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