crypto_bigint/uint/
sub.rs

1//! [`Uint`] subtraction operations.
2
3use super::Uint;
4use crate::{
5    Checked, CheckedSub, Choice, CtOption, Limb, Sub, SubAssign, Wrapping, WrappingSub, word,
6};
7
8impl<const LIMBS: usize> Uint<LIMBS> {
9    /// Computes `self - (rhs + borrow)`, returning the result along with the new borrow.
10    #[deprecated(since = "0.7.0", note = "please use `borrowing_sub` instead")]
11    #[must_use]
12    pub const fn sbb(&self, rhs: &Self, borrow: Limb) -> (Self, Limb) {
13        self.borrowing_sub(rhs, borrow)
14    }
15
16    /// Computes `self - (rhs + borrow)`, returning the result along with the new borrow.
17    #[inline(always)]
18    #[must_use]
19    pub const fn borrowing_sub(&self, rhs: &Self, mut borrow: Limb) -> (Self, Limb) {
20        let mut limbs = [Limb::ZERO; LIMBS];
21        let mut i = 0;
22
23        while i < LIMBS {
24            let (w, b) = self.limbs[i].borrowing_sub(rhs.limbs[i], borrow);
25            limbs[i] = w;
26            borrow = b;
27            i += 1;
28        }
29
30        (Self { limbs }, borrow)
31    }
32
33    /// Perform wrapping subtraction, returning the truthy value as the second element of
34    /// the tuple if an underflow has occurred.
35    #[must_use]
36    pub(crate) const fn conditional_borrowing_sub(
37        &self,
38        rhs: &Self,
39        choice: Choice,
40    ) -> (Self, Choice) {
41        let mut limbs = [Limb::ZERO; LIMBS];
42        let mask = Limb::select(Limb::ZERO, Limb::MAX, choice);
43        let mut borrow = Limb::ZERO;
44
45        let mut i = 0;
46        while i < LIMBS {
47            let masked_rhs = rhs.limbs[i].bitand(mask);
48            (limbs[i], borrow) = self.limbs[i].borrowing_sub(masked_rhs, borrow);
49            i += 1;
50        }
51
52        (Self { limbs }, borrow.lsb_to_choice())
53    }
54
55    /// Perform saturating subtraction, returning `ZERO` on underflow.
56    #[must_use]
57    pub const fn saturating_sub(&self, rhs: &Self) -> Self {
58        let (res, underflow) = self.borrowing_sub(rhs, Limb::ZERO);
59        Self::select(&res, &Self::ZERO, word::choice_from_mask(underflow.0))
60    }
61
62    /// Perform wrapping subtraction, discarding underflow and wrapping around
63    /// the boundary of the type.
64    #[must_use]
65    pub const fn wrapping_sub(&self, rhs: &Self) -> Self {
66        self.borrowing_sub(rhs, Limb::ZERO).0
67    }
68}
69
70impl<const LIMBS: usize> CheckedSub for Uint<LIMBS> {
71    fn checked_sub(&self, rhs: &Self) -> CtOption<Self> {
72        let (result, underflow) = self.borrowing_sub(rhs, Limb::ZERO);
73        CtOption::new(result, underflow.is_zero())
74    }
75}
76
77impl<const LIMBS: usize> Sub for Uint<LIMBS> {
78    type Output = Self;
79
80    fn sub(self, rhs: Self) -> Self {
81        self.sub(&rhs)
82    }
83}
84
85impl<const LIMBS: usize> Sub<&Uint<LIMBS>> for Uint<LIMBS> {
86    type Output = Self;
87
88    fn sub(self, rhs: &Self) -> Self {
89        self.checked_sub(rhs)
90            .expect("attempted to subtract with underflow")
91    }
92}
93
94impl<const LIMBS: usize> SubAssign<Uint<LIMBS>> for Uint<LIMBS> {
95    fn sub_assign(&mut self, rhs: Uint<LIMBS>) {
96        *self = self.sub(&rhs);
97    }
98}
99
100impl<const LIMBS: usize> SubAssign<&Uint<LIMBS>> for Uint<LIMBS> {
101    fn sub_assign(&mut self, rhs: &Uint<LIMBS>) {
