Day 9

src/bin/09.rs

@@ -0,0 +1,92 @@
+fn main() {
+    println!("First solution: {}", input().risk());
+    println!("Second solution: {}", input().part2());
+}
+
+struct Field(Vec<Vec<u32>>);
+
+impl Field {
+    fn risk(&self) -> u32 {
+        let mut risk = 0;
+        for row in 0..self.0.len() {
+            for col in 0..self.0[0].len() {
+                let current = self.0[row][col];
+                if self.neighbor_values(row, col).iter().all(|&n| n > current) {
+                    risk += current + 1;
+                }
+            }
+        }
+        risk
+    }
+
+    fn neighbor_values(&self, row: usize, col: usize) -> Vec<u32> {
+        self.neighbor_positions(row, col)
+            .into_iter()
+            .map(|(row, col)| self.0[row][col])
+            .collect()
+    }
+
+    fn neighbor_positions(&self, row: usize, col: usize) -> Vec<(usize, usize)> {
+        let mut result = Vec::new();
+        if row > 0 {
+            result.push((row - 1, col));
+        }
+        if row < self.0.len() - 1 {
+            result.push((row + 1, col));
+        }
+        if col > 0 {
+            result.push((row, col - 1));
+        }
+        if col < self.0[0].len() - 1 {
+            result.push((row, col + 1));
+        }
+        result
+    }
+
+    fn part2(&mut self) -> u32 {
+        let mut basin_sizes = Vec::new();
+
+        loop {
+            let mut fill_start = None;
+            'search: for row in 0..self.0.len() {
+                for col in 0..self.0[0].len() {
+                    if self.0[row][col] < 9 {
+                        fill_start = Some((row, col));
+                        break 'search;
+                    }
+                }
+            }
+
+            let start = match fill_start {
+                Some(f) => f,
+                None => break,
+            };
+
+            let mut fill_stack = vec![start];
+            let mut size = 0;
+            while let Some((row, col)) = fill_stack.pop() {
+                if self.0[row][col] >= 9 {
+                    continue;
+                }
+                size += 1;
+                self.0[row][col] = 9; // Mark field as processed
+                for (nr, nc) in self.neighbor_positions(row, col) {
+                    fill_stack.push((nr, nc));
+                }
+            }
+            basin_sizes.push(size);
+        }
+
+        basin_sizes.sort_unstable();
+        basin_sizes.into_iter().rev().take(3).product()
+    }
+}
+
+fn input() -> Field {
+    Field(
+        include_str!("../../input/09.txt")
+            .lines()
+            .map(|l| l.chars().map(|c| c.to_digit(10).unwrap()).collect())
+            .collect(),
+    )
+}