How can I create a random array between a range of two other arrays? - random

Question

I need to generate an array of random 20 bytes between a given range of arrays. Since arrays are comparable in Rust, this works:
let low = [0u8; 20];
let high = [2u8; 20];
assert_eq!(true, low < high);
assert_eq!(false, low > high);
assert_eq!(true, low == [0u8; 20]);
For these bounds:
let low: [u8; 20] = [98, 0, 1, 0, 2, 6, 99, 3, 0, 5, 23, 3, 5, 6, 11, 8, 0, 2, 0, 17];
let high: [u8; 20] = [99, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];
These would be a valid result:
[98, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[99, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
These are not:
[98, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[99, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2]
I want to do something like:
use rand::prelude::*;
fn main() {
let low = [0u8; 20];
let high = [2u8; 20];
let value = rand::thread_rng().gen_range(low, high);
println!("{:?}", value);
}
but I get following error:
error[E0277]: the trait bound `[u8; 20]: rand::distributions::uniform::SampleUniform` is not satisfied
--> src\main.rs:6:36
|
6 | let value = rand::thread_rng().gen_range(low, high);
| ^^^^^^^^^ the trait `rand::distributions::uniform::SampleUniform` is not implemented for `[u8; 20]`
I tried implementing SampleUniform and UniformSampler without much success. Is there a simple way to implement this?

Answer 1

If you want to treat the byte arrays as big integers, use the
num-bigint crate with the rand feature enabled:
use bigint::{ToBigInt, RandBigInt};
let low = -10000.to_bigint().unwrap();
let high = 10000.to_bigint().unwrap();
let b = rng.gen_bigint_range(&low, &high);
You could also use unsigned integers instead of signed. There are methods to convert to and from big endian byte arrays:
from_bytes_be
to_bytes_be
See also:
How do I generate a random num::BigUint?