mirror of
https://github.com/bertptrs/adventofcode.git
synced 2025-12-26 05:10:32 +01:00
Bert Peters
859034b09d
Using 32 bit integers is significantly faster due to better memory locality, although it requires a bit of casting to move between indices and values as needed.
62 lines
1.5 KiB
Rust
62 lines
1.5 KiB
Rust
use std::io::Read;
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use crate::common::numbers_and_stuff;
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use crate::Solution;
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fn nth_number(start: &[u32], n: u32) -> u32 {
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// Don't want to special case this
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assert!(n as usize >= start.len());
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let mut history = vec![0; n as usize];
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let mut prev = 0;
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for (i, &n) in start.iter().enumerate() {
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history[prev as usize] = i as u32;
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prev = n;
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}
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for i in (start.len() as u32)..n {
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let last_seen = history[prev as usize];
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history[prev as usize] = i;
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prev = if last_seen == 0 { 0 } else { i - last_seen };
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}
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prev
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}
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#[derive(Default)]
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pub struct Day15;
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impl Solution for Day15 {
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fn part1(&mut self, input: &mut dyn Read) -> String {
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let numbers: Vec<u32> = numbers_and_stuff(input);
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nth_number(&numbers, 2020).to_string()
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}
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fn part2(&mut self, input: &mut dyn Read) -> String {
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let numbers: Vec<u32> = numbers_and_stuff(input);
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nth_number(&numbers, 30000000).to_string()
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn test_nth_number() {
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assert_eq!(0, nth_number(&[0, 3, 6], 10));
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assert_eq!(436, nth_number(&[0, 3, 6], 2020));
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assert_eq!(1, nth_number(&[1, 3, 2], 2020));
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assert_eq!(10, nth_number(&[2, 1, 3], 2020));
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assert_eq!(27, nth_number(&[1, 2, 3], 2020));
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assert_eq!(78, nth_number(&[2, 3, 1], 2020));
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assert_eq!(438, nth_number(&[3, 2, 1], 2020));
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assert_eq!(1836, nth_number(&[3, 1, 2], 2020));
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}
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}
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