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|
//! [`Uint`] bitwise left shift operations.
use crate::{CtChoice, Limb, Uint, Word};
use core::ops::{Shl, ShlAssign};
impl<const LIMBS: usize> Uint<LIMBS> {
/// Computes `self << shift` where `0 <= shift < Limb::BITS`,
/// returning the result and the carry.
#[inline(always)]
pub(crate) const fn shl_limb(&self, n: usize) -> (Self, Limb) {
let mut limbs = [Limb::ZERO; LIMBS];
let nz = Limb(n as Word).ct_is_nonzero();
let lshift = n as Word;
let rshift = Limb::ct_select(Limb::ZERO, Limb((Limb::BITS - n) as Word), nz).0;
let carry = Limb::ct_select(
Limb::ZERO,
Limb(self.limbs[LIMBS - 1].0.wrapping_shr(Word::BITS - n as u32)),
nz,
);
let mut i = LIMBS - 1;
while i > 0 {
let mut limb = self.limbs[i].0 << lshift;
let hi = self.limbs[i - 1].0 >> rshift;
limb |= nz.if_true(hi);
limbs[i] = Limb(limb);
i -= 1
}
limbs[0] = Limb(self.limbs[0].0 << lshift);
(Uint::<LIMBS>::new(limbs), carry)
}
/// Computes `self << shift`.
///
/// NOTE: this operation is variable time with respect to `n` *ONLY*.
///
/// When used with a fixed `n`, this function is constant-time with respect
/// to `self`.
#[inline(always)]
pub const fn shl_vartime(&self, n: usize) -> Self {
let mut limbs = [Limb::ZERO; LIMBS];
if n >= Limb::BITS * LIMBS {
return Self { limbs };
}
let shift_num = n / Limb::BITS;
let rem = n % Limb::BITS;
let mut i = LIMBS;
while i > shift_num {
i -= 1;
limbs[i] = self.limbs[i - shift_num];
}
let (new_lower, _carry) = (Self { limbs }).shl_limb(rem);
new_lower
}
/// Computes a left shift on a wide input as `(lo, hi)`.
///
/// NOTE: this operation is variable time with respect to `n` *ONLY*.
///
/// When used with a fixed `n`, this function is constant-time with respect
/// to `self`.
#[inline(always)]
pub const fn shl_vartime_wide(lower_upper: (Self, Self), n: usize) -> (Self, Self) {
let (lower, mut upper) = lower_upper;
let new_lower = lower.shl_vartime(n);
upper = upper.shl_vartime(n);
if n >= Self::BITS {
upper = upper.bitor(&lower.shl_vartime(n - Self::BITS));
} else {
upper = upper.bitor(&lower.shr_vartime(Self::BITS - n));
}
(new_lower, upper)
}
/// Computes `self << n`.
/// Returns zero if `n >= Self::BITS`.
pub const fn shl(&self, shift: usize) -> Self {
let overflow = CtChoice::from_usize_lt(shift, Self::BITS).not();
let shift = shift % Self::BITS;
let mut result = *self;
let mut i = 0;
while i < Self::LOG2_BITS {
let bit = CtChoice::from_lsb((shift as Word >> i) & 1);
result = Uint::ct_select(&result, &result.shl_vartime(1 << i), bit);
i += 1;
}
Uint::ct_select(&result, &Self::ZERO, overflow)
}
}
impl<const LIMBS: usize> Shl<usize> for Uint<LIMBS> {
type Output = Uint<LIMBS>;
/// NOTE: this operation is variable time with respect to `rhs` *ONLY*.
///
/// When used with a fixed `rhs`, this function is constant-time with respect
/// to `self`.
fn shl(self, rhs: usize) -> Uint<LIMBS> {
Uint::<LIMBS>::shl(&self, rhs)
}
}
impl<const LIMBS: usize> Shl<usize> for &Uint<LIMBS> {
type Output = Uint<LIMBS>;
/// NOTE: this operation is variable time with respect to `rhs` *ONLY*.
///
/// When used with a fixed `rhs`, this function is constant-time with respect
/// to `self`.
fn shl(self, rhs: usize) -> Uint<LIMBS> {
self.shl(rhs)
}
}
impl<const LIMBS: usize> ShlAssign<usize> for Uint<LIMBS> {
/// NOTE: this operation is variable time with respect to `rhs` *ONLY*.
///
/// When used with a fixed `rhs`, this function is constant-time with respect
/// to `self`.
fn shl_assign(&mut self, rhs: usize) {
*self = self.shl(rhs)
}
}
#[cfg(test)]
mod tests {
use crate::{Limb, Uint, U128, U256};
const N: U256 =
U256::from_be_hex("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141");
const TWO_N: U256 =
U256::from_be_hex("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFD755DB9CD5E9140777FA4BD19A06C8282");
const FOUR_N: U256 =
U256::from_be_hex("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFAEABB739ABD2280EEFF497A3340D90504");
const SIXTY_FIVE: U256 =
U256::from_be_hex("FFFFFFFFFFFFFFFD755DB9CD5E9140777FA4BD19A06C82820000000000000000");
const EIGHTY_EIGHT: U256 =
U256::from_be_hex("FFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD03641410000000000000000000000");
const SIXTY_FOUR: U256 =
U256::from_be_hex("FFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD03641410000000000000000");
#[test]
fn shl_simple() {
let mut t = U256::from(1u8);
assert_eq!(t << 1, U256::from(2u8));
t = U256::from(3u8);
assert_eq!(t << 8, U256::from(0x300u16));
}
#[test]
fn shl1() {
assert_eq!(N << 1, TWO_N);
}
#[test]
fn shl2() {
assert_eq!(N << 2, FOUR_N);
}
#[test]
fn shl65() {
assert_eq!(N << 65, SIXTY_FIVE);
}
#[test]
fn shl88() {
assert_eq!(N << 88, EIGHTY_EIGHT);
}
#[test]
fn shl256() {
assert_eq!(N << 256, U256::default());
}
#[test]
fn shl64() {
assert_eq!(N << 64, SIXTY_FOUR);
}
#[test]
fn shl_wide_1_1_128() {
assert_eq!(
Uint::shl_vartime_wide((U128::ONE, U128::ONE), 128),
(U128::ZERO, U128::ONE)
);
}
#[test]
fn shl_wide_max_0_1() {
assert_eq!(
Uint::shl_vartime_wide((U128::MAX, U128::ZERO), 1),
(U128::MAX.sbb(&U128::ONE, Limb::ZERO).0, U128::ONE)
);
}
#[test]
fn shl_wide_max_max_256() {
assert_eq!(
Uint::shl_vartime_wide((U128::MAX, U128::MAX), 256),
(U128::ZERO, U128::ZERO)
);
}
}
|
