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//! [`Uint`] negation modulus operations.
use crate::{Limb, NegMod, Uint};
impl<const LIMBS: usize> Uint<LIMBS> {
/// Computes `-a mod p` in constant time.
/// Assumes `self` is in `[0, p)`.
pub const fn neg_mod(&self, p: &Self) -> Self {
let z = self.ct_is_nonzero();
let mut ret = p.sbb(self, Limb::ZERO).0;
let mut i = 0;
while i < LIMBS {
// Set ret to 0 if the original value was 0, in which
// case ret would be p.
ret.limbs[i].0 = z.if_true(ret.limbs[i].0);
i += 1;
}
ret
}
/// Computes `-a mod p` in constant time for the special modulus
/// `p = MAX+1-c` where `c` is small enough to fit in a single [`Limb`].
pub const fn neg_mod_special(&self, c: Limb) -> Self {
Self::ZERO.sub_mod_special(self, c)
}
}
impl<const LIMBS: usize> NegMod for Uint<LIMBS> {
type Output = Self;
fn neg_mod(&self, p: &Self) -> Self {
debug_assert!(self < p);
self.neg_mod(p)
}
}
#[cfg(test)]
mod tests {
use crate::U256;
#[test]
fn neg_mod_random() {
let x =
U256::from_be_hex("8d16e171674b4e6d8529edba4593802bf30b8cb161dd30aa8e550d41380007c2");
let p =
U256::from_be_hex("928334a4e4be0843ec225a4c9c61df34bdc7a81513e4b6f76f2bfa3148e2e1b5");
let actual = x.neg_mod(&p);
let expected =
U256::from_be_hex("056c53337d72b9d666f86c9256ce5f08cabc1b63b207864ce0d6ecf010e2d9f3");
assert_eq!(expected, actual);
}
#[test]
fn neg_mod_zero() {
let x =
U256::from_be_hex("0000000000000000000000000000000000000000000000000000000000000000");
let p =
U256::from_be_hex("928334a4e4be0843ec225a4c9c61df34bdc7a81513e4b6f76f2bfa3148e2e1b5");
let actual = x.neg_mod(&p);
let expected =
U256::from_be_hex("0000000000000000000000000000000000000000000000000000000000000000");
assert_eq!(expected, actual);
}
}
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