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[code] small improvements

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* readability: changing variable names/adding variables for clarity
* initialize primes Vec with reasonable space to reduce allocations
* return type of find_next_prime more idiomatic (Option)
* avoid overflow by using checked_mul
This commit is contained in:
ddidderr 2024-02-09 22:38:11 +01:00
parent 2005b7d5ca
commit 1a7e57404e
Signed by: ddidderr
GPG Key ID: 3841F1C27E6F0E14
1 changed files with 14 additions and 9 deletions
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23
src/sieve.rs
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@ -1,7 +1,7 @@
pub fn get_primes(max_nr: usize) -> Vec<u64> {
let mut sieve = vec![true; max_nr + 1];
let mut primes = Vec::new();
let mut primes = Vec::with_capacity(1_000_000);
sieve[0] = false;
sieve[1] = false;
@ -12,7 +12,7 @@ pub fn get_primes(max_nr: usize) -> Vec<u64> {
loop {
check_off_multiples_of_nr(nr, &mut sieve);
if !find_next_prime(&mut nr, &mut sieve) {
if find_next_prime(&mut nr, &mut sieve).is_none() {
break;
}
@ -22,24 +22,29 @@ pub fn get_primes(max_nr: usize) -> Vec<u64> {
primes
}
fn find_next_prime(nr: &mut u64, sieve: &mut [bool]) -> bool {
fn find_next_prime(nr: &mut u64, sieve: &mut [bool]) -> Option<u64> {
let start_idx = usize::try_from(*nr).ok()? + 1;
if let Some((idx, _)) = sieve
.iter()
.enumerate()
.skip(usize::try_from(*nr).unwrap() + 1)
.find(|(_, val)| **val)
.skip(start_idx)
.find(|&(_, &is_prime)| is_prime)
{
*nr = idx as u64;
return true;
return Some(*nr);
}
false
None
}
fn check_off_multiples_of_nr(nr: u64, sieve: &mut [bool]) {
let nr = usize::try_from(nr).unwrap();
sieve
.iter_mut()
.skip(usize::try_from(nr * 2).unwrap())
.step_by(usize::try_from(nr).unwrap())
.skip(nr.checked_mul(2).unwrap())
.step_by(nr)
.for_each(|nr| *nr = false);
}