65 lines
1.5 KiB
Rust
65 lines
1.5 KiB
Rust
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#![forbid(elided_lifetimes_in_paths, unsafe_code)]
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use aoc23::read;
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use chumsky::{prelude::*, text::int};
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struct Game {
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time: i64,
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distance: i64
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}
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fn parser() -> impl Parser<char, Vec<Game>, Error = Simple<char>> {
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let int_list = just(" ")
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.repeated()
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.ignore_then(int(10).separated_by(just(" ").repeated().at_least(1)));
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just("Time:")
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.ignore_then(int_list)
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.then_ignore(just("\nDistance:"))
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.then(int_list)
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.map(|(times, dists): (Vec<String>, Vec<String>)| {
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times
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.into_iter()
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.zip(dists)
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.map(|(time, distance)| Game {
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time: time.parse().unwrap(),
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distance: distance.parse().unwrap()
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})
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.collect()
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})
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.then_ignore(just("\n"))
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.then_ignore(end())
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}
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fn main() -> anyhow::Result<()> {
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let games = read("inputs/day6.txt", parser())?;
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let mut result = 1;
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for game in &games {
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let t = game.time;
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let d = game.distance + 1;
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// we want the minimum a s.t. (t-a)*a > d,
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// and the maximum a s.t. (t-a)*a < d
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// that means we have a quadratic function -a² + ta - d and we want to find
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// its intersections with the axis
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let discriminant = t * t - 4 * d;
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if discriminant < 0 {
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continue;
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}
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let sqrt = (discriminant as f64).sqrt();
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let min = ((t as f64 - sqrt) / 2.0).ceil() as i64;
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let max = ((t as f64 + sqrt) / 2.0).floor() as i64;
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eprintln!(
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"t={t}, d={d}, min={min} ({}), max={max} ({})",
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(t - min) * min,
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(t - max) * max
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);
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assert!(max >= min);
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result *= max - min + 1;
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}
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println!("{result}");
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Ok(())
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}
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