qfall_math/integer_mod_q/z_q/
set.rs

1// Copyright 2026 Jan Niklas Siemer
2//
3// This file is part of qFALL-math.
4//
5// qFALL-math is free software: you can redistribute it and/or modify it under
6// the terms of the Mozilla Public License Version 2.0 as published by the
7// Mozilla Foundation. See <https://mozilla.org/en-US/MPL/2.0/>.
8
9//! Implementations to manipulate a [`Zq`] polynomial.
10
11use crate::integer_mod_q::{Modulus, Zq};
12
13impl Zq {
14    /// Changes the modulus of the given [`Zq`] value to the new modulus.
15    /// It takes the representation of `self` in [0, q) as the new
16    /// value and reduces it by the new modulus.
17    ///
18    /// Parameters:
19    /// - `modulus`: the new modulus of the [`Zq`] value
20    ///
21    /// # Examples
22    /// ```
23    /// use qfall_math::integer_mod_q::{Zq, Modulus};
24    /// use std::str::FromStr;
25    ///
26    /// let mut value = Zq::from_str("2 mod 3").unwrap();
27    /// value.change_modulus(2);
28    ///
29    /// assert_eq!("0 mod 2", value.to_string());
30    /// ```
31    ///
32    /// # Panics ...
33    /// - if `modulus` is smaller than `2`.
34    pub fn change_modulus(&mut self, modulus: impl Into<Modulus>) {
35        self.modulus = modulus.into();
36        self.value = self.get_representative_least_nonnegative_residue();
37        self.reduce();
38    }
39}
40
41#[cfg(test)]
42mod test_change_modulus {
43    use super::Zq;
44    use crate::integer_mod_q::Modulus;
45    use std::str::FromStr;
46
47    /// Ensures that the modulus is changed correctly.
48    #[test]
49    fn modulus_correct() {
50        let mut matrix = Zq::from_str("6 mod 7").unwrap();
51        let modulus = Modulus::from(5);
52
53        matrix.change_modulus(&modulus);
54
55        assert_eq!("1 mod 5", matrix.to_string());
56    }
57
58    /// Ensures that the modulus is changed correctly, if the modulus is big.
59    #[test]
60    fn big_modulus_correct() {
61        let mut matrix = Zq::from_str(&format!("6 mod {}", i64::MAX)).unwrap();
62        let modulus = Modulus::from(u64::MAX);
63
64        matrix.change_modulus(&modulus);
65
66        assert_eq!(format!("6 mod {}", u64::MAX), matrix.to_string());
67    }
68
69    /// Ensures that the matrix is reduced correctly.
70    #[test]
71    fn reduced_correct() {
72        let mut matrix = Zq::from_str("6 mod 7").unwrap();
73        let modulus = Modulus::from(2);
74
75        matrix.change_modulus(&modulus);
76
77        assert_eq!("0 mod 2", matrix.to_string());
78    }
79}
qfall_math/integer_mod_q/z_q/set.rs

qfall_math/integer_mod_q/z_q/
set.rs
