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impl SeedableRng for StepRng #1496

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impl SeedableRng for StepRng #1496

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impl SeedableRng for StepRng
benjamin-lieser Sep 23, 2024
42c30a6
changelog
benjamin-lieser Sep 23, 2024
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1 change: 1 addition & 0 deletions rand_core/CHANGELOG.md
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Expand Up @@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
## [Unreleased]
- Bump the MSRV to 1.61.0
- The `serde1` feature has been renamed `serde` (#1477)
- Implement `SeedableRng` for `StepRng`

## [0.9.0-alpha.1] - 2024-03-18

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15 changes: 14 additions & 1 deletion src/rngs/mock.rs
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Expand Up @@ -8,10 +8,11 @@

//! Mock random number generator

use rand_core::{impls, RngCore};
use rand_core::{impls, RngCore, SeedableRng};

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};
use zerocopy::transmute;

/// A mock generator yielding very predictable output
///
Expand Down Expand Up @@ -75,6 +76,18 @@ impl RngCore for StepRng {
}
}

impl SeedableRng for StepRng {
type Seed = [u8; 16];

fn from_seed(seed: Self::Seed) -> Self {
let seed_u64: [u64; 2] = transmute!(seed);
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Concern: we should make results portable where (easily enough) possible. transmute! presumably does not alter the value, so we would need to use u64::from_le (or u64::from_le_bytes with as_chunks) here. Note that rand_core::impls::fill_bytes_via_next uses to_le_bytes so this would cancel out with a word-based-generator on a BE platform.

StepRng {
v: seed_u64[0],
a: seed_u64[1],
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Concern: if this were to randomly sample a 0, the output would be constant; if it randomly samples an even number, half the output values will be unreachable. I'm not entirely sure what behaviour people would expect here but suggest we do seed_u64[1] | 1.

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@benjamin-lieser benjamin-lieser Sep 23, 2024 •
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This makes me think this impl is a bad idea. The usecases for StepRng are probably the easy predictability and SeedableRng goes against this

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It may be that my concern is entirely invalid given that this is a mock RNG, but you may also be right.

}
}
}

rand_core::impl_try_rng_from_rng_core!(StepRng);

#[cfg(test)]
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