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Deterministic random number generator specification
The deterministic random number generator used in pyRCV2 is based on an algorithm by Ronald L Rivest.
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 state 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.
In revision 86695aa and older, a slightly different formula was in use to avoid modulo bias – see the documentation of previous revisions for details. It is theoretically possible, though overwhelmingly unlikely, that this may result in different outcomes.