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2017 primality test performance improvement #10

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Merged
merged 2 commits into from
Jul 8, 2025
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2017 primality test performance improvement #10

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e50ebb4
perf: skip even numbers in primality test -> 2 will always be a facto…
Rolv-Apneseth Jul 8, 2025
6e39cc3
perf: more efficient square root limit for the primality test
Rolv-Apneseth Jul 8, 2025
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7 changes: 5 additions & 2 deletions src/year2017/day23.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -51,7 +51,7 @@
//! [`Day 18`]: crate::year2017::day18
use crate::util::parse::*;

/// We only need the vrey first number from the input.
/// We only need the very first number from the input.
pub fn parse(input: &str) -> u32 {
input.unsigned()
}
Expand All @@ -69,7 +69,10 @@ pub fn part2(input: &u32) -> usize {
/// Simple [prime number check](https://en.wikipedia.org/wiki/Primality_test)
/// of all factors from 2 to √n inclusive.
fn composite(n: u32) -> Option<u32> {
for f in 2..=n.isqrt() {
if n % 2 == 0 {
return Some(n);
};
for f in (3..).step_by(2).take_while(|m| m * m <= n) {
if n % f == 0 {
return Some(n);
}
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