12th day

Author: majcn

src/bin/12.rs

@@ -1,16 +1,23 @@
 advent_of_code::solution!(12);
 
 use advent_of_code::maneatingape::grid::*;
+use advent_of_code::maneatingape::hash::*;
 use advent_of_code::maneatingape::point::*;
 
+struct Garden {
+    area: u32,
+    perimeter: u32,
+    edges: FastSet<(Point, Point)>,
+}
+
 fn parse_data(input: &str) -> Grid<u8> {
     Grid::parse(input)
 }
 
-fn find_data_with_flood(grid: &Grid<u8>) -> Vec<(usize, usize)> {
+fn find_gardens(grid: &Grid<u8>) -> Vec<Garden> {
     let mut visited = grid.same_size_with(false);
 
-    let mut result = vec![];
+    let mut gardens = vec![];
 
     for y in 0..grid.height {
         for x in 0..grid.width {
@@ -24,6 +31,8 @@ fn find_data_with_flood(grid: &Grid<u8>) -> Vec<(usize, usize)> {
             let mut area = 0;
             let mut perimeter = 0;
 
+            let mut edges = FastSet::new();
+
             let mut queue = vec![start_location];
             while let Some(position) = queue.pop() {
                 if visited[position] {
@@ -34,49 +43,101 @@ fn find_data_with_flood(grid: &Grid<u8>) -> Vec<(usize, usize)> {
                 area += 1;
                 perimeter += 4;
 
-                for new_position in ORTHOGONAL.map(|o| position + o) {
+                for direction in ORTHOGONAL {
+                    let new_position = position + direction;
                     if grid.contains(new_position) && grid[new_position] == plot {
                         queue.push(new_position);
                         perimeter -= 1;
+                    } else {
+                        edges.insert((position, direction));
                     }
                 }
             }
 
-            result.push((area, perimeter));
+            gardens.push(Garden {
+                area,
+                perimeter,
+                edges,
+            });
         }
     }
 
-    result
+    gardens
+}
+
+fn find_sides(mut edges: FastSet<(Point, Point)>) -> u32 {
+    let next_corner_edge = |edges: &FastSet<(Point, Point)>| {
+        let mut edge = edges.iter().next().copied()?;
+        loop {
+            let new_edge = (edge.0 + edge.1.clockwise(), edge.1);
+            if !edges.contains(&new_edge) {
+                return Some(edge);
+            }
+
+            edge = new_edge;
+        }
+    };
+
+    let mut sides = 0;
+    let mut next_edge = next_corner_edge(&edges);
+    while let Some(edge @ (p, d)) = next_edge {
+        edges.remove(&edge);
+
+        let left_edge = (p + d.counter_clockwise(), d);
+        if edges.contains(&left_edge) {
+            next_edge = Some(left_edge);
+            continue;
+        }
+
+        let right_edge = (p + d.clockwise(), d);
+        if edges.contains(&right_edge) {
+            next_edge = Some(right_edge);
+            continue;
+        }
+
+        next_edge = next_corner_edge(&edges);
+        sides += 1;
+    }
+
+    sides
 }
 
 pub fn part_one(input: &str) -> Option<u32> {
     let grid = parse_data(input);
 
-    let result = find_data_with_flood(&grid)
+    let result = find_gardens(&grid)
         .into_iter()
-        .map(|(area, perimeter)| (area * perimeter) as u32)
+        .map(|garden| garden.area * garden.perimeter)
         .sum();
 
     Some(result)
 }
 
-pub fn part_two(_input: &str) -> Option<u32> {
-    None
+pub fn part_two(input: &str) -> Option<u32> {
+    let grid = parse_data(input);
+
+    let result = find_gardens(&grid)
+        .into_iter()
+        .map(|garden| garden.area * find_sides(garden.edges))
+        .sum();
+
+    Some(result)
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
-
     #[test]
     fn test_part_one() {
-        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        let input = advent_of_code::template::read_file("examples", DAY);
+        let result = part_one(&input);
         assert_eq!(result, Some(1930));
     }
 
     #[test]
     fn test_part_two() {
-        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
-        assert_eq!(result, None);
+        let input = advent_of_code::template::read_file("examples", DAY);
+        let result = part_two(&input);
+        assert_eq!(result, Some(1206));
     }
 }