Complete day 3

day-3/src/main.rs

@@ -0,0 +1,91 @@
+use std::fs;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("input")?;
+    let slope = parse_slope(&input)?;
+
+    // Part 1
+    let encounters = count_encounters(&slope, (1, 3));
+    println!("{}", encounters);
+
+    // Part 2
+    let encounters: usize = [(1, 1), (1, 3), (1, 5), (1, 7), (2, 1)]
+        .iter()
+        .map(|angle| count_encounters(&slope, *angle))
+        .product();
+    println!("{}", encounters);
+
+    Ok(())
+}
+
+fn parse_slope(input: &str) -> Result<Vec<Vec<usize>>, String> {
+    input
+        .lines()
+        .map(|line| {
+            line.chars()
+                .map(|c| match c {
+                    '.' => Ok(0),
+                    '#' => Ok(1),
+                    other => Err(format!("Invalid slope; contains character: {}", other)),
+                })
+                .collect()
+        })
+        .collect()
+}
+
+fn count_encounters(input: &Vec<Vec<usize>>, angle: (usize, usize)) -> usize {
+    input
+        .iter()
+        .enumerate()
+        .step_by(angle.0)
+        .map(|(i, row)| row[(i * angle.1 / angle.0) % row.len()])
+        .sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use indoc::indoc;
+
+    #[test]
+    fn test_simple() -> Result<(), Box<dyn std::error::Error>> {
+        let example = indoc!(
+            "..##.......
+             #...#...#..
+             .#....#..#.
+             ..#.#...#.#
+             .#...##..#.
+             ..#.##.....
+             .#.#.#....#
+             .#........#
+             #.##...#...
+             #...##....#
+             .#..#...#.#"
+        );
+
+        let parsed = parse_slope(example)?;
+        assert_eq!(count_encounters(&parsed, (1, 3)), 7);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple2() -> Result<(), Box<dyn std::error::Error>> {
+        let example = indoc!(
+            "..##.......
+             #...#...#..
+             .#....#..#.
+             ..#.#...#.#
+             .#...##..#.
+             ..#.##.....
+             .#.#.#....#
+             .#........#
+             #.##...#...
+             #...##....#
+             .#..#...#.#"
+        );
+
+        let parsed = parse_slope(example)?;
+        assert_eq!(count_encounters(&parsed, (2, 1)), 2);
+        Ok(())
+    }
+}