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Add more bit-counting functions #282

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merged 1 commit into from
Oct 14, 2023
Merged

Add more bit-counting functions #282

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5 changes: 5 additions & 0 deletions src/limb/bits.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,4 +15,9 @@ impl Limb {
pub const fn trailing_zeros(self) -> usize {
self.0.trailing_zeros() as usize
}

/// Calculate the number of trailing ones the binary representation of this number.
pub const fn trailing_ones(self) -> usize {
self.0.trailing_ones() as usize
}
}
176 changes: 163 additions & 13 deletions src/uint/bits.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,15 +12,14 @@ impl<const LIMBS: usize> Uint<LIMBS> {
}

/// Calculate the number of bits needed to represent this number.
#[allow(trivial_numeric_casts)]
pub const fn bits_vartime(&self) -> usize {
let mut i = LIMBS - 1;
while i > 0 && self.limbs[i].0 == 0 {
i -= 1;
}

let limb = self.limbs[i].0;
Limb::BITS * (i + 1) - limb.leading_zeros() as usize
let limb = self.limbs[i];
Limb::BITS * (i + 1) - limb.leading_zeros()
}

/// Calculate the number of leading zeros in the binary representation of this number.
Expand All @@ -42,6 +41,26 @@ impl<const LIMBS: usize> Uint<LIMBS> {
count as usize
}

/// Calculate the number of leading zeros in the binary representation of this number,
/// variable time in `self`.
pub const fn leading_zeros_vartime(&self) -> usize {
let limbs = self.as_limbs();

let mut count = 0;
let mut i = LIMBS;
while i > 0 {
i -= 1;
let l = limbs[i];
let z = l.leading_zeros();
count += z;
if z != Limb::BITS {
break;
}
}

count
}

/// Calculate the number of trailing zeros in the binary representation of this number.
pub const fn trailing_zeros(&self) -> usize {
let limbs = self.as_limbs();
Expand All @@ -61,6 +80,65 @@ impl<const LIMBS: usize> Uint<LIMBS> {
count as usize
}

/// Calculate the number of trailing zeros in the binary representation of this number,
/// variable time in `self`.
pub const fn trailing_zeros_vartime(&self) -> usize {
let limbs = self.as_limbs();

let mut count = 0;
let mut i = 0;
while i < LIMBS {
let l = limbs[i];
let z = l.trailing_zeros();
count += z;
if z != Limb::BITS {
break;
}
i += 1;
}

count
}

/// Calculate the number of trailing ones in the binary representation of this number.
pub const fn trailing_ones(&self) -> usize {
let limbs = self.as_limbs();

let mut count: Word = 0;
let mut i = 0;
let mut nonmax_limb_not_encountered = CtChoice::TRUE;
while i < LIMBS {
let l = limbs[i];
let z = l.trailing_ones() as Word;
count += nonmax_limb_not_encountered.if_true(z);
nonmax_limb_not_encountered =
nonmax_limb_not_encountered.and(Limb::ct_eq(l, Limb::MAX));
i += 1;
}

count as usize
}

/// Calculate the number of trailing ones in the binary representation of this number,
/// variable time in `self`.
pub const fn trailing_ones_vartime(&self) -> usize {
let limbs = self.as_limbs();

let mut count = 0;
let mut i = 0;
while i < LIMBS {
let l = limbs[i];
let z = l.trailing_ones();
count += z;
if z != Limb::BITS {
break;
}
i += 1;
}

count
}

/// Calculate the number of bits needed to represent this number.
pub const fn bits(&self) -> usize {
Self::BITS - self.leading_zeros()
Expand Down Expand Up @@ -145,37 +223,109 @@ mod tests {
#[test]
fn leading_zeros() {
let u = uint_with_bits_at(&[256 - 16, 256 - 79, 256 - 207]);
assert_eq!(u.leading_zeros() as u32, 15);
assert_eq!(u.leading_zeros(), 15);

let u = uint_with_bits_at(&[256 - 79, 256 - 207]);
assert_eq!(u.leading_zeros(), 78);

