173 lines
3.6 KiB
Rust
173 lines
3.6 KiB
Rust
use std::fs;
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fn main() -> Result<(), Box<dyn std::error::Error>> {
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let input = fs::read_to_string("input")?;
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let (time, ids) = parse_notes(&input)?;
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// Part 1
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let (earliest, arrival) = find_closest_bus(time, &ids);
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println!("{}", (arrival - time) * earliest);
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// Part 2
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let timestamp = find_gauss(&ids);
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println!("{}", timestamp);
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Ok(())
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}
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fn parse_notes(input: &str) -> Result<(u32, Vec<u32>), &str> {
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let mut lines = input.lines();
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let estimate = lines
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.next()
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.ok_or("Input too short")?
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.parse()
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.map_err(|_| "Estimate is not a number")?;
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let ids = lines
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.next()
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.ok_or("Input too short")?
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.split(',')
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.map(|id| id.parse().unwrap_or(1))
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.collect();
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Ok((estimate, ids))
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}
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fn find_closest_bus(time: u32, ids: &Vec<u32>) -> (u32, u32) {
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ids.iter()
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.filter(|id| **id != 1)
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.map(|id| {
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let mut arrival = *id;
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while arrival < time {
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arrival += id
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}
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(*id, arrival)
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})
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.min_by_key(|id| id.1)
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.expect("Must have at least one bus ID")
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}
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fn find_gauss(ids: &Vec<u32>) -> u128 {
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let ids: Vec<u128> = ids.iter().map(|id| *id as u128).collect();
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let product: u128 = ids.iter().product();
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let gauss = |mut n: u128, mut d: u128, m: u128| -> u128 {
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while d != 1 {
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if m == 1 {
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return 0;
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};
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let multiplier = if m > d {
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m / d + (m % d != 0) as u128
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} else {
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1
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};
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n = (n * multiplier) % m;
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d = (d * multiplier) % m;
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}
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n / d
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};
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(0..ids.len())
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.map(|a| {
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let b = product / ids[a];
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let bi = gauss(1, b, ids[a]);
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let a = (ids[a] * a as u128 - a as u128) % ids[a] as u128;
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a * b * bi
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})
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.sum::<u128>()
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% product
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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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use indoc::indoc;
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#[test]
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fn test_simple() -> Result<(), Box<dyn std::error::Error>> {
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let input = indoc!(
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"939
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7,13,x,x,59,x,31,19
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"
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);
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let (time, ids) = parse_notes(input)?;
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let (earliest, arrival) = find_closest_bus(time, &ids);
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assert_eq!(arrival, 944);
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Ok(())
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}
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#[test]
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fn test_simple2() -> Result<(), Box<dyn std::error::Error>> {
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let input = indoc!(
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"939
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17,x,13,19
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"
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);
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let (time, ids) = parse_notes(input)?;
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let timestamp = find_gauss(&ids);
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assert_eq!(timestamp, 3417);
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Ok(())
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}
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#[test]
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fn test_simple3() -> Result<(), Box<dyn std::error::Error>> {
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let input = indoc!(
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"939
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7,13,x,x,59,x,31,19
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"
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);
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let (time, ids) = parse_notes(input)?;
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let timestamp = find_gauss(&ids);
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assert_eq!(timestamp, 1068781);
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Ok(())
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}
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#[test]
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fn test_simple4() -> Result<(), Box<dyn std::error::Error>> {
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let input = indoc!(
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"939
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67,7,59,61
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"
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);
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let (time, ids) = parse_notes(input)?;
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let timestamp = find_gauss(&ids);
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assert_eq!(timestamp, 754018);
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Ok(())
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}
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#[test]
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fn test_simple5() -> Result<(), Box<dyn std::error::Error>> {
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let input = indoc!(
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"939
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67,x,7,59,61
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"
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);
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let (time, ids) = parse_notes(input)?;
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let timestamp = find_gauss(&ids);
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assert_eq!(timestamp, 779210);
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Ok(())
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}
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}
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