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|
use nom::{
branch::alt,
bytes::complete::tag,
character::complete::{alpha1, char as nom_char, line_ending},
combinator::{map, value},
multi::{many1, separated_list1},
sequence::{pair, preceded, separated_pair, terminated},
IResult,
};
use std::{collections::BTreeMap, fs, ops::Range};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let input = fs::read_to_string("inputs/day_8.txt")?;
let mut directions = Directions::parser(&input).unwrap().1;
dbg!(directions.steps_from_a_to_z());
dbg!(directions.ghost_steps_from_a_to_z());
Ok(())
}
#[derive(Debug)]
struct Directions {
turns: Vec<Turn>,
packed_map: Vec<PackedFork>,
packed_ghost_starts: Range<u16>,
packed_ghost_destinations: Range<u16>,
distance_to_ghost_dest_cache: BTreeMap<(u16, usize), (u16, usize)>,
}
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Turn {
Left,
Right,
}
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
struct Location(String);
#[derive(Debug)]
struct Fork {
left: Location,
right: Location,
}
#[derive(Debug)]
struct PackedFork {
left: u16,
right: u16,
}
impl Directions {
fn parser(input: &str) -> IResult<&str, Self> {
map(
separated_pair(
many1(Turn::parser),
pair(line_ending, line_ending),
separated_list1(
line_ending,
separated_pair(Location::parser, tag(" = "), Fork::parser),
),
),
|(turns, map)| {
let map: BTreeMap<Location, Fork> = map.into_iter().collect();
let mut locations: Vec<Location> = map.keys().cloned().collect();
locations.sort_by_key(|l| l.0.chars().rev().collect::<String>());
Directions {
turns,
packed_map: locations
.iter()
.map(|l| {
let unpacked_fork = map.get(l).unwrap();
PackedFork {
left: locations
.iter()
.position(|f| *f == unpacked_fork.left)
.unwrap() as u16,
right: locations
.iter()
.position(|f| *f == unpacked_fork.right)
.unwrap() as u16,
}
})
.collect(),
packed_ghost_starts: 0
..locations.iter().position(|l| !l.ghost_start()).unwrap() as u16,
packed_ghost_destinations: locations.iter().position(|l| l.ghost_end()).unwrap()
as u16
..locations.len() as u16,
distance_to_ghost_dest_cache: BTreeMap::new(),
}
},
)(input)
}
fn steps_from_a_to_z(&self) -> usize {
let mut step_count = 0;
let mut current_location: u16 = 0;
for dir in self.turns.iter().cycle() {
let current_fork: &PackedFork = &self.packed_map[current_location as usize];
current_location = match dir {
Turn::Left => current_fork.left,
Turn::Right => current_fork.right,
};
step_count += 1;
if current_location == self.packed_map.len() as u16 - 1 {
return step_count;
}
}
unreachable!()
}
fn ghost_steps_from_a_to_z(&mut self) -> usize {
let mut current_locations: Vec<(u16, usize)> = self
.packed_ghost_starts
.clone()
.map(|start| self.ghost_step_to_next_end(start, 0))
.collect();
let mut any_stepped = true;
while any_stepped {
any_stepped = false;
let current_max = current_locations
.iter()
.map(|(_, steps)| steps.clone())
.max()
.unwrap();
for current_location in &mut current_locations {
if current_location.1 < current_max {
any_stepped = true;
let (new_location, extra_steps) =
self.ghost_step_to_next_end(current_location.0, current_location.1);
current_location.0 = new_location;
current_location.1 += extra_steps;
}
}
}
current_locations[0].1
}
fn ghost_step_to_next_end(&mut self, start: u16, current_step_count: usize) -> (u16, usize) {
self.distance_to_ghost_dest_cache
.entry((start, current_step_count % self.turns.len()))
.or_insert_with(|| {
let mut step_count = 0;
let mut current_location: u16 = start;
for dir in self
.turns
.iter()
.skip(current_step_count % self.turns.len())
.cycle()
{
let current_fork: &PackedFork = &self.packed_map[current_location as usize];
current_location = match dir {
Turn::Left => current_fork.left,
Turn::Right => current_fork.right,
};
step_count += 1;
if self.packed_ghost_destinations.contains(¤t_location) {
break;
}
}
(current_location, step_count)
})
.clone()
}
}
impl Turn {
fn parser(input: &str) -> IResult<&str, Self> {
alt((
value(Turn::Left, nom_char('L')),
value(Turn::Right, nom_char('R')),
))(input)
}
}
impl Location {
fn parser(input: &str) -> IResult<&str, Self> {
map(alpha1, |l: &str| Location(l.to_string()))(input)
}
fn ghost_start(&self) -> bool {
self.0.ends_with("A")
}
fn ghost_end(&self) -> bool {
self.0.ends_with("Z")
}
}
impl Fork {
fn parser(input: &str) -> IResult<&str, Self> {
map(
preceded(
tag("("),
terminated(
separated_pair(Location::parser, tag(", "), Location::parser),
tag(")"),
),
),
|(left, right)| Fork { left, right },
)(input)
}
}
|
