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