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factor::miller_rabin: Avoid repeatedly transforming 1 and -1

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Approx. 25% speedup
This commit is contained in:
nicoo 2020-06-15 23:04:46 +02:00
parent cb6051c580
commit 4851619d62
1 changed files with 7 additions and 4 deletions
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11
src/uu/factor/src/miller_rabin.rs
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@ -38,6 +38,9 @@ pub(crate) fn test<A: Arithmetic>(m: A) -> Result {
let i = (n - 1).trailing_zeros(); let i = (n - 1).trailing_zeros();
let r = (n - 1) >> i; let r = (n - 1) >> i;
let one = m.one();
let minus_one = m.minus_one();
for _a in BASIS.iter() { for _a in BASIS.iter() {
let _a = _a % n; let _a = _a % n;
if _a == 0 { if _a == 0 {
@ -55,21 +58,21 @@ pub(crate) fn test<A: Arithmetic>(m: A) -> Result {
for _ in 0..i { for _ in 0..i {
y = m.mul(y, y) y = m.mul(y, y)
} }
if y != m.one() { if y != one {
return Pseudoprime; return Pseudoprime;
}; };
} }
if x == m.one() || x == m.minus_one() { if x == one || x == minus_one {
break; break;
} }
loop { loop {
let y = m.mul(x, x); let y = m.mul(x, x);
if y == m.one() { if y == one {
return Composite(gcd(m.to_u64(x) - 1, m.modulus())); return Composite(gcd(m.to_u64(x) - 1, m.modulus()));
} }
if y == m.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; break;