Implement 2023 day 10 part 2

2023/src/common.rs

@@ -7,6 +7,7 @@ use std::ops::Index;
 use std::ops::IndexMut;
 use std::ops::Sub;
 
+use anyhow::Context;
 use anyhow::Result;
 use nom::combinator::map;
 use nom::error::ErrorKind;
@@ -53,6 +54,7 @@ pub fn parse_input<'a, O>(
 ///
 /// This implementation is based on [`nom::multi::fold_many1`] with minor differences. If
 /// successful, this should probably be upstreamed.
+#[allow(unused)]
 pub fn reduce_many1<I, O, E, F>(
     mut f: F,
     mut g: impl FnMut(O, O) -> O,
@@ -104,6 +106,7 @@ where
 }
 
 /// Add an index to repeated successful invocations of the embedded parser.
+#[allow(unused)]
 pub fn enumerate<I, O, E>(f: impl Parser<I, O, E>) -> impl FnMut(I) -> IResult<I, (usize, O), E> {
     let mut index = 0usize;
 
@@ -115,6 +118,7 @@ pub fn enumerate<I, O, E>(f: impl Parser<I, O, E>) -> impl FnMut(I) -> IResult<I
 }
 
 /// Return the minimum and maximum of two unordered variables
+#[allow(unused)]
 pub fn minmax<T>(a: T, b: T) -> (T, T)
 where
     T: PartialOrd,
@@ -127,6 +131,7 @@ where
 }
 
 /// Some magic to get two mutable references into the same slice
+#[allow(unused)]
 pub fn get_both<T>(slice: &mut [T], first: usize, second: usize) -> (&mut T, &mut T) {
     match first.cmp(&second) {
         Ordering::Greater => {
@@ -186,9 +191,11 @@ impl IndexSet {
     }
 }
 
+#[allow(unused)]
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Vec2(pub [i32; 2]);
 
+#[allow(unused)]
 impl Vec2 {
     pub fn l1(self) -> i32 {
         self.0.into_iter().map(i32::abs).sum()
@@ -234,3 +241,54 @@ impl IndexMut<usize> for Vec2 {
         &mut self.0[index]
     }
 }
+
+pub struct Grid<'a> {
+    width: usize,
+    data: &'a [u8],
+}
+
+impl<'a> Grid<'a> {
+    pub fn new(data: &'a [u8]) -> anyhow::Result<Self> {
+        let width = 1 + data
+            .iter()
+            .position(|&c| c == b'\n')
+            .context("Failed to find end of line in grid")?;
+
+        anyhow::ensure!(
+            data.len() % width == 0,
+            "Grid should divide equally into rows"
+        );
+
+        Ok(Self { width, data })
+    }
+
+    pub fn height(&self) -> usize {
+        self.data.len() / self.width
+    }
+
+    pub fn width(&self) -> usize {
+        self.width - 1
+    }
+
+    pub fn rows(&self) -> impl Iterator<Item = &'a [u8]> {
+        let width = self.width();
+        self.data
+            .chunks_exact(self.width)
+            .map(move |row| &row[..width])
+    }
+
+    pub fn find(&self, c: u8) -> Option<(usize, usize)> {
+        let pos = self.data.iter().position(|&d| d == c)?;
+
+        Some((pos % self.width, pos / self.width))
+    }
+}
+
+impl<'a> Index<usize> for Grid<'a> {
+    type Output = [u8];
+
+    fn index(&self, y: usize) -> &Self::Output {
+        let offset = y * self.width;
+        &self.data[offset..(offset + self.width())]
+    }
+}

2023/src/day03.rs

@@ -1,52 +1,6 @@
 use std::collections::HashMap;
-use std::ops::Index;
 
-use anyhow::Context;
-
-struct Grid<'a> {
-    width: usize,
-    data: &'a [u8],
-}
-
-impl<'a> Grid<'a> {
-    pub fn new(data: &'a [u8]) -> anyhow::Result<Self> {
-        let width = 1 + data
-            .iter()
-            .position(|&c| c == b'\n')
-            .context("Failed to find end of line in grid")?;
-
-        anyhow::ensure!(
-            data.len() % width == 0,
-            "Grid should divide equally into rows"
-        );
-
-        Ok(Self { width, data })
-    }
-
-    pub fn height(&self) -> usize {
-        self.data.len() / self.width
-    }
-
-    pub fn width(&self) -> usize {
-        self.width - 1
-    }
-
-    pub fn rows(&self) -> impl Iterator<Item = &'a [u8]> {
-        let width = self.width();
-        self.data
-            .chunks_exact(self.width)
-            .map(move |row| &row[..width])
-    }
-}
-
-impl<'a> Index<usize> for Grid<'a> {
-    type Output = [u8];
-
-    fn index(&self, y: usize) -> &Self::Output {
-        let offset = y * self.width;
-        &self.data[offset..(offset + self.width())]
-    }
-}
+use crate::common::Grid;
 
 fn is_surrounded(grid: &Grid<'_>, y: usize, start: usize, last: usize) -> bool {
     fn is_symbol(c: u8) -> bool {

