Module portable_random 

Sampling routines whose results depend only on the bytes an RNG produces, never on which version of rand is in the dependency graph.

rand’s own shuffle, choose, choose_multiple and choose_multiple_weighted are free to change between releases, and they have: 0.9 replaced the range sampler and the weighted-selection wrapper, so the same seed produces a different permutation. That is fine for a test fixture and unacceptable for a permutation every validator has to agree on.

The routines here reproduce rand 0.8’s algorithms and are fixed by tests against captured vectors, so callers that need a permutation to stay the same forever can bump rand freely.

Two rules for callers:

• Seed [rand::rngs::ChaCha12Rng], not StdRng. StdRng is documented as non-portable — rand reserves the right to swap the algorithm under it — whereas ChaCha12Rng names the generator and is what StdRng currently happens to be.
• Draw floats through f64_unit rather than rand’s random::<f64>(), which is another conversion rand owns and could change.

Functions

choose

Returns a uniformly drawn element, or None if slice is empty.

f64_unit

Draws a f64 uniformly from [0, 1), using the top 53 bits of one 64-bit word.

index

Draws an index uniformly from 0..ubound.

sample_indices

Draws amount distinct indices from 0..length, in random order.

sample_weighted

Draws amount distinct indices from 0..length with probability proportional to weight, in descending order of the keys drawn.

shuffle

Permutes slice in place (Fisher-Yates, from the back).