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use std::collections::HashMap;
static DAY: u8 = 8;
fn main() {
let input = advent::read_lines(DAY);
println!("{DAY}a: {}", required_steps(&input));
println!("{DAY}b: {}", required_steps_ghost(&input));
}
#[derive(Clone, Copy)]
enum Direction {
Left,
Right,
}
impl Direction {
fn from(c: char) -> Direction {
match c {
'L' => Direction::Left,
'R' => Direction::Right,
_ => panic!("invalid direction"),
}
}
}
struct Choice {
left: String,
right: String,
}
impl Choice {
fn from(input: &str) -> Choice {
/* "(AAA, ZZZ)" */
let tmp = input.chars()
.skip(1)
.take(input.len()-2)
.collect::<String>();
let (left, right) = tmp.split_once(", ").unwrap();
Choice { left: left.to_string(), right: right.to_string() }
}
}
struct Map {
instructions: Vec<Direction>,
map: HashMap<String, Choice>,
}
impl Map {
fn new(input: &[String]) -> Map {
let instructions = input[0].chars()
.map(Direction::from)
.collect();
let mut map = HashMap::new();
for line in input.iter().skip(2) {
let (from, to) = line.split_once(" = ").unwrap();
let choice = Choice::from(to);
map.insert(from.to_string(), choice);
}
Map { instructions, map }
}
fn get_direction(&self, step: usize) -> Direction {
let i = step % self.instructions.len();
self.instructions[i]
}
fn next_pos(&self, pos: &str, steps: usize) -> String {
let choice = self.map.get(pos).unwrap();
match self.get_direction(steps) {
Direction::Left => choice.left.clone(),
Direction::Right => choice.right.clone(),
}
}
fn number_steps(&self, from: &str, to: &str) -> usize {
let mut steps = 0;
let mut pos = from.to_string();
while pos != to {
pos = self.next_pos(&pos, steps);
steps += 1;
}
steps
}
fn number_steps_to_z(&self, from: &str) -> (usize, String) {
let mut steps = 0;
let mut pos = from.to_string();
loop {
pos = self.next_pos(&pos, steps);
steps += 1;
if pos.ends_with('Z') {
break;
}
}
(steps, pos)
}
fn number_steps_ghost(&self) -> usize {
let positions = self.map.keys()
.filter(|pos| pos.ends_with('A'))
.cloned()
.collect::<Vec<_>>();
let distances_a_to_z = positions.iter()
.map(|pos| self.number_steps_to_z(pos))
.collect::<Vec<_>>();
let distances_z_to_z = distances_a_to_z.iter()
.map(|(_, pos)| self.number_steps_to_z(pos))
.collect::<Vec<_>>();
/* it turns out that distances from xxA to yyZ are the same
as from yyZ to yyZ; so no need to deal with offsets. */
assert_eq!(distances_a_to_z, distances_z_to_z);
let cycle_lens = distances_a_to_z.iter()
.map(|(steps, _)| *steps)
.collect::<Vec<_>>();
let mut steps = lcm(cycle_lens[0], cycle_lens[1]);
for cycle_len in cycle_lens.iter().skip(2) {
steps = lcm(steps, *cycle_len);
}
steps
}
}
fn gcd(a: usize, b: usize) -> usize {
if b == 0 {
a
} else {
gcd(b, a % b)
}
}
fn lcm(a: usize, b: usize) -> usize {
(a*b) / gcd(a, b)
}
fn required_steps(input: &[String]) -> usize {
let map = Map::new(input);
map.number_steps("AAA", "ZZZ")
}
fn required_steps_ghost(input: &[String]) -> usize {
let map = Map::new(input);
map.number_steps_ghost()
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test() {
let input = [
"RL",
"",
"AAA = (BBB, CCC)",
"BBB = (DDD, EEE)",
"CCC = (ZZZ, GGG)",
"DDD = (DDD, DDD)",
"EEE = (EEE, EEE)",
"GGG = (GGG, GGG)",
"ZZZ = (ZZZ, ZZZ)",
].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
assert_eq!(required_steps(&input), 2);
let input = [
"LLR",
"",
"AAA = (BBB, BBB)",
"BBB = (AAA, ZZZ)",
"ZZZ = (ZZZ, ZZZ)",
].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
assert_eq!(required_steps(&input), 6);
let input = [
"LR",
"",
"11A = (11B, XXX)",
"11B = (XXX, 11Z)",
"11Z = (11B, XXX)",
"22A = (22B, XXX)",
"22B = (22C, 22C)",
"22C = (22Z, 22Z)",
"22Z = (22B, 22B)",
"XXX = (XXX, XXX)",
].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
assert_eq!(required_steps_ghost(&input), 6);
}
}
|
