Day 16

Author: ochsnerd

src/day15.rs

@@ -2,7 +2,7 @@ use core::fmt;
 
 use crate::{
     grid::{Direction, Grid, GridParseError, GridParser, Index, Point, Vector},
-    util::bfs,
+    graph::bfs,
 };
 
 pub fn solve(input: &str) -> (usize, usize) {

src/day16.rs

@@ -0,0 +1,169 @@
+use itertools::Itertools;
+
+use crate::{
+    graph::{dijkstra, dijkstra_all},
+    grid::{Direction, Grid, GridParseError, GridParser, Index},
+    util::IteratorExt,
+};
+
+pub fn solve(input: &str) -> (usize, usize) {
+    let (start, end, map) = parse(input).unwrap();
+
+    // println!(
+    //     "{}",
+    //     map.display(
+    //         |_| None,
+    //         |t| match t {
+    //             Thing::Wall => '#',
+    //             Thing::Floor => '.',
+    //         }
+    //     )
+    // );
+
+    let (end_dir, cost) = part1(start.clone(), end, &map);
+    let count_on_best_path = part2(
+        start,
+        Pose {
+            position: end,
+            heading: end_dir,
+        },
+        &map,
+    );
+
+    (cost, count_on_best_path)
+}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+struct Pose {
+    position: Index,
+    heading: Direction,
+}
+
+impl Ord for Pose {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        let pos: (_, _) = self.position.into();
+        pos.cmp(&other.position.into())
+            .then_with(|| self.heading.cmp(&other.heading))
+    }
+}
+
+impl PartialOrd for Pose {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(&other))
+    }
+}
+
+#[derive(Hash, PartialEq, Eq, Clone, Copy)]
+enum Thing {
+    Wall,
+    Floor,
+}
+
+fn cost(path: &[Pose]) -> usize {
+    path.iter()
+        .tuple_windows()
+        .map(|(p1, p2)| if p1.heading != p2.heading { 1000 } else { 1 })
+        .sum()
+}
+
+fn part1(start: Pose, end: Index, map: &Grid<Thing>) -> (Direction, usize) {
+    // need to check all 4 possible end-headings
+    Direction::all()
+        .iter()
+        .map(|&d| Pose {
+            position: end,
+            heading: d,
+        })
+        .map(|e| {
+            (
+                e.heading,
+                dijkstra(start.clone(), |p| movements(p, map), e)
+                    .unwrap() // we're sure there's a way to the end
+                    .collect::<Vec<_>>(),
+            )
+        })
+        .map(|(dir, path)| (dir, cost(&path)))
+        .min_by_key(|(_, cost)| *cost)
+        .unwrap() // we know the iterator is not empty
+}
+
+fn movements(pose: &Pose, map: &Grid<Thing>) -> impl IntoIterator<Item = (usize, Pose)> {
+    let straight = pose.position.neighbor(pose.heading);
+    [
+        map.at(straight).and_then(|t| match t {
+            Thing::Floor => Some((
+                1,
+                Pose {
+                    position: straight,
+                    heading: pose.heading,
+                },
+            )),
+            Thing::Wall => None,
+        }),
+        Some((
+            1000,
+            Pose {
+                position: pose.position,
+                heading: pose.heading.clockwise(),
+            },
+        )),
+        Some((
+            1000,
+            Pose {
+                position: pose.position,
+                heading: pose.heading.anti_clockwise(),
+            },
+        )),
+    ]
+    .into_iter()
+    .filter_map(|p| p)
+}
+
+fn part2(start: Pose, end: Pose, map: &Grid<Thing>) -> usize {
+    dijkstra_all(start, |p| movements(p, map), end)
+        .into_iter()
+        .flatten()
+        .map(|p| p.position)
+        .uniques()
+        .count()
+}
+
+fn parse(input: &str) -> Result<(Pose, Index, Grid<Thing>), GridParseError> {
+    let map = GridParser::new(|c| match c {
+        '#' => Ok(Thing::Wall),
+        '.' => Ok(Thing::Floor),
+        'S' => Ok(Thing::Floor),
+        'E' => Ok(Thing::Floor),
+        _ => Err(GridParseError),
+    })
+    .parse(input)?;
+
+    let start = input
+        .chars()
+        .filter(|c| !c.is_whitespace())
+        .enumerate()
+        .filter_map(|(i, c)| match c {
+            'S' => Some(map.make_index(i)),
+            _ => None,
+        })
+        .next()
+        .ok_or(GridParseError)?;
+
+    let start = Pose {
+        position: start,
+        heading: Direction::Right,
+    };
+
+    let end = input
+        .chars()
+        .filter(|c| !c.is_whitespace())
+        .enumerate()
+        .filter_map(|(i, c)| match c {
+            'E' => Some(map.make_index(i)),
+            _ => None,
+        })
+        .next()
+        .ok_or(GridParseError)?;
+
+    Ok((start, end, map))
+}