102        *self = self.sub(rhs);
103    }
104}
105
106impl<const LIMBS: usize> SubAssign for Wrapping<Uint<LIMBS>> {
107    fn sub_assign(&mut self, other: Self) {
108        *self = *self - other;
109    }
110}
111
112impl<const LIMBS: usize> SubAssign<&Wrapping<Uint<LIMBS>>> for Wrapping<Uint<LIMBS>> {
113    fn sub_assign(&mut self, other: &Self) {
114        *self = *self - other;
115    }
116}
117
118impl<const LIMBS: usize> SubAssign for Checked<Uint<LIMBS>> {
119    fn sub_assign(&mut self, other: Self) {
120        *self = *self - other;
121    }
122}
123
124impl<const LIMBS: usize> SubAssign<&Checked<Uint<LIMBS>>> for Checked<Uint<LIMBS>> {
125    fn sub_assign(&mut self, other: &Self) {
126        *self = *self - other;
127    }
128}
129
130impl<const LIMBS: usize> WrappingSub for Uint<LIMBS> {
131    fn wrapping_sub(&self, v: &Self) -> Self {
132        self.wrapping_sub(v)
133    }
134}
135
136#[cfg(test)]
137mod tests {
138    use ctutils::Choice;
139
140    use crate::{CheckedSub, Limb, U128};
141
142    #[test]
143    fn borrowing_sub_no_borrow() {
144        let (res, borrow) = U128::ONE.borrowing_sub(&U128::ONE, Limb::ZERO);
145        assert_eq!(res, U128::ZERO);
146        assert_eq!(borrow, Limb::ZERO);
147    }
148
149    #[test]
150    fn borrowing_sub_with_borrow() {
151        let (res, borrow) = U128::ZERO.borrowing_sub(&U128::ONE, Limb::ZERO);
152
153        assert_eq!(res, U128::MAX);
154        assert_eq!(borrow, Limb::MAX);
155    }
156
157    #[test]
158    fn conditional_borrowing_sub_no_sub() {
159        let (res, borrow) = U128::ONE.conditional_borrowing_sub(&U128::ONE, Choice::FALSE);
160        assert_eq!(res, U128::ONE);
161        assert!(!borrow.to_bool_vartime());
162    }
163
164    #[test]
165    fn conditional_borrowing_sub_no_borrow() {
166        let (res, borrow) = U128::ONE.conditional_borrowing_sub(&U128::ZERO, Choice::TRUE);
167        assert_eq!(res, U128::ONE);
168        assert!(!borrow.to_bool_vartime());
169    }
170
171    #[test]
172    fn conditional_borrowing_sub_borrow() {
173        let (res, borrow) = U128::ZERO.conditional_borrowing_sub(&U128::ONE, Choice::TRUE);
174        assert_eq!(res, U128::MAX);
175        assert!(borrow.to_bool_vartime());
176    }
177
178    #[test]
179    fn saturating_sub_no_borrow() {
180        assert_eq!(
181            U128::from(5u64).saturating_sub(&U128::ONE),
182            U128::from(4u64)
183        );
184    }
185
186    #[test]
187    fn saturating_sub_with_borrow() {
188        assert_eq!(
189            U128::from(4u64).saturating_sub(&U128::from(5u64)),
190            U128::ZERO
191        );
192    }
193
194    #[test]
195    fn wrapping_sub_no_borrow() {
196        assert_eq!(U128::ONE.wrapping_sub(&U128::ONE), U128::ZERO);
197    }
198
199    #[test]
200    fn wrapping_sub_with_borrow() {
201        assert_eq!(U128::ZERO.wrapping_sub(&U128::ONE), U128::MAX);
202    }
203
204    #[test]
205    fn checked_sub_ok() {
206        let result = U128::ONE.checked_sub(&U128::ONE);
207        assert_eq!(result.unwrap(), U128::ZERO);
208    }
209
210    #[test]
211    fn checked_sub_overflow() {
212        let result = U128::ZERO.checked_sub(&U128::ONE);
213        assert!(!bool::from(result.is_some()));
214    }
215}
crypto_bigint/uint/sub.rs

crypto_bigint/uint/
sub.rs
