1 files changed, 67 insertions, 0 deletions

diff --git a/2021/day9/src/main.rs b/2021/day9/src/main.rs
new file mode 100644
index 0000000..75bf1e9
--- /dev/null
+++ b/2021/day9/src/main.rs
@@ -0,0 +1,67 @@
+use std::env;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+fn main() {
+    let reader = open_file();
+
+    let mut points: Vec<Vec<u32>> = Vec::new();
+
+    for line in reader.lines() {
+        points.push(
+            line.unwrap()
+                .trim()
+                .chars()
+                .map(|x| x.to_digit(10).unwrap())
+                .collect(),
+        );
+    }
+
+    let nine = vec![9];
+
+    let mut risk_level: u32 = 0;
+
+    for (idx, height) in points.iter().enumerate() {
+        for (ndx, num) in height.iter().enumerate() {
+            let up = points
+                .get(((idx as isize) - 1) as usize)
+                .unwrap_or_else(|| &nine)
+                .get(ndx)
+                .unwrap_or_else(|| &9);
+
+            let down = match points.get(idx + 1) {
+                Some(line) => match line.get(ndx) {
+                    Some(number) => number,
+                    None => &9,
+                },
+                None => &9,
+            };
+            let left = match height.get((ndx as isize - 1) as usize) {
+                Some(number) => number,
+                None => &9,
+            };
+            let right = match height.get(ndx + 1) {
+                Some(number) => number,
+                None => &9,
+            };
+            if num < up && num < down && num < right && num < left {
+                risk_level += 1 + *num;
+            }
+        }
+    }
+    println!("{}", risk_level);
+}
+
+fn open_file() -> BufReader<File> {
+    let args: Vec<String> = env::args().collect();
+
+    if args.len() != 2 {
+        eprintln!("Usage: {} <filename>", args[0]);
+        std::process::exit(1);
+    }
+
+    let filename = &args[1];
+
+    let file = File::open(filename).unwrap_or_else(|_| panic!("No such file: {}", filename));
+    BufReader::new(file)
+}