented by 1.`
Implement --ties random 2021-06-13 03:15:15 +10:00
			`In order to avoid modulo bias, if k ≥ ⌊M/n⌋ × n (where M = 2^256), k is discarded and the algorithm is repeated.`

			`Otherwise, the result is k modulo n.`