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|
//! [`Uint`] addition operations.
use super::Uint;
use crate::{Checked, CheckedSub, CtChoice, Limb, Wrapping, Zero};
use core::ops::{Sub, SubAssign};
use subtle::CtOption;
impl<const LIMBS: usize> Uint<LIMBS> {
/// Computes `a - (b + borrow)`, returning the result along with the new borrow.
#[inline(always)]
pub const fn sbb(&self, rhs: &Self, mut borrow: Limb) -> (Self, Limb) {
let mut limbs = [Limb::ZERO; LIMBS];
let mut i = 0;
while i < LIMBS {
let (w, b) = self.limbs[i].sbb(rhs.limbs[i], borrow);
limbs[i] = w;
borrow = b;
i += 1;
}
(Self { limbs }, borrow)
}
/// Perform saturating subtraction, returning `ZERO` on underflow.
pub const fn saturating_sub(&self, rhs: &Self) -> Self {
let (res, underflow) = self.sbb(rhs, Limb::ZERO);
Self::ct_select(&res, &Self::ZERO, CtChoice::from_mask(underflow.0))
}
/// Perform wrapping subtraction, discarding underflow and wrapping around
/// the boundary of the type.
pub const fn wrapping_sub(&self, rhs: &Self) -> Self {
self.sbb(rhs, Limb::ZERO).0
}
/// Perform wrapping subtraction, returning the truthy value as the second element of the tuple
/// if an underflow has occurred.
pub(crate) const fn conditional_wrapping_sub(
&self,
rhs: &Self,
choice: CtChoice,
) -> (Self, CtChoice) {
let actual_rhs = Uint::ct_select(&Uint::ZERO, rhs, choice);
let (res, borrow) = self.sbb(&actual_rhs, Limb::ZERO);
(res, CtChoice::from_mask(borrow.0))
}
}
impl<const LIMBS: usize> CheckedSub<&Uint<LIMBS>> for Uint<LIMBS> {
type Output = Self;
fn checked_sub(&self, rhs: &Self) -> CtOption<Self> {
let (result, underflow) = self.sbb(rhs, Limb::ZERO);
CtOption::new(result, underflow.is_zero())
}
}
impl<const LIMBS: usize> Sub for Wrapping<Uint<LIMBS>> {
type Output = Self;
fn sub(self, rhs: Self) -> Wrapping<Uint<LIMBS>> {
Wrapping(self.0.wrapping_sub(&rhs.0))
}
}
impl<const LIMBS: usize> Sub<&Wrapping<Uint<LIMBS>>> for Wrapping<Uint<LIMBS>> {
type Output = Wrapping<Uint<LIMBS>>;
fn sub(self, rhs: &Wrapping<Uint<LIMBS>>) -> Wrapping<Uint<LIMBS>> {
Wrapping(self.0.wrapping_sub(&rhs.0))
}
}
impl<const LIMBS: usize> Sub<Wrapping<Uint<LIMBS>>> for &Wrapping<Uint<LIMBS>> {
type Output = Wrapping<Uint<LIMBS>>;
fn sub(self, rhs: Wrapping<Uint<LIMBS>>) -> Wrapping<Uint<LIMBS>> {
Wrapping(self.0.wrapping_sub(&rhs.0))
}
}
impl<const LIMBS: usize> Sub<&Wrapping<Uint<LIMBS>>> for &Wrapping<Uint<LIMBS>> {
type Output = Wrapping<Uint<LIMBS>>;
fn sub(self, rhs: &Wrapping<Uint<LIMBS>>) -> Wrapping<Uint<LIMBS>> {
Wrapping(self.0.wrapping_sub(&rhs.0))
}
}
impl<const LIMBS: usize> SubAssign for Wrapping<Uint<LIMBS>> {
