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//! Limb comparisons
use super::{Limb, SignedWord, WideSignedWord, Word, HI_BIT};
use core::cmp::Ordering;
use subtle::{Choice, ConstantTimeEq, ConstantTimeGreater, ConstantTimeLess};
impl Limb {
/// Is this limb an odd number?
#[inline]
pub fn is_odd(&self) -> Choice {
Choice::from(self.0 as u8 & 1)
}
/// Perform a comparison of the inner value in variable-time.
///
/// Note that the [`PartialOrd`] and [`Ord`] impls wrap constant-time
/// comparisons using the `subtle` crate.
pub fn cmp_vartime(&self, other: &Self) -> Ordering {
self.0.cmp(&other.0)
}
/// Performs an equality check in variable-time.
pub const fn eq_vartime(&self, other: &Self) -> bool {
self.0 == other.0
}
/// Returns all 1's if `a`!=0 or 0 if `a`==0.
///
/// Const-friendly: we can't yet use `subtle` in `const fn` contexts.
#[inline]
pub(crate) const fn is_nonzero(self) -> Word {
let inner = self.0 as SignedWord;
((inner | inner.saturating_neg()) >> HI_BIT) as Word
}
#[inline]
pub(crate) const fn ct_cmp(lhs: Self, rhs: Self) -> SignedWord {
let a = lhs.0 as WideSignedWord;
let b = rhs.0 as WideSignedWord;
let gt = ((b - a) >> Limb::BIT_SIZE) & 1;
let lt = ((a - b) >> Limb::BIT_SIZE) & 1 & !gt;
(gt as SignedWord) - (lt as SignedWord)
}
}
impl ConstantTimeEq for Limb {
#[inline]
fn ct_eq(&self, other: &Self) -> Choice {
self.0.ct_eq(&other.0)
}
}
impl ConstantTimeGreater for Limb {
#[inline]
fn ct_gt(&self, other: &Self) -> Choice {
self.0.ct_gt(&other.0)
}
}
impl ConstantTimeLess for Limb {
#[inline]
fn ct_lt(&self, other: &Self) -> Choice {
self.0.ct_lt(&other.0)
}
}
impl Eq for Limb {}
impl Ord for Limb {
fn cmp(&self, other: &Self) -> Ordering {
let mut n = 0i8;
n -= self.ct_lt(other).unwrap_u8() as i8;
n += self.ct_gt(other).unwrap_u8() as i8;
match n {
-1 => Ordering::Less,
1 => Ordering::Greater,
_ => {
debug_assert_eq!(n, 0);
debug_assert!(bool::from(self.ct_eq(other)));
Ordering::Equal
}
}
}
}
impl PartialOrd for Limb {
#[inline]
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
impl PartialEq for Limb {
#[inline]
fn eq(&self, other: &Self) -> bool {
self.ct_eq(other).into()
}
}
#[cfg(test)]
mod tests {
use crate::{Limb, Zero};
use core::cmp::Ordering;
use subtle::{ConstantTimeEq, ConstantTimeGreater, ConstantTimeLess};
#[test]
fn is_zero() {
assert!(bool::from(Limb::ZERO.is_zero()));
assert!(!bool::from(Limb::ONE.is_zero()));
assert!(!bool::from(Limb::MAX.is_zero()));
}
#[test]
fn is_odd() {
assert!(!bool::from(Limb::ZERO.is_odd()));
assert!(bool::from(Limb::ONE.is_odd()));
assert!(bool::from(Limb::MAX.is_odd()));
}
#[test]
fn ct_eq() {
let a = Limb::ZERO;
let b = Limb::MAX;
assert!(bool::from(a.ct_eq(&a)));
assert!(!bool::from(a.ct_eq(&b)));
assert!(!bool::from(b.ct_eq(&a)));
assert!(bool::from(b.ct_eq(&b)));
}
#[test]
fn ct_gt() {
let a = Limb::ZERO;
let b = Limb::ONE;
let c = Limb::MAX;
assert!(bool::from(b.ct_gt(&a)));
assert!(bool::from(c.ct_gt(&a)));
assert!(bool::from(c.ct_gt(&b)));
assert!(!bool::from(a.ct_gt(&a)));
assert!(!bool::from(b.ct_gt(&b)));
assert!(!bool::from(c.ct_gt(&c)));
assert!(!bool::from(a.ct_gt(&b)));
assert!(!bool::from(a.ct_gt(&c)));
assert!(!bool::from(b.ct_gt(&c)));
}
#[test]
fn ct_lt() {
let a = Limb::ZERO;
let b = Limb::ONE;
let c = Limb::MAX;
assert!(bool::from(a.ct_lt(&b)));
assert!(bool::from(a.ct_lt(&c)));
assert!(bool::from(b.ct_lt(&c)));
assert!(!bool::from(a.ct_lt(&a)));
assert!(!bool::from(b.ct_lt(&b)));
assert!(!bool::from(c.ct_lt(&c)));
assert!(!bool::from(b.ct_lt(&a)));
assert!(!bool::from(c.ct_lt(&a)));
assert!(!bool::from(c.ct_lt(&b)));
}
#[test]
fn cmp() {
assert_eq!(Limb::ZERO.cmp(&Limb::ONE), Ordering::Less);
assert_eq!(Limb::ONE.cmp(&Limb::ONE), Ordering::Equal);
assert_eq!(Limb::MAX.cmp(&Limb::ONE), Ordering::Greater);
}
}
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