I'm bad at this

2024/rust/src/day12.rs

@@ -1,4 +1,5 @@
 mod prelude;
+
 pub use crate::prelude::*;
 
 fn main() {
@@ -6,7 +7,6 @@ fn main() {
     show_answers(part1(&input), part2(&input));
 }
 
-#[derive(Debug, Hash, PartialEq, Eq)]
 enum Dir {
     N,
     S,
@@ -17,7 +17,7 @@ enum Dir {
 #[derive(Debug)]
 struct Region {
     locations: HashSet<(usize, usize)>,
-    perimeters: HashSet<(usize, usize, Dir)>,
+    perimeters: usize,
 }
 
 impl Region {
@@ -26,24 +26,24 @@ impl Region {
         let (height, width) = (bytes.len(), bytes[0].len());
         let mut candidates = vec![(sx, sy)];
         let mut locations = HashSet::from_iter(candidates.clone());
-        let mut perimeters = HashSet::new();
+        let mut perimeters = 0;
 
         while let Some((x, y)) = candidates.pop() {
             print!("AT {x},{y}: ");
-            for (nx, ny, d) in [
-                (x, y.saturating_sub(1), Dir::N),
-                (x, y + 1, Dir::S),
-                (x + 1, y, Dir::E),
-                (x.saturating_sub(1), y, Dir::W),
+            for (nx, ny) in [
+                (x, y.saturating_sub(1)),
+                (x, y + 1),
+                (x + 1, y),
+                (x.saturating_sub(1), y),
             ] {
                 if (nx, ny) == (x, y) || nx >= width || ny >= height {
                     print!("EDGE@{nx},{ny}; ");
-                    perimeters.insert((nx, ny, d));
+                    perimeters += 1;
                     continue;
                 }
                 if bytes[ny][nx] != target {
                     print!("EDGE@{nx},{ny}; ");
-                    perimeters.insert((nx, ny, d));
+                    perimeters += 1;
                     continue;
                 }
                 if !locations.contains(&(nx, ny)) {
@@ -60,7 +60,7 @@ impl Region {
     }
 }
 
-fn part1(input: &str) -> usize {
+fn regions(input: &str) -> Vec<Region> {
     let mut regions = vec![];
     let bytes: Vec<Vec<u8>> = input
         .trim()
@@ -74,25 +74,71 @@ fn part1(input: &str) -> usize {
                 continue;
             }
             let region = Region::measure_at(&bytes, x, y);
-            println!(
-                "New Region: (Byte: {}, Area: {}, Perimeter: {})",
-                bytes[y][x],
-                &region.locations.len(),
-                &region.perimeters.len(),
-            );
             regionated.extend(&region.locations);
             regions.push(region);
         }
     }
-    println!("{regions:?}");
     regions
+}
+
+fn part1(input: &str) -> usize {
+    regions(input)
         .iter()
-        .map(|r| r.locations.len() * r.perimeters.len())
+        .map(|r| r.locations.len() * r.perimeters)
         .sum()
 }
 
 fn part2(input: &str) -> usize {
-    0
+    regions(input)
+        .iter()
+        .map(|r| {
+            println!("{r:?}");
+            // find top edge in region
+            if r.locations.len() <= 2 {
+                return r.locations.len() * 4;
+            }
+            let (sx, sy) = {
+                let (x, mut y) = *r.locations.iter().next().unwrap();
+                y = (1..y)
+                    .rev()
+                    .find(|y| r.locations.contains(&(x, y - 1)))
+                    .unwrap_or(0);
+                (x, y)
+            };
+
+            // walk along region edge and count turns until we end up where we started
+            let mut turns = 0;
+            let ops = [
+                |(x, y)| (x + 1, y),
+                |(x, y)| (x, y + 1),
+                |(x, y): (usize, usize)| (x.saturating_sub(1), y),
+                |(x, y): (usize, usize)| (x, y.saturating_sub(1)),
+            ];
+            let mut dir = 0;
+            let (mut x, mut y) = (sx, sy);
+            println!("start at {x},{y}");
+            loop {
+                let initial_dir = dir;
+                while !r.locations.contains(&ops[dir]((x, y))) || ops[dir]((x, y)) == (x, y) {
+                    println!("turning @ {x},{y}");
+                    dir = (dir + 1) % ops.len();
+                    if dir == initial_dir {
+                        panic!("turned too many times in one place");
+                    }
+                    turns += 1;
+                }
+                (x, y) = ops[dir]((x, y));
+                println!("move to {x},{y}");
+                if turns >= 3 && x == sx && y == sy {
+                    break;
+                }
+            }
+            println!("turns {turns}");
+
+            // walk each side, counting turns
+            r.locations.len() * turns
+        })
+        .sum()
 }
 
 #[cfg(test)]
@@ -118,7 +164,7 @@ MMMISSJEEE"#;
             .collect();
         let region = Region::measure_at(&bytes, 0, 0);
         assert_eq!(region.locations.len(), 12);
-        assert_eq!(region.perimeters.len(), 18);
+        assert_eq!(region.perimeters, 18);
         assert_eq!(part1(input), 1930);
         assert_eq!(part2(input), 1206);
     }