solution: day 12

data/examples/12.txt

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

src/bin/12.rs

@@ -0,0 +1,109 @@
+use hashbrown::HashSet;
+use itertools::Itertools;
+
+fn count_sides(perim: &mut [(isize, isize)]) -> usize {
+    perim.sort();
+
+    let mut sides = 0;
+
+    let mut k = 0;
+    while k < perim.len() {
+        let (mut i, mut j) = perim[k];
+
+        while let Some((ni, nj)) = perim.get(k + 1).copied() {
+            if i != ni || nj - j != 3 {
+                break;
+            }
+
+            (i, j) = (ni, nj);
+            k += 1;
+        }
+
+        sides += 1;
+        k += 1;
+    }
+
+    sides
+}
+
+pub fn solve(input: &str) -> (usize, usize) {
+    let grid = input.lines().map(|x| x.chars().collect_vec()).collect_vec();
+
+    let mut price = 0;
+    let mut discounted = 0;
+
+    let mut stack = Vec::new();
+    let mut visited = HashSet::new();
+
+    let mut ver_sides = Vec::new();
+    let mut hor_sides = Vec::new();
+
+    for p in (0..grid.len()).flat_map(|x| (0..grid[x].len()).map(move |y| (x, y))) {
+        if visited.contains(&p) {
+            continue;
+        }
+
+        let matcher = grid[p.0][p.1];
+
+        let mut area = 0;
+        ver_sides.clear();
+        hor_sides.clear();
+
+        stack.push(p);
+        while let Some((i, j)) = stack.pop() {
+            if !visited.insert((i, j)) {
+                continue;
+            }
+            area += 1;
+
+            for (di, dj) in [(0, 1), (1, 0), (-1, 0), (0, -1)] {
+                let ni = i.wrapping_add(di as usize);
+                let nj = j.wrapping_add(dj as usize);
+
+                let fi = i as isize * 3 + di;
+                let fj = j as isize * 3 + dj;
+
+                match grid.get(ni).and_then(|row| row.get(nj)) {
+                    Some(m) if *m == matcher => stack.push((ni, nj)),
+                    _ if dj == 0 => hor_sides.push((fi, fj)),
+                    _ if dj != 0 => ver_sides.push((fj, fi)),
+                    _ => unreachable!(),
+                }
+            }
+        }
+
+        price += area * (hor_sides.len() + ver_sides.len());
+        discounted += area * (count_sides(&mut hor_sides) + count_sides(&mut ver_sides));
+    }
+
+    (price, discounted)
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    solve(input).0.into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    solve(input).1.into()
+}
+
+aoc::solution!(12);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(
+            part_one(&aoc::template::read_file("examples", 12)),
+            Some(1930)
+        );
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(
+            part_two(&aoc::template::read_file("examples", 12)),
+            Some(1206)
+        );
+    }
+}