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use crate::{AddMod, Limb, UInt};
impl<const LIMBS: usize> UInt<LIMBS> {
pub const fn add_mod(&self, rhs: &UInt<LIMBS>, p: &UInt<LIMBS>) -> UInt<LIMBS> {
let (w, carry) = self.adc(rhs, Limb::ZERO);
let (w, borrow) = w.sbb(p, Limb::ZERO);
let (_, borrow) = carry.sbb(Limb::ZERO, borrow);
let mut i = 0;
let mut res = Self::ZERO;
let mut carry = Limb::ZERO;
while i < LIMBS {
let rhs = p.limbs[i].bitand(borrow);
let (limb, c) = w.limbs[i].adc(rhs, carry);
res.limbs[i] = limb;
carry = c;
i += 1;
}
res
}
pub const fn add_mod_special(&self, rhs: &Self, c: Limb) -> Self {
let (out, carry) = self.adc(rhs, c);
let l = carry.0.wrapping_sub(1) & c.0;
let (out, _) = out.sbb(&UInt::from_word(l), Limb::ZERO);
out
}
}
impl<const LIMBS: usize> AddMod for UInt<LIMBS> {
type Output = Self;
fn add_mod(&self, rhs: &Self, p: &Self) -> Self {
debug_assert!(self < p);
debug_assert!(rhs < p);
self.add_mod(rhs, p)
}
}
#[cfg(all(test, feature = "rand"))]
mod tests {
use crate::{Limb, NonZero, Random, RandomMod, UInt, U256};
use rand_core::SeedableRng;
#[test]
fn add_mod_nist_p256() {
let a =
U256::from_be_hex("44acf6b7e36c1342c2c5897204fe09504e1e2efb1a900377dbc4e7a6a133ec56");
let b =
U256::from_be_hex("d5777c45019673125ad240f83094d4252d829516fac8601ed01979ec1ec1a251");
let n =
U256::from_be_hex("ffffffff00000000ffffffffffffffffbce6faada7179e84f3b9cac2fc632551");
let actual = a.add_mod(&b, &n);
let expected =
U256::from_be_hex("1a2472fde50286541d97ca6a3592dd75beb9c9646e40c511b82496cfc3926956");
assert_eq!(expected, actual);
}
macro_rules! test_add_mod_special {
($size:expr, $test_name:ident) => {
#[test]
fn $test_name() {
let mut rng = rand_chacha::ChaCha8Rng::seed_from_u64(1);
let moduli = [
NonZero::<Limb>::random(&mut rng),
NonZero::<Limb>::random(&mut rng),
];
for special in &moduli {
let p = &NonZero::new(UInt::ZERO.wrapping_sub(&UInt::from_word(special.0)))
.unwrap();
let minus_one = p.wrapping_sub(&UInt::ONE);
let base_cases = [
(UInt::ZERO, UInt::ZERO, UInt::ZERO),
(UInt::ONE, UInt::ZERO, UInt::ONE),
(UInt::ZERO, UInt::ONE, UInt::ONE),
(minus_one, UInt::ONE, UInt::ZERO),
(UInt::ONE, minus_one, UInt::ZERO),
];
for (a, b, c) in &base_cases {
let x = a.add_mod_special(b, *special.as_ref());
assert_eq!(*c, x, "{} + {} mod {} = {} != {}", a, b, p, x, c);
}
for _i in 0..100 {
let a = UInt::<$size>::random_mod(&mut rng, p);
let b = UInt::<$size>::random_mod(&mut rng, p);
let c = a.add_mod_special(&b, *special.as_ref());
assert!(c < **p, "not reduced: {} >= {} ", c, p);
let expected = a.add_mod(&b, p);
assert_eq!(c, expected, "incorrect result");
}
}
}
};
}
test_add_mod_special!(1, add_mod_special_1);
test_add_mod_special!(2, add_mod_special_2);
test_add_mod_special!(3, add_mod_special_3);
test_add_mod_special!(4, add_mod_special_4);
test_add_mod_special!(5, add_mod_special_5);
test_add_mod_special!(6, add_mod_special_6);
test_add_mod_special!(7, add_mod_special_7);
test_add_mod_special!(8, add_mod_special_8);
test_add_mod_special!(9, add_mod_special_9);
test_add_mod_special!(10, add_mod_special_10);
test_add_mod_special!(11, add_mod_special_11);
test_add_mod_special!(12, add_mod_special_12);
}