2023/src/day10.rs

@@ -1,7 +1,144 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::collections::VecDeque;
+
+use anyhow::Context;
+
+use crate::common::Grid;
+use crate::common::IndexSet;
+
+const LEFT: u8 = 1;
+const RIGHT: u8 = 2;
+const UP: u8 = 4;
+const DOWN: u8 = 8;
+
+fn get_connections(c: u8) -> u8 {
+    match c {
+        b'|' => UP | DOWN,
+        b'-' => LEFT | RIGHT,
+        b'F' => DOWN | RIGHT,
+        b'J' => LEFT | UP,
+        b'L' => RIGHT | UP,
+        b'7' => LEFT | DOWN,
+        _ => 0,
+    }
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+fn find_cycle(map: &Grid<'_>) -> anyhow::Result<(usize, bool, IndexSet)> {
+    let (start_x, start_y) = map.find(b'S').context("Couldn't find starting point")?;
+    let mut visited = IndexSet::with_capacity(map.width() * map.height());
+    let mut todo = VecDeque::new();
+
+    visited.insert(start_y * map.width() + start_x);
+
+    let mut start_up = false;
+
+    if start_x > 0 && (get_connections(map[start_y][start_x - 1]) & RIGHT) != 0 {
+        todo.push_back((1, (start_x - 1, start_y)));
+        visited.insert(start_y * map.width() + start_x - 1);
+    }
+
+    if start_x + 1 < map.width() && (get_connections(map[start_y][start_x + 1]) & LEFT) != 0 {
+        todo.push_back((1, (start_x + 1, start_y)));
+        visited.insert(start_y * map.width() + start_x + 1);
+    }
+
+    if start_y > 0 && (get_connections(map[start_y - 1][start_x]) & DOWN) != 0 {
+        todo.push_back((1, (start_x, start_y - 1)));
+        visited.insert((start_y - 1) * map.width() + start_x);
+        start_up = true;
+    }
+
+    if start_y + 1 < map.height() && (get_connections(map[start_y + 1][start_x]) & DOWN) != 0 {
+        todo.push_back((1, (start_x, start_y + 1)));
+        visited.insert((start_y + 1) * map.width() + start_x);
+    }
+
+    let mut max_dist = 1;
+
+    while let Some((dist, (x, y))) = todo.pop_front() {
+        let mut enqueue = |x, y| {
+            if visited.insert(y * map.width() + x) {
+                todo.push_back((dist + 1, (x, y)));
+                // Can elide comparison because we do a BFS and length is strictly increasing
+                max_dist = dist + 1;
+            }
+        };
+
+        let connections = get_connections(map[y][x]);
+
+        if (connections & LEFT) != 0 {
+            enqueue(x - 1, y);
+        }
+
+        if (connections & RIGHT) != 0 {
+            enqueue(x + 1, y);
+        }
+
+        if (connections & UP) != 0 {
+            enqueue(x, y - 1);
+        }
+
+        if (connections & DOWN) != 0 {
+            enqueue(x, y + 1);
+        }
+    }
+
+    Ok((max_dist, start_up, visited))
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let map = Grid::new(input)?;
+
+    find_cycle(&map).map(|(max_dist, _, _)| max_dist.to_string())
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    let map = Grid::new(input)?;
+    let (_, s_up, visited) = find_cycle(&map)?;
+
+    let mut inside = 0;
+
+    for (y, row) in map.rows().enumerate() {
+        let y_offset = y * map.width();
+        let mut pipes = 0;
+
+        for (x, &c) in row.iter().enumerate() {
+            if visited.contains(y_offset + x) {
+                let is_up = match c {
+                    b'|' | b'J' | b'L' => true,
+                    b'S' => s_up,
+                    _ => false,
+                };
+
+                if is_up {
+                    pipes += 1;
+                }
+            } else {
+                inside += pipes % 2;
+            }
+        }
+    }
+
+    Ok(inside.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/10.1.txt");
+    const SAMPLE2: &[u8] = include_bytes!("samples/10.2.txt");
+    const SAMPLE3: &[u8] = include_bytes!("samples/10.3.txt");
+    const SAMPLE4: &[u8] = include_bytes!("samples/10.4.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("8", part1(SAMPLE).unwrap());
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!("4", part2(SAMPLE2).unwrap());
+        assert_eq!("8", part2(SAMPLE3).unwrap());
+        assert_eq!("10", part2(SAMPLE4).unwrap());
+    }
 }

2023/src/samples/10.1.txt

@@ -0,0 +1,5 @@
+..F7.
+.FJ|.
+SJ.L7
+|F--J
+LJ...

2023/src/samples/10.2.txt

@@ -0,0 +1,9 @@
+...........
+.S-------7.
+.|F-----7|.
+.||.....||.
+.||.....||.
+.|L-7.F-J|.
+.|..|.|..|.
+.L--J.L--J.
+...........

2023/src/samples/10.3.txt

@@ -0,0 +1,10 @@
+.F----7F7F7F7F-7....
+.|F--7||||||||FJ....
+.||.FJ||||||||L7....
+FJL7L7LJLJ||LJ.L-7..
+L--J.L7...LJS7F-7L7.
+....F-J..F7FJ|L7L7L7
+....L7.F7||L7|.L7L7|
+.....|FJLJ|FJ|F7|.LJ
+....FJL-7.||.||||...
+....L---J.LJ.LJLJ...

2023/src/samples/10.4.txt

@@ -0,0 +1,10 @@
+FF7FSF7F7F7F7F7F---7
+L|LJ||||||||||||F--J
+FL-7LJLJ||||||LJL-77
+F--JF--7||LJLJ7F7FJ-
+L---JF-JLJ.||-FJLJJ7
+|F|F-JF---7F7-L7L|7|
+|FFJF7L7F-JF7|JL---7
+7-L-JL7||F7|L7F-7F7|
+L.L7LFJ|||||FJL7||LJ
+L7JLJL-JLJLJL--JLJ.L