random_range seems broken

[michalmoc]

Summary

It seems that random_range() always returns its lower bound. I've checked it with debugger, and it seems that value returned from Rng is for unknown reason replaced with 0, and then forced into range.

Rand 0.9.0, Rust 1.84.1, Windows 11.

Code sample

#[test]
fn gen() {
    let mut rng = StepRng::new(1, 1);
    let res = [
        rng.random_range(1..=6),
        rng.random_range(1..=6),
        rng.random_range(1..=6),
    ];
    assert_eq!(res, [1, 2, 3]);
}

results in

assertion `left == right` failed
  left: [1, 1, 1]
 right: [1, 2, 3]

[newpavlov]

The problem here is StepRng, not random_range. Try the same code with a proper RNG, e.g. SmallRng seeded with a fixed seed.

[michalmoc]

The problem here is StepRng, not random_range. Try the same code with a proper RNG, e.g. SmallRng seeded with a fixed seed.

Hmm. Indeed it works with another rng, but shouldn't it work with StepRng too? I'm doing some testing and I need predictable randomness.

[newpavlov]

No, StepRng is an extremely bad PRNG which can cause effects like in your snippet above. Use SmallRng or, better, a concrete PRNG (e.g. from rand_pcg).

[dhardy]

predictable randomness

StepRng does not deliver randomness.

If by predictable you mean repeatable, then see reproducibility.