Day 13

Author: Alextopher

Cargo.lock

@@ -43,7 +43,7 @@ members = [
     "y23/d10-pipe-maze",
     "y23/d11-cosmic-expansion",
     "y23/d12-hot-springs",
-    "y23/d13",
+    "y23/d13-point-of-incidence",
     "y23/d14",
     "y23/d15",
     "y23/d16",

Cargo.toml

@@ -10,4 +10,4 @@ aoc-macro = { path = "../aoc-macro" }
 
 binary-heap-plus = "0.5.0"
 itertools = "0.12.0"
-num-traits = "0.2.17"
+num-traits = "0.2.17"

aoc/Cargo.toml

@@ -167,4 +167,31 @@ pub trait IterJunk: Iterator {
     {
         self.k_largest_by(k, |a, b| f(a).cmp(&f(b)))
     }
+
+    /// `partial_sums` is a function that returns an iterator over the partial sums of the iterator.
+    ///
+    /// The first element is the first element of the iterator, the second element is the sum of the
+    /// first two elements, and so on.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use aoc::iterstuff::IterJunk;
+    ///
+    /// let v = vec![1, 2, 3, 4, 5];
+    /// let sums: Vec<i32> = v.into_iter().partial_sums().collect();
+    ///
+    /// assert_eq!(sums, vec![1, 3, 6, 10, 15]);
+    /// ```
+    #[must_use = "iterators are lazy and do nothing unless consumed"]
+    fn partial_sums(self) -> impl Iterator<Item = <Self as Iterator>::Item>
+    where
+        Self: Sized,
+        Self::Item: std::ops::AddAssign + Copy + Default,
+    {
+        self.scan(Default::default(), |sum, x| {
+            *sum += x;
+            Some(*sum)
+        })
+    }
 }

aoc/src/iterstuff.rs

@@ -6,3 +6,5 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+aoc = { path = "../../aoc" }
+itertools = "0.12.0"

