day 3 solutions

src/day03.rs

@@ -0,0 +1,35 @@
+// ==== day 3
+
+use crate::utils;
+use anyhow::Result;
+
+fn count_trees(data: &str, right: usize, down: usize) -> Result<usize> {
+    Ok(data
+        .lines()
+        .step_by(down)
+        .enumerate()
+        .filter(|(i, s)| {
+            s.chars()
+                .nth((i * right) % s.len())
+                .expect("string index error")
+                == '#'
+        })
+        .count())
+}
+
+pub fn run() -> Result<(usize, usize)> {
+    let data = utils::read_file("data/input-03.txt")?;
+    let answers = utils::read_i64s("data/output-03.txt")?;
+
+    let p1 = count_trees(&data, 3, 1)?;
+    assert_eq!(p1, answers[0] as usize);
+
+    let p2 = count_trees(&data, 1, 1)?
+        * p1
+        * count_trees(&data, 5, 1)?
+        * count_trees(&data, 7, 1)?
+        * count_trees(&data, 1, 2)?;
+    assert_eq!(p2, answers[1] as usize);
+
+    Ok((p1, p2))
+}

src/main.rs

@@ -2,6 +2,7 @@ use anyhow::Result;
 
 mod day01;
 mod day02;
+mod day03;
 mod utils;
 
 // TODO
@@ -14,5 +15,6 @@ mod utils;
 fn main() -> Result<()> {
     println!("Day 01: {:?}", day01::run()?);
     println!("Day 02: {:?}", day02::run()?);
+    println!("Day 03: {:?}", day03::run()?);
     Ok(())
 }