fn sub_assign(&mut self, other: Self) {
*self = *self - other;
}
}
impl<const LIMBS: usize> SubAssign<&Wrapping<Uint<LIMBS>>> for Wrapping<Uint<LIMBS>> {
fn sub_assign(&mut self, other: &Self) {
*self = *self - other;
}
}
impl<const LIMBS: usize> Sub for Checked<Uint<LIMBS>> {
type Output = Self;
fn sub(self, rhs: Self) -> Checked<Uint<LIMBS>> {
Checked(
self.0
.and_then(|lhs| rhs.0.and_then(|rhs| lhs.checked_sub(&rhs))),
)
}
}
impl<const LIMBS: usize> Sub<&Checked<Uint<LIMBS>>> for Checked<Uint<LIMBS>> {
type Output = Checked<Uint<LIMBS>>;
fn sub(self, rhs: &Checked<Uint<LIMBS>>) -> Checked<Uint<LIMBS>> {
Checked(
self.0
.and_then(|lhs| rhs.0.and_then(|rhs| lhs.checked_sub(&rhs))),
)
}
}
impl<const LIMBS: usize> Sub<Checked<Uint<LIMBS>>> for &Checked<Uint<LIMBS>> {
type Output = Checked<Uint<LIMBS>>;
fn sub(self, rhs: Checked<Uint<LIMBS>>) -> Checked<Uint<LIMBS>> {
Checked(
self.0
.and_then(|lhs| rhs.0.and_then(|rhs| lhs.checked_sub(&rhs))),
)
}
}
impl<const LIMBS: usize> Sub<&Checked<Uint<LIMBS>>> for &Checked<Uint<LIMBS>> {
type Output = Checked<Uint<LIMBS>>;
fn sub(self, rhs: &Checked<Uint<LIMBS>>) -> Checked<Uint<LIMBS>> {
Checked(
self.0
.and_then(|lhs| rhs.0.and_then(|rhs| lhs.checked_sub(&rhs))),
)
}
}
impl<const LIMBS: usize> SubAssign for Checked<Uint<LIMBS>> {
fn sub_assign(&mut self, other: Self) {
*self = *self - other;
}
}
impl<const LIMBS: usize> SubAssign<&Checked<Uint<LIMBS>>> for Checked<Uint<LIMBS>> {
fn sub_assign(&mut self, other: &Self) {
*self = *self - other;
}
}
#[cfg(test)]
mod tests {
use crate::{CheckedSub, Limb, U128};
#[test]
fn sbb_no_borrow() {
let (res, borrow) = U128::ONE.sbb(&U128::ONE, Limb::ZERO);
assert_eq!(res, U128::ZERO);
assert_eq!(borrow, Limb::ZERO);
}
#[test]
fn sbb_with_borrow() {
let (res, borrow) = U128::ZERO.sbb(&U128::ONE, Limb::ZERO);
assert_eq!(res, U128::MAX);
assert_eq!(borrow, Limb::MAX);
}
#[test]
fn saturating_sub_no_borrow() {
assert_eq!(
U128::from(5u64).saturating_sub(&U128::ONE),
U128::from(4u64)
);
}
#[test]
fn saturating_sub_with_borrow() {
assert_eq!(
U128::from(4u64).saturating_sub(&U128::from(5u64)),
U128::ZERO
);
}
#[test]
fn wrapping_sub_no_borrow() {
assert_eq!(U128::ONE.wrapping_sub(&U128::ONE), U128::ZERO);
}
#[test]
fn wrapping_sub_with_borrow() {
assert_eq!(U128::ZERO.wrapping_sub(&U128::ONE), U128::MAX);
}
#[test]
fn checked_sub_ok() {
let result = U128::ONE.checked_sub(&U128::ONE);
assert_eq!(result.unwrap(), U128::ZERO);
}
#[test]
fn checked_sub_overflow() {
let result = U128::ZERO.checked_sub(&U128::ONE);
assert!(!bool::from(result.is_some()));
}
}
|
