2024 day 12

Author: ictrobot

crates/year2024/examples/day12_example0.txt

@@ -0,0 +1,4 @@
+AAAA
+BBCD
+BBCC
+EEEC

crates/year2024/examples/day12_example1.txt

@@ -0,0 +1,5 @@
+OOOOO
+OXOXO
+OOOOO
+OXOXO
+OOOOO

crates/year2024/examples/day12_example2.txt

@@ -0,0 +1,10 @@
+RRRRIICCFF
+RRRRIICCCF
+VVRRRCCFFF
+VVRCCCJFFF
+VVVVCJJCFE
+VVIVCCJJEE
+VVIIICJJEE
+MIIIIIJJEE
+MIIISIJEEE
+MMMISSJEEE

crates/year2024/examples/day12_example3.txt

@@ -0,0 +1,5 @@
+EEEEE
+EXXXX
+EEEEE
+EXXXX
+EEEEE

crates/year2024/examples/day12_example4.txt

@@ -0,0 +1,6 @@
+AAAAAA
+AAABBA
+AAABBA
+ABBAAA
+ABBAAA
+AAAAAA

crates/year2024/src/day12.rs

@@ -0,0 +1,128 @@
+use utils::grid;
+use utils::prelude::*;
+
+/// Counting area, perimeter and sides of shapes in a grid.
+#[derive(Clone, Debug)]
+pub struct Day12 {
+    grid: Vec<u8>,
+    offsets: [isize; 4],
+}
+
+impl Day12 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let (_, cols, grid) =
+            grid::from_str_padded(input, 2, 0, |b| b.is_ascii_uppercase().then_some(b))?;
+        let offsets = [-(cols as isize), 1, cols as isize, -1];
+        Ok(Self { grid, offsets })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        let mut visited = vec![false; self.grid.len()];
+        let mut total = 0;
+        for i in 0..self.grid.len() {
+            if self.grid[i] == 0 || visited[i] {
+                continue;
+            }
+
+            let (area, perimeter) = self.flood_fill(i, &mut visited);
+            total += area * perimeter;
+        }
+        total
+    }
+
+    fn flood_fill(&self, i: usize, visited: &mut [bool]) -> (u64, u64) {
+        let plant = self.grid[i];
+        visited[i] = true;
+
+        let (mut area, mut perimeter) = (1, 0);
+        for &offset in &self.offsets {
+            let next = i.wrapping_add_signed(offset);
+            if self.grid[next] == plant {
+                if !visited[next] {
+                    let (a, p) = self.flood_fill(next, visited);
+                    area += a;
+                    perimeter += p;
+                }
+            } else {
+                perimeter += 1;
+            }
+        }
+
+        (area, perimeter)
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u64 {
+        let mut visited = vec![false; self.grid.len()];
+        let mut edges = vec![[false; 4]; self.grid.len()];
+        let mut total = 0;
+        for i in 0..self.grid.len() {
+            if self.grid[i] == 0 || visited[i] {
+                continue;
+            }
+
+            let (area, min_idx, max_idx) = self.edge_fill(i, &mut visited, &mut edges);
+
+            let mut sides = 0;
+            for dir in 0..4 {
+                for j in min_idx..=max_idx {
+                    if edges[j][dir] {
+                        sides += 1;
+                        self.edge_unset(j, dir, &mut edges);
+                    }
+                }
+            }
+
+            total += area * sides;
+        }
+        total
+    }
+
+    fn edge_fill(
+        &self,
+        i: usize,
+        visited: &mut [bool],
+        edges: &mut [[bool; 4]],
+    ) -> (u64, usize, usize) {
+        let plant = self.grid[i];
+        visited[i] = true;
+
+        let (mut area, mut min_idx, mut max_idx) = (1, usize::MAX, 0);
+        for dir in 0..4 {
+            let next = i.wrapping_add_signed(self.offsets[dir]);
+            if self.grid[next] == plant {
+                if !visited[next] {
+                    let r = self.edge_fill(next, visited, edges);
+                    area += r.0;
+                    min_idx = min_idx.min(r.1);
+                    max_idx = max_idx.max(r.2);
+                }
+            } else {
+                edges[next][dir] = true;
+                min_idx = min_idx.min(next);
+                max_idx = max_idx.max(next);
+            }
+        }
+
+        (area, min_idx, max_idx)
+    }
+
+    fn edge_unset(&self, i: usize, dir: usize, edges: &mut [[bool; 4]]) {
+        edges[i][dir] = false;
+        for offset in [self.offsets[(dir + 1) % 4], self.offsets[(dir + 3) % 4]] {
+            let next = i.wrapping_add_signed(offset);
+            if edges[next][dir] {
+                self.edge_unset(next, dir, edges);
+            }
+        }
+    }
+}
+
+examples!(Day12 -> (u64, u64) [
+    {file: "day12_example0.txt", part1: 140, part2: 80},
+    {file: "day12_example1.txt", part1: 772},
+    {file: "day12_example2.txt", part1: 1930, part2: 1206},
+    {file: "day12_example3.txt", part2: 236},
+    {file: "day12_example4.txt", part2: 368},
+]);

crates/year2024/src/lib.rs

@@ -13,4 +13,5 @@ utils::year!(2024 => year2024, ${
     9 => day09::Day09<'_>,
     10 => day10::Day10,
     11 => day11::Day11,
+    12 => day12::Day12,
 });