12 lines
876 B
Markdown
12 lines
876 B
Markdown
# Deterministic random number generator specification
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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).
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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.
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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.
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In order to avoid modulo bias, if *k* ≥ ⌊*M*/*n*⌋ × *n* (where *M* = 2^256), *k* is discarded and the algorithm is repeated.
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Otherwise, the result is *k* modulo *n*.
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