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use std::{cmp::Ordering, collections::HashSet, fmt::Display, str::FromStr};
use crate::{Problem, Solution};
type Unit = isize;
type Position = (Unit, Unit);
enum Motion {
Up(Unit),
Down(Unit),
Left(Unit),
Right(Unit),
}
impl FromStr for Motion {
type Err = anyhow::Error;
fn from_str(s: &str) -> Result<Self, Self::Err> {
match s.split_once(' ') {
Some(("U", n)) => Ok(Motion::Up(n.parse()?)),
Some(("D", n)) => Ok(Motion::Down(n.parse()?)),
Some(("L", n)) => Ok(Motion::Left(n.parse()?)),
Some(("R", n)) => Ok(Motion::Right(n.parse()?)),
_ => Err(anyhow::format_err!("Improper motion format: {}", s)),
}
}
}
impl Display for Motion {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match self {
Motion::Up(n) => write!(f, "U {}", n),
Motion::Down(n) => write!(f, "D {}", n),
Motion::Left(n) => write!(f, "L {}", n),
Motion::Right(n) => write!(f, "R {}", n),
}
}
}
#[derive(Clone)]
enum List {
Cons(Position, Box<List>),
None,
}
impl List {
fn new() -> Self {
Self::None
}
fn prepend(self) -> List {
Self::Cons(Default::default(), Box::new(self))
}
fn update_from_motion(&mut self, motion: Motion) -> Vec<Position> {
let Self::Cons(position, next) = self else {
panic!("update_from_motion must be called on a head with at least one tail.");
};
let (dx, dy, count) = match motion {
Motion::Up(n) => (0, 1, n),
Motion::Down(n) => (0, -1, n),
Motion::Left(n) => (-1, 0, n),
Motion::Right(n) => (1, 0, n),
};
let mut visited = Vec::new();
for _ in 0..count {
position.0 += dx;
position.1 += dy;
visited.push(next.as_mut().update_from_position(position));
}
visited
}
fn update_from_position(&mut self, head: &Position) -> Position {
let Self::Cons(position, next) = self else {
return *head;
};
if position.0.abs_diff(head.0) > 1 || position.1.abs_diff(head.1) > 1 {
match position.0.cmp(&head.0) {
Ordering::Less => position.0 += 1,
Ordering::Equal => {}
Ordering::Greater => position.0 -= 1,
}
match position.1.cmp(&head.1) {
Ordering::Less => position.1 += 1,
Ordering::Equal => {}
Ordering::Greater => position.1 -= 1,
}
}
next.as_mut().update_from_position(position)
}
}
pub struct Day09;
impl Problem for Day09 {
const DAY: u8 = 9;
const INPUT: &'static str = include_str!("../input/day_9.txt");
}
impl Solution for Day09 {
type Answer1 = usize;
type Answer2 = usize;
fn part_1(input: &str) -> Result<Self::Answer1, anyhow::Error> {
let mut head = List::new();
head = head.prepend();
head = head.prepend();
input
.lines()
.flat_map(Motion::from_str)
.flat_map(|motion| head.update_from_motion(motion))
.try_fold(HashSet::new(), |mut set, pos| {
set.insert(pos);
Ok(set)
})
.map(|s| s.len())
}
fn part_2(input: &str) -> Result<Self::Answer2, anyhow::Error> {
let mut head = List::new();
for _ in 0..10 {
head = head.prepend();
}
input
.lines()
.flat_map(Motion::from_str)
.flat_map(|motion| head.update_from_motion(motion))
.try_fold(HashSet::new(), |mut set, pos| {
set.insert(pos);
Ok(set)
})
.map(|s| s.len())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_part_1_example() -> Result<(), anyhow::Error> {
let test_input = indoc::indoc! {r#"
R 4
U 4
L 3
D 1
R 4
D 1
L 5
R 2
"#};
Ok(assert_eq!(13, Day09::part_1(test_input)?))
}
#[test]
fn test_part_2_example() -> Result<(), anyhow::Error> {
let test_input = indoc::indoc! {r#"
R 5
U 8
L 8
D 3
R 17
D 10
L 25
U 20
"#};
Ok(assert_eq!(36, Day09::part_2(test_input)?))
}
}
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