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Faster random_mod #50

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Sc00bz wants to merge 3 commits into from
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Faster random_mod #50

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bcf8d72
Faster random_mod for Limb
Sc00bz Nov 16, 2021
423bf15
Faster random_mod for UInt<LIMBS>
Sc00bz Nov 16, 2021
b31b976
Fixed random_mod for UInt<LIMBS>
Sc00bz Nov 16, 2021
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15 changes: 5 additions & 10 deletions src/limb/rand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,20 +23,15 @@ impl RandomMod for Limb {
fn random_mod(mut rng: impl CryptoRng + RngCore, modulus: &NonZero<Self>) -> Self {
let mut bytes = <Self as Encoding>::Repr::default();

// TODO(tarcieri): use `div_ceil` when available
// See: https://github.com/rust-lang/rust/issues/88581
let mut n_bytes = modulus.bits() / 8;

// Ensure the randomly generated value can always be larger than
// the modulus in order to ensure a uniform distribution
if n_bytes < Self::BYTE_SIZE {
n_bytes += 1;
}
let n_bits = modulus.bits();
let n_bytes = (n_bits + 7) / 8;
let mask = 0xff >> (8 * n_bytes - n_bits);

loop {
rng.fill_bytes(&mut bytes[..n_bytes]);
let n = Limb::from_le_bytes(bytes);
bytes[0] = bytes[0] & mask;

let n = Limb::from_be_bytes(bytes);
if n.ct_lt(modulus).into() {
return n;
}
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23 changes: 5 additions & 18 deletions src/uint/rand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -35,28 +35,15 @@ impl<const LIMBS: usize> RandomMod for UInt<LIMBS> {
fn random_mod(mut rng: impl CryptoRng + RngCore, modulus: &NonZero<Self>) -> Self {
let mut n = Self::ZERO;

// TODO(tarcieri): use `div_ceil` when available
// See: https://github.com/rust-lang/rust/issues/88581
let mut n_limbs = modulus.bits() / Limb::BIT_SIZE;
if n_limbs < LIMBS {
n_limbs += 1;
}

// Compute the highest limb of `modulus` as a `NonZero`.
// Add one to ensure `Limb::random_mod` returns values inclusive of this limb.
let modulus_hi =
NonZero::new(modulus.limbs[n_limbs.saturating_sub(1)].saturating_add(Limb::ONE))
.unwrap(); // Always at least one due to `saturating_add`
let n_bits = modulus.bits();
let n_limbs = (n_bits + Limb::BIT_SIZE - 1) / Limb::BIT_SIZE;
let mask = Limb(Limb::MAX.0 >> (Limb::BIT_SIZE * n_limbs - n_bits));

loop {
for i in 0..n_limbs {
n.limbs[i] = if (i + 1 == n_limbs) && (*modulus_hi != Limb::MAX) {
// Highest limb
Limb::random_mod(&mut rng, &modulus_hi)
} else {
Limb::random(&mut rng)
}
n.limbs[i] = Limb::random(&mut rng);
}
n.limbs[n_limbs - 1] = n.limbs[n_limbs - 1].bitand(mask);

if n.ct_lt(modulus).into() {
return n;
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