add modulus switching to GLWE ciphertexts (and Zq,Rq)

arith/src/ringq.rs

@@ -49,9 +49,6 @@ impl<const Q: u64, const N: usize> Ring for Rq<Q, N> {
     }
 }
 
-// TODO define a trait "PolynomialRingTrait" or similar, so that when other structs use it can just
-// use the trait and not need to add '<Q, N>' to their params
-
 impl<const Q: u64, const N: usize> From<crate::ring::R<N>> for Rq<Q, N> {
     fn from(r: crate::ring::R<N>) -> Self {
         Self::from_vec(
@@ -165,7 +162,7 @@ impl<const Q: u64, const N: usize> Rq<Q, N> {
     }
 
     /// perform the mod switch operation from Q to Q', where Q2=Q'
-    fn mod_switch<const Q2: u64>(&self) -> Rq<Q2, N> {
+    pub fn mod_switch<const Q2: u64>(&self) -> Rq<Q2, N> {
         Rq::<Q2, N> {
             coeffs: array::from_fn(|i| self.coeffs[i].mod_switch::<Q2>()),
             evals: None,

arith/src/traits.rs

@@ -4,7 +4,7 @@ use std::fmt::Debug;
 use std::iter::Sum;
 use std::ops::{Add, AddAssign, Mul, Sub, SubAssign};
 
-/// represents a ring element
+/// Represents a ring element. Currently implemented by ring.rs#R and ringq.rs#Rq.
 pub trait Ring:
     Sized
     + Add<Output = Self>
@@ -25,6 +25,6 @@ pub trait Ring:
 
     fn coeffs(&self) -> Vec<Self::C>;
     fn zero() -> Self;
-    fn rand(rng: impl Rng, dist: impl Distribution<f64>) -> Self;
     // note/wip/warning: dist (0,q) with f64, will output more '0=q' elements than other values
+    fn rand(rng: impl Rng, dist: impl Distribution<f64>) -> Self;
 }