crypto_bigint/uint/
neg.rs

1use crate::{ConstChoice, Limb, Uint, WideWord, Word, WrappingNeg};
2
3impl<const LIMBS: usize> Uint<LIMBS> {
4    /// Perform wrapping negation.
5    pub const fn wrapping_neg(&self) -> Self {
6        self.carrying_neg().0
7    }
8
9    /// Perform negation: map `self` to `(self ^ 1111...1111) + 0000...0001`.
10    /// Additionally return whether the carry flag is set on the addition.
11    ///
12    /// Note: the carry is set if and only if `self == Self::ZERO`.
13    pub const fn carrying_neg(&self) -> (Self, ConstChoice) {
14        let mut ret = [Limb::ZERO; LIMBS];
15        let mut carry = 1;
16        let mut i = 0;
17        while i < LIMBS {
18            let r = (!self.limbs[i].0 as WideWord) + carry;
19            ret[i] = Limb(r as Word);
20            carry = r >> Limb::BITS;
21            i += 1;
22        }
23        (Uint::new(ret), ConstChoice::from_word_lsb(carry as Word))
24    }
25
26    /// Perform wrapping negation, if `negate` is truthy. Otherwise, return `self`.
27    pub const fn wrapping_neg_if(&self, negate: ConstChoice) -> Self {
28        Uint::select(self, &self.wrapping_neg(), negate)
29    }
30}
31
32impl<const LIMBS: usize> WrappingNeg for Uint<LIMBS> {
33    #[inline]
34    fn wrapping_neg(&self) -> Self {
35        self.wrapping_neg()
36    }
37}
38
39#[cfg(test)]
40mod tests {
41    use crate::{ConstChoice, U256};
42
43    #[test]
44    fn wrapping_neg() {
45        assert_eq!(U256::ZERO.wrapping_neg(), U256::ZERO);
46        assert_eq!(U256::MAX.wrapping_neg(), U256::ONE);
47        assert_eq!(
48            U256::from_u64(13).wrapping_neg(),
49            U256::from_u64(13).not().saturating_add(&U256::ONE)
50        );
51        assert_eq!(
52            U256::from_u64(42).wrapping_neg(),
53            U256::from_u64(42).saturating_sub(&U256::ONE).not()
54        );
55    }
56
57    #[test]
58    fn carrying_neg() {
59        assert_eq!(U256::ZERO.carrying_neg(), (U256::ZERO, ConstChoice::TRUE));
60        assert_eq!(U256::ONE.carrying_neg(), (U256::MAX, ConstChoice::FALSE));
61        assert_eq!(U256::MAX.carrying_neg(), (U256::ONE, ConstChoice::FALSE));
62    }
63
64    #[test]
65    fn wrapping_neg_if() {
66        let negate = ConstChoice::TRUE;
67        assert_eq!(U256::ZERO.wrapping_neg_if(negate), U256::ZERO);
68        assert_eq!(U256::ONE.wrapping_neg_if(negate), U256::MAX);
69        assert_eq!(U256::MAX.wrapping_neg_if(negate), U256::ONE);
70
71        let do_not_negate = ConstChoice::FALSE;
72        assert_eq!(U256::ZERO.wrapping_neg_if(do_not_negate), U256::ZERO);
73        assert_eq!(U256::ONE.wrapping_neg_if(do_not_negate), U256::ONE);
74        assert_eq!(U256::MAX.wrapping_neg_if(do_not_negate), U256::MAX);
75    }
76}
crypto_bigint/uint/neg.rs

crypto_bigint/uint/
neg.rs
