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use subtle::Choice;
use crate::Word;
/// A boolean value returned by constant-time `const fn`s.
// TODO: should be replaced by `subtle::Choice` or `CtOption`
// when `subtle` starts supporting const fns.
#[derive(Debug, Copy, Clone)]
pub struct CtChoice(Word);
impl CtChoice {
/// The falsy value.
pub const FALSE: Self = Self(0);
/// The truthy value.
pub const TRUE: Self = Self(Word::MAX);
/// Returns the truthy value if `value == Word::MAX`, and the falsy value if `value == 0`.
/// Panics for other values.
pub(crate) const fn from_mask(value: Word) -> Self {
debug_assert!(value == Self::FALSE.0 || value == Self::TRUE.0);
Self(value)
}
/// Returns the truthy value if `value == 1`, and the falsy value if `value == 0`.
/// Panics for other values.
pub(crate) const fn from_lsb(value: Word) -> Self {
debug_assert!(value == 0 || value == 1);
Self(value.wrapping_neg())
}
pub(crate) const fn not(&self) -> Self {
Self(!self.0)
}
pub(crate) const fn and(&self, other: Self) -> Self {
Self(self.0 & other.0)
}
/// Return `b` if `self` is truthy, otherwise return `a`.
pub(crate) const fn select(&self, a: Word, b: Word) -> Word {
a ^ (self.0 & (a ^ b))
}
/// Return `x` if `self` is truthy, otherwise return 0.
pub(crate) const fn if_true(&self, x: Word) -> Word {
x & self.0
}
pub(crate) const fn is_true_vartime(&self) -> bool {
self.0 == CtChoice::TRUE.0
}
}
impl From<CtChoice> for Choice {
fn from(choice: CtChoice) -> Self {
Choice::from(choice.0 as u8 & 1)
}
}
impl From<CtChoice> for bool {
fn from(choice: CtChoice) -> Self {
choice.is_true_vartime()
}
}
#[cfg(test)]
mod tests {
use super::CtChoice;
use crate::Word;
#[test]
fn select() {
let a: Word = 1;
let b: Word = 2;
assert_eq!(CtChoice::TRUE.select(a, b), b);
assert_eq!(CtChoice::FALSE.select(a, b), a);
}
}
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