y23/d13/Cargo.toml renamed to y23/d13-point-of-incidence/Cargo.toml

@@ -0,0 +1,207 @@
+use std::iter::repeat;
+
+use aoc::{input_str, IterJunk};
+use itertools::Itertools;
+
+// Read in 2d array of ash and rocks
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Tile {
+    Ash,
+    Rock,
+}
+
+impl Tile {
+    fn from_char(c: char) -> Self {
+        match c {
+            '.' => Self::Ash,
+            '#' => Self::Rock,
+            _ => panic!("Invalid tile"),
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Map {
+    tiles: Vec<Tile>,
+    transposed: Vec<Tile>,
+    width: usize,
+    height: usize,
+}
+
+impl Map {
+    fn from_str(s: &str) -> Self {
+        let mut height = 0;
+        let tiles = s
+            .lines()
+            .map(|s| s.chars())
+            .flat_map(|line| {
+                height += 1;
+                line
+            })
+            .map(Tile::from_char)
+            .collect_vec();
+
+        let width = tiles.len() / height;
+
+        let transposed = (0..width)
+            .flat_map(|x| (0..height).zip(repeat(x)))
+            .map(|(y, x)| tiles[y * width + x])
+            .collect_vec();
+
+        Self {
+            tiles,
+            transposed,
+            width,
+            height,
+        }
+    }
+
+    fn get(&self, x: usize, y: usize) -> Tile {
+        self.tiles[y * self.width + x]
+    }
+
+    fn get_row(&self, y: usize) -> &[Tile] {
+        &self.tiles[y * self.width..(y + 1) * self.width]
+    }
+
+    fn get_col(&self, x: usize) -> &[Tile] {
+        &self.transposed[x * self.height..(x + 1) * self.height]
+    }
+
+    fn detect_mirror(&self) -> usize {
+        fn is_mirror((left, right): (&[Tile], &[Tile])) -> bool {
+            left.iter().rev().zip(right.iter()).all(|(l, r)| l == r)
+        }
+
+        if let Some(col) = (1..self.width).find(|&col| {
+            (0..self.height)
+                .map(|r| self.get_row(r).split_at(col))
+                .all(is_mirror)
+        }) {
+            return col;
+        }
+
+        if let Some(row) = (1..self.height).find(|&row| {
+            (0..self.width)
+                .map(|c| self.get_col(c).split_at(row))
+                .all(is_mirror)
+        }) {
+            return 100 * row;
+        }
+
+        panic!("No mirror found");
+    }
+
+    // In part 2, there is a second mirror that is _almost_ correct.
+    // It is off by exactly one tile
+    fn detect_with_smudge(&self, smudges: usize) -> usize {
+        // Helper function that returns how close 2 sets of tiles are to being a mirror
+        // (returns how many tiles are different)
+        fn mirror_diff(left: &[Tile], right: &[Tile]) -> usize {
+            left.iter()
+                .rev()
+                .zip(right.iter())
+                .filter(|(l, r)| l != r)
+                .count()
+        }
+
+        // Find a column that is almost a mirror with a difference of 1
+        if let Some(col) = (1..self.width).find(|&col| {
+            (0..self.height)
+                .map(|r| self.get_row(r).split_at(col))
+                .map(|(left, right)| mirror_diff(left, right))
+                .partial_sums()
+                .find_or_last(|&x| x > smudges)
+                .eq(&Some(smudges))
+        }) {
+            return col;
+        }
+
+        // Find a row that is almost a mirror with a difference of 1
+        if let Some(row) = (1..self.height).find(|&row| {
+            (0..self.width)
+                .map(|c| self.get_col(c).split_at(row))
+                .map(|(left, right)| mirror_diff(left, right))
+                .partial_sums()
+                .find_or_last(|&x| x > smudges)
+                .eq(&Some(smudges))
+        }) {
+            return 100 * row;
+        }
+
+        panic!("No mirror found");
+    }
+}
+
+impl std::fmt::Display for Map {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        for y in 0..self.height {
+            for x in 0..self.width {
+                let c = match self.get(x, y) {
+                    Tile::Ash => '.',
+                    Tile::Rock => '#',
+                };
+                write!(f, "{}", c)?;
+            }
+            writeln!(f)?;
+        }
+        Ok(())
+    }
+}
+
+fn main() {
+    let input = input_str!(2023, 13);
+
+    // Each map uses multiple lines, maps are separated by a blank line
+    let start = std::time::Instant::now();
+    let maps = input.split("\n\n").map(Map::from_str).collect_vec();
+    println!("Parsed: {:?}", start.elapsed());
+
+    // Part 1 - sum the mirror values
+    let start = std::time::Instant::now();
+    let sum: usize = maps.iter().map(|m| m.detect_mirror()).sum();
+    println!("Part 1: {}", sum);
+    println!("Part 1: {:?}", start.elapsed());
+
+    // Part 2 - sum the mirror values with the smudge
+    let start = std::time::Instant::now();
+    let sum: usize = maps.iter().map(|m| m.detect_with_smudge(1)).sum();
+    println!("Part 2: {}", sum);
+    println!("Part 2: {:?}", start.elapsed());
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_map() {
+        let input = "#.##..##.\n..#.##.#.\n##......#\n##......#\n..#.##.#.\n..##..##.\n#.#.##.#.";
+        let map = Map::from_str(input);
+
+        assert_eq!(map.width, 9);
+        assert_eq!(map.height, 7);
+
+        // Detect the mirror
+        assert_eq!(map.detect_mirror(), 5);
+
+        let input = "#...##..#\n#....#..#\n..##..###\n#####.##.\n#####.##.\n..##..###\n#....#..#";
+        let map = Map::from_str(input);
+
+        assert_eq!(map.width, 9);
+        assert_eq!(map.height, 7);
+
+        // Detect the mirror
+        assert_eq!(map.detect_mirror(), 400);
+    }
+
+    #[test]
+    fn test_regressions() {
+        let input = "##.#.##...####.\n#####..#...##..\n##.##.#.#####.#\n.##.#..##..##..\n###..#.#####.#.\n###..#.#####.#.\n.##.#..##..##..\n##.##.#.#####.#\n#####..#...##..\n##.#.##..#####.\n...#...#....###\n##.#.###.###..#\n..#.###.#.#....\n####...#.#.#...\n.#..#....##.#.#\n....####.###.##\n....####.###.##";
+        let map = Map::from_str(input);
+
+        // Detect the mirror (row 4)
+        println!("{} {}", map.width, map.height);
+        assert_eq!(map.detect_mirror(), 1600);
+    }
+}