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// This code is more complicated than it should be. The reason is that
// for part 1 I decided to ignore the start and finish position, and not
// to treat them as other cells. This way I could get away with not
// considering wall cells '#' at all.
// But then for part 2 it was more convenient to include them, so I
// added an enum Position {} that treats start and finish as special
// cases. This is quite messy.
use std::collections::{VecDeque, HashSet};
#[derive(PartialEq, Debug)]
pub enum Direction { Up, Down, Left, Right, None }
impl Direction {
pub fn from_byte(b: u8) -> Direction {
match b {
b'^' => Direction::Up,
b'v' => Direction::Down,
b'>' => Direction::Right,
b'<' => Direction::Left,
b'.' => Direction::None,
x => panic!("unexpected char '{}'", x as char)
}
}
}
#[derive(Hash, Eq, PartialEq, Copy, Clone, Debug)]
pub enum Position {
Start,
Finish,
Other { i: usize, j: usize }
}
impl Position {
pub fn dist(&self, other: Position) -> usize {
match (*self, other) {
(Position::Start, Position::Start) |
(Position::Finish, Position::Finish) =>
0,
(Position::Start, Position::Finish) |
(Position::Finish, Position::Start) =>
usize::MAX, // Not technically correct, but it works
(Position::Start | Position::Finish, Position::Other { i, j }) |
(Position::Other { i, j }, Position::Start | Position::Finish) =>
i + j,
(Position::Other { i: i1, j: j1 },
Position::Other { i: i2, j: j2 }) => {
let i = (i1 as i64 - i2 as i64).abs() as usize;
let j = (j1 as i64 - j2 as i64).abs() as usize;
i + j
}
}
}
}
pub struct Map {
pub tiles: Vec<Vec<Direction>>,
pub imax: usize,
pub jmax: usize
}
#[derive(Hash, Eq, PartialEq, Copy, Clone, Debug)]
struct Node {
p: Position,
t: usize
}
impl Map {
fn submod(i: usize, t: usize, m: usize) -> usize {
let maybeneg = ((i as i64) - (t as i64)) % m as i64;
(maybeneg as usize + m) % m
}
pub fn is_busy(&self, p: Position, t: usize) -> bool {
match p {
Position::Start | Position::Finish => false,
Position::Other { i, j } => {
let iup = Self::submod(i, t, self.imax);
let idown = (i + t) % self.imax;
let jleft = Map::submod(j, t, self.jmax);
let jright = (j + t) % self.jmax;
self.tiles[iup][j] == Direction::Down ||
self.tiles[idown][j] == Direction::Up ||
self.tiles[i][jleft] == Direction::Right ||
self.tiles[i][jright] == Direction::Left
}
}
}
#[allow(dead_code)]
pub fn print(&self, t: usize) {
print!("#.");
for _ in 0..self.jmax { print!("#"); }
println!();
for i in 0..self.imax {
print!("#");
for j in 0..self.jmax {
let p = Position::Other { i, j };
print!("{}", if self.is_busy(p, t) { '*' } else { '.' });
}
println!("#");
}
for _ in 0..self.jmax { print!("#"); }
print!(".#");
println!();
}
fn mv(&self, p: Position, d: Direction) -> Option<Position> {
if d == Direction::None { return Some(p); }
match p {
Position::Start => if d == Direction::Down {
Some(Position::Other { i: 0, j: 0 })
} else { None },
Position::Finish => if d == Direction::Up {
Some(Position::Other { i: self.imax-1, j: self.jmax-1 })
} else { None },
Position::Other { i, j } => match d {
Direction::Up => if i > 0 {
Some(Position::Other { i: i-1, j })
} else {
if i == 0 && j == 0 {
Some(Position::Start)
} else { None }
},
Direction::Down => if i < self.imax-1 {
Some(Position::Other { i: i+1, j })
} else {
if i == self.imax-1 && j == self.jmax-1 {
Some(Position::Finish)
} else { None }
},
Direction::Left => if j > 0 {
Some(Position::Other { i, j: j-1 })
} else { None },
Direction::Right => if j < self.jmax-1 {
Some(Position::Other { i, j: j+1 })
} else { None },
Direction::None => panic!("stupid rust")
}
}
}
fn maybe_push(&self, x: Node, d: Direction, v: &mut HashSet<Node>) {
if let Some(p) = self.mv(x.p, d) {
if !self.is_busy(p, x.t+1) {
v.insert(Node { p, t: x.t+1 });
}
}
}
pub fn shortest_path(
&self,
start: Position,
finish: Position,
t0: usize
) -> usize {
let mut vnext = HashSet::<Node>::new();
vnext.insert(Node { p: start, t: t0 });
while !vnext.is_empty() {
let mut v = vnext.clone().into_iter().collect::<Vec<_>>();
// Heuristic: explore first the positions closer to the target
v.sort_by(|a, b| a.p.dist(finish).cmp(&(b.p.dist(finish))));
let mut v = VecDeque::<Node>::from(v);
vnext.clear();
while !v.is_empty() {
let x = v.pop_front().unwrap();
if x.p == finish { return x.t; }
self.maybe_push(x, Direction::Up, &mut vnext);
self.maybe_push(x, Direction::Down, &mut vnext);
self.maybe_push(x, Direction::Left, &mut vnext);
self.maybe_push(x, Direction::Right, &mut vnext);
if !self.is_busy(x.p, x.t+1) {
self.maybe_push(x, Direction::None, &mut vnext);
}
}
}
panic!("queue is empty");
}
}
pub fn read_map_from_stdin() -> Map {
let mut tiles = vec![];
let mut line = String::new();
while std::io::stdin().read_line(&mut line).unwrap() > 0 {
let bytes = line.as_bytes();
if bytes[2] == b'#' {
line.clear();
continue;
}
let i = tiles.len();
tiles.push(vec![]);
for j in 1..bytes.len()-2 {
tiles[i].push(Direction::from_byte(bytes[j]));
}
line.clear();
}
let imax = tiles.len();
let jmax = tiles[0].len();
Map { tiles, imax, jmax }
}
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