OpenTally/src/sharandom.rs

67 lines
1.8 KiB
Rust

/* OpenTally: Open-source election vote counting
* Copyright © 2021 Lee Yingtong Li (RunasSudo)
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
use ibig::UBig;
use sha2::{Digest, Sha256};
/// Deterministic random number generator using SHA256
#[derive(Clone)]
pub struct SHARandom<'r> {
seed: &'r str,
counter: usize,
}
impl<'r> SHARandom<'r> {
/// Return a new [SHARandom] with the given seed
pub fn new(seed: &'r str) -> Self {
Self {
seed: seed,
counter: 0,
}
}
/// Draw a random number *n*, such that 0 ≤ *n* < `max`
pub fn next(&mut self, max: usize) -> usize {
let mut hasher = Sha256::new();
hasher.update(format!("{},{}", self.seed, self.counter).as_bytes());
self.counter += 1;
let hash = hasher.finalize();
let hash = UBig::from_be_bytes(&hash);
if hash >= UBig::from(2_u32).pow(256) / UBig::from(max) * UBig::from(max) {
return self.next(max);
}
return (hash % UBig::from(max)).to_string().parse().unwrap();
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn sharandom1() {
let mut random = SHARandom::new(&"foobar");
assert_eq!(random.next(10), 0);
assert_eq!(random.next(42), 30);
assert_eq!(random.next(64), 13);
}
}