1 files changed, 90 insertions, 0 deletions
diff --git a/2021/day9/src/lib.rs b/2021/day9/src/lib.rs
new file mode 100644
index 0000000..3391a74
--- /dev/null
+++ b/2021/day9/src/lib.rs
@@ -0,0 +1,90 @@
+use std::str::FromStr;
+
+#[derive(Debug)]
+struct Heightmap {
+    data: Vec<Vec<u8>>,
+    rows: usize,
+    cols: usize,
+}
+
+impl FromStr for Heightmap {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let rows: Vec<_> = s.trim().split('\n').collect::<Vec<&str>>();
+        let mut data: Vec<Vec<u8>> = vec![];
+        for row in &rows {
+            data.push(
+                row.chars()
+                    .map(|x| x.to_digit(10).unwrap() as u8)
+                    .collect::<Vec<u8>>(),
+            );
+        }
+        Ok(Heightmap {
+            data,
+            rows: rows.len(),
+            cols: rows[0].chars().count(),
+        })
+    }
+}
+
+impl Heightmap {
+    fn resolve(&self) -> usize {
+        let mut count: usize = 0;
+
+        for i in 0..self.rows {
+            for j in 0..self.cols {
+                let top = if i > 0 { self.data[i - 1][j] } else { 10 };
+
+                let bottom = if i < self.rows - 1 {
+                    self.data[i + 1][j]
+                } else {
+                    10
+                };
+
+                let left = if j > 0 { self.data[i][j - 1] } else { 10 };
+
+                let right = if j < self.cols - 1 {
+                    self.data[i][j + 1]
+                } else {
+                    10
+                };
+                let center = self.data[i][j];
+
+                if center < top && center < bottom && center < left && center < right {
+                    count += (center + 1) as usize;
+                }
+            }
+        }
+
+        count
+    }
+}
+
+pub fn part1(input: &str) -> usize {
+    let grid: Heightmap = input.parse().unwrap();
+    let result = grid.resolve();
+
+    result
+}
+
+#[cfg(test)]
+mod day9_test {
+    use super::*;
+
+    #[test]
+    fn test_with_example_data() {
+        let data = include_str!("../example.txt");
+        let result = part1(data);
+
+        assert_eq!(result, 15);
+    }
+
+    #[test]
+    fn test_puzzle_input_part1() {
+        let data = include_str!("../input.txt");
+        let result = part1(data);
+
+        assert_eq!(result, 541);
+    }
+}