let u = uint_with_bits_at(&[256 - 207]);
assert_eq!(u.leading_zeros(), 206);

let u = uint_with_bits_at(&[256 - 1, 256 - 75, 256 - 150]);
assert_eq!(u.leading_zeros(), 0);

let u = U256::ZERO;
assert_eq!(u.leading_zeros(), 256);
}

#[test]
fn leading_zeros_vartime() {
let u = uint_with_bits_at(&[256 - 16, 256 - 79, 256 - 207]);
assert_eq!(u.leading_zeros_vartime(), 15);

let u = uint_with_bits_at(&[256 - 79, 256 - 207]);
assert_eq!(u.leading_zeros() as u32, 78);
assert_eq!(u.leading_zeros_vartime(), 78);

let u = uint_with_bits_at(&[256 - 207]);
assert_eq!(u.leading_zeros() as u32, 206);
assert_eq!(u.leading_zeros_vartime(), 206);

let u = uint_with_bits_at(&[256 - 1, 256 - 75, 256 - 150]);
assert_eq!(u.leading_zeros() as u32, 0);
assert_eq!(u.leading_zeros_vartime(), 0);

let u = U256::ZERO;
assert_eq!(u.leading_zeros() as u32, 256);
assert_eq!(u.leading_zeros_vartime(), 256);
}

#[test]
fn trailing_zeros() {
let u = uint_with_bits_at(&[16, 79, 150]);
assert_eq!(u.trailing_zeros() as u32, 16);
assert_eq!(u.trailing_zeros(), 16);

let u = uint_with_bits_at(&[79, 150]);
assert_eq!(u.trailing_zeros(), 79);

let u = uint_with_bits_at(&[150, 207]);
assert_eq!(u.trailing_zeros(), 150);

let u = uint_with_bits_at(&[0, 150, 207]);
assert_eq!(u.trailing_zeros(), 0);

let u = U256::ZERO;
assert_eq!(u.trailing_zeros(), 256);
}

#[test]
fn trailing_zeros_vartime() {
let u = uint_with_bits_at(&[16, 79, 150]);
assert_eq!(u.trailing_zeros_vartime(), 16);

let u = uint_with_bits_at(&[79, 150]);
assert_eq!(u.trailing_zeros() as u32, 79);
assert_eq!(u.trailing_zeros_vartime(), 79);

let u = uint_with_bits_at(&[150, 207]);
assert_eq!(u.trailing_zeros() as u32, 150);
assert_eq!(u.trailing_zeros_vartime(), 150);

let u = uint_with_bits_at(&[0, 150, 207]);
assert_eq!(u.trailing_zeros() as u32, 0);
assert_eq!(u.trailing_zeros_vartime(), 0);

let u = U256::ZERO;
assert_eq!(u.trailing_zeros() as u32, 256);
assert_eq!(u.trailing_zeros_vartime(), 256);
}

#[test]
fn trailing_ones() {
let u = !uint_with_bits_at(&[16, 79, 150]);
assert_eq!(u.trailing_ones(), 16);

let u = !uint_with_bits_at(&[79, 150]);
assert_eq!(u.trailing_ones(), 79);

let u = !uint_with_bits_at(&[150, 207]);
assert_eq!(u.trailing_ones(), 150);

let u = !uint_with_bits_at(&[0, 150, 207]);
assert_eq!(u.trailing_ones(), 0);

let u = U256::MAX;
assert_eq!(u.trailing_ones(), 256);
}

#[test]
fn trailing_ones_vartime() {
let u = !uint_with_bits_at(&[16, 79, 150]);
assert_eq!(u.trailing_ones_vartime(), 16);

let u = !uint_with_bits_at(&[79, 150]);
assert_eq!(u.trailing_ones_vartime(), 79);

let u = !uint_with_bits_at(&[150, 207]);
assert_eq!(u.trailing_ones_vartime(), 150);

let u = !uint_with_bits_at(&[0, 150, 207]);
assert_eq!(u.trailing_ones_vartime(), 0);

let u = U256::MAX;
assert_eq!(u.trailing_ones_vartime(), 256);
}

#[test]
Expand Down