WyRand::generate_range is off by one.

[StyMaar]

fn example(){
    use nanorand::{Rng, WyRand};

    let mut rng = WyRand::new();
    println!("Random number: {}", rng.generate_range(1..=3));
    println!("Random number: {}", rng.generate_range(1..=3));
    println!("Random number: {}", rng.generate_range(1..=3));
    println!("Random number: {}", rng.generate_range(1..=3));
    println!("Random number: {}", rng.generate_range(1..=3));
    println!("Random number: {}", rng.generate_range(1..=3));

}

prints:

Random number: 1
Random number: 2
Random number: 1
Random number: 1
Random number: 0
Random number: 2

[jklott]

I am not able to reproduce the off by one error you mention. What version are you on? Do any tests failed when running cargo test? Some of the tests in gen.rs should fail if a number goes outside of the range specified.

I tried to recreate using:

for _ in 0..1_000_000 {
        let x  = rng.generate_range(1..=3);
        if x == 0 {
            break
        }
    }

But a 0 never occured.

[StT191]

I encounter the same problem. It occurs only with unsigned integer types though.

use nanorand::{tls_rng, Rng};

assert_eq!(tls_rng().generate_range(1..2 as u32), 1); // -> succeeds
assert_eq!(tls_rng().generate_range(1..2 as i32), 1); // -> panicks

thread 'main' panicked at src/main.rs:4:1:
assertion `left == right` failed
  left: 0
 right: 1

I confirmed it happening on platforms x86_64-unknown-linux-gnu and wasm32-unknown-unknown.

I enabled these features in Cargo.toml:

nanorand = { version = "0.7", default-features = false, features = ["tls", "getrandom"] }
getrandom = { version = "*", features = ["js"] }

[ruuda]

I also observed this and similar snippets (where rng is a WyRand) hitting the unreachable case:

let value = match self.rng.generate_range(0..4) {
    0 => 0,
    1 => 1,
    2 => self.rng.generate_range(0..10),
    3 => self.rng.generate(),
    _ => unreachable!(),
};

[cyeganeh01248]

I have also noticed this issue on 0.7.0 but not 0.6.x.

However, I experience it with signed types only.

My PoC.

use nanorand::{Rng, WyRand};

fn main() {
    let mut test_seed = 0;
    let mi = 50;
    let ma = 100;
    'outer: loop {
        let mut rng = WyRand::new_seed(test_seed);
        for j in 0..128 {
            let v = rng.generate_range(mi..ma);
            if v < mi || v >= ma {
                println!("{} {} {}", test_seed, j, v);
                // break 'outer;
            }
        }
        test_seed += 1;
    }
}