crypto_bigint/uint/
neg.rs

1use crate::{Choice, Limb, Uint, WideWord, Word, WrappingNeg, word};
2
3impl<const LIMBS: usize> Uint<LIMBS> {
4    /// Perform wrapping negation.
5    #[must_use]
6    pub const fn wrapping_neg(&self) -> Self {
7        self.carrying_neg().0
8    }
9
10    /// Perform negation: map `self` to `(self ^ 1111...1111) + 0000...0001`.
11    /// Additionally return whether the carry flag is set on the addition.
12    ///
13    /// Note: the carry is set if and only if `self == Self::ZERO`.
14    #[must_use]
15    #[allow(clippy::cast_possible_truncation)]
16    pub const fn carrying_neg(&self) -> (Self, Choice) {
17        let mut ret = [Limb::ZERO; LIMBS];
18        let mut carry = 1;
19        let mut i = 0;
20        while i < LIMBS {
21            let r = (!self.limbs[i].0 as WideWord) + carry;
22            ret[i] = Limb(r as Word);
23            carry = r >> Limb::BITS;
24            i += 1;
25        }
26        (Uint::new(ret), word::choice_from_lsb(carry as Word))
27    }
28
29    /// Perform wrapping negation, if `negate` is truthy. Otherwise, return `self`.
30    #[must_use]
31    pub const fn wrapping_neg_if(&self, negate: Choice) -> Self {
32        Uint::select(self, &self.wrapping_neg(), negate)
33    }
34}
35
36impl<const LIMBS: usize> WrappingNeg for Uint<LIMBS> {
37    #[inline]
38    fn wrapping_neg(&self) -> Self {
39        self.wrapping_neg()
40    }
41}
42
43#[cfg(test)]
44mod tests {
45    use crate::{Choice, U256};
46
47    #[test]
48    fn wrapping_neg() {
49        assert_eq!(U256::ZERO.wrapping_neg(), U256::ZERO);
50        assert_eq!(U256::MAX.wrapping_neg(), U256::ONE);
51        assert_eq!(
52            U256::from_u64(13).wrapping_neg(),
53            U256::from_u64(13).not().saturating_add(&U256::ONE)
54        );
55        assert_eq!(
56            U256::from_u64(42).wrapping_neg(),
57            U256::from_u64(42).saturating_sub(&U256::ONE).not()
58        );
59    }
60
61    #[test]
62    fn carrying_neg() {
63        let (ret, carry) = U256::ZERO.carrying_neg();
64        assert_eq!(ret, U256::ZERO);
65        assert!(carry.to_bool());
66
67        let (ret, carry) = U256::ONE.carrying_neg();
68        assert_eq!(ret, U256::MAX);
69        assert!(!carry.to_bool());
70
71        let (ret, carry) = U256::MAX.carrying_neg();
72        assert_eq!(ret, U256::ONE);
73        assert!(!carry.to_bool());
74    }
75
76    #[test]
77    fn wrapping_neg_if() {
78        let negate = Choice::TRUE;
79        assert_eq!(U256::ZERO.wrapping_neg_if(negate), U256::ZERO);
80        assert_eq!(U256::ONE.wrapping_neg_if(negate), U256::MAX);
81        assert_eq!(U256::MAX.wrapping_neg_if(negate), U256::ONE);
82
83        let do_not_negate = Choice::FALSE;
84        assert_eq!(U256::ZERO.wrapping_neg_if(do_not_negate), U256::ZERO);
85        assert_eq!(U256::ONE.wrapping_neg_if(do_not_negate), U256::ONE);
86        assert_eq!(U256::MAX.wrapping_neg_if(do_not_negate), U256::MAX);
87    }
88}
crypto_bigint/uint/neg.rs

crypto_bigint/uint/
neg.rs
