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Merge pull request #1562 from nbraud/factor/faster/miller-rabbin

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factor::miller_rabin: minor refactoring
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Roy Ivy III 2020-07-24 21:54:09 -05:00 committed by GitHub
commit c04c7a14e3
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1 changed files with 7 additions and 16 deletions
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23
src/uu/factor/src/miller_rabin.rs
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@ -25,6 +25,7 @@ impl Basis for Montgomery<u32> {
} }
#[derive(Eq, PartialEq)] #[derive(Eq, PartialEq)]
#[must_use = "Ignoring the output of a primality test."]
pub(crate) enum Result { pub(crate) enum Result {
Prime, Prime,
Pseudoprime, Pseudoprime,
@ -54,7 +55,7 @@ pub(crate) fn test<A: Arithmetic + Basis>(m: A) -> Result {
let one = m.one(); let one = m.one();
let minus_one = m.minus_one(); let minus_one = m.minus_one();
for _a in A::BASIS.iter() { 'witness: for _a in A::BASIS.iter() {
let _a = _a % n; let _a = _a % n;
if _a == 0 { if _a == 0 {
continue; continue;
@ -65,33 +66,23 @@ pub(crate) fn test<A: Arithmetic + Basis>(m: A) -> Result {
// x = a^r mod n // x = a^r mod n
let mut x = m.pow(a, r); let mut x = m.pow(a, r);
{
// y = ((x²)²...)² i times = x ^ (2ⁱ) = a ^ (r 2ⁱ) = x ^ (n - 1)
let mut y = x;
for _ in 0..i {
y = m.mul(y, y)
}
if y != one {
return Pseudoprime;
};
}
if x == one || x == minus_one { if x == one || x == minus_one {
continue; continue;
} }
loop { for _ in 1..i {
let y = m.mul(x, x); let y = m.mul(x, x);
if y == one { if y == one {
return Composite(gcd(m.to_u64(x) - 1, m.modulus())); return Composite(gcd(m.to_u64(x) - 1, m.modulus()));
} } else if y == minus_one {
if y == minus_one {
// This basis element is not a witness of `n` being composite. // This basis element is not a witness of `n` being composite.
// Keep looking. // Keep looking.
break; continue 'witness;
} }
x = y; x = y;
} }
return Pseudoprime;
} }
Prime Prime