1 files changed, 71 insertions, 0 deletions
diff --git a/src/bin/2021/day9.rs b/src/bin/2021/day9.rs
new file mode 100644
index 0000000..c0c1b47
--- /dev/null
+++ b/src/bin/2021/day9.rs
@@ -0,0 +1,71 @@
+use advent_of_code::prelude::*;
+
+pub fn parse(fh: File) -> Result<Grid<u8>> {
+    Ok(parse::digit_grid(parse::raw_lines(fh)))
+}
+
+pub fn part1(map: Grid<u8>) -> Result<u64> {
+    let mut risk = 0;
+    for ((row, col), pos) in map.indexed_cells() {
+        if map
+            .adjacent(row, col, false)
+            .map(|(row, col)| map[row][col])
+            .all(|n| *pos < n)
+        {
+            risk += 1 + u64::from(*pos);
+        }
+    }
+    Ok(risk)
+}
+
+pub fn part2(map: Grid<u8>) -> Result<u64> {
+    let mut low = vec![];
+    for ((row, col), pos) in map.indexed_cells() {
+        if map
+            .adjacent(row, col, false)
+            .map(|(row, col)| map[row][col])
+            .all(|n| *pos < n)
+        {
+            low.push((row, col));
+        }
+    }
+
+    let mut sizes = vec![];
+    for (row, col) in low {
+        let mut basin: Grid<bool> = Grid::default();
+        basin.grow(map.rows(), map.cols());
+        let mut check = vec![(row, col)];
+        let mut count = 0;
+        while let Some((row, col)) = check.pop() {
+            if basin[row][col] || map[row][col] == 9 {
+                continue;
+            }
+
+            basin[row][col] = true;
+            count += 1;
+
+            for (row, col) in basin.adjacent(row, col, false) {
+                if !basin[row][col] {
+                    check.push((row, col));
+                }
+            }
+        }
+        sizes.push(count);
+    }
+    sizes.sort_unstable();
+    Ok(sizes[sizes.len() - 1]
+        * sizes[sizes.len() - 2]
+        * sizes[sizes.len() - 3])
+}
+
+#[test]
+fn test() {
+    assert_eq!(
+        part1(parse(parse::data(2021, 9).unwrap()).unwrap()).unwrap(),
+        570
+    );
+    assert_eq!(
+        part2(parse(parse::data(2021, 9).unwrap()).unwrap()).unwrap(),
+        899392
+    );
+}