Implementation day 3

2020/src/common.rs

@@ -34,6 +34,30 @@ where
     buf.trim().parse().unwrap()
 }
 
+pub fn read_char_grid(input: &mut dyn Read) -> Vec<Vec<u8>> {
+    let mut reader = BufReader::new(input);
+    let mut buffer = Vec::new();
+
+    let mut grid = Vec::new();
+
+    while let Ok(read) = reader.read_until(b'\n', &mut buffer) {
+        if read == 0 {
+            break;
+        }
+
+        let line: &[u8] = if let Some(&b'\n') = buffer.last() {
+            &buffer[..(buffer.len() - 1)]
+        } else {
+            &buffer[..]
+        };
+
+        grid.push(line.to_owned());
+        buffer.clear();
+    }
+
+    grid
+}
+
 /// An interface to count elements in particular categories.
 pub trait GroupingCount {
     /// The type of the categories under inspection

2020/src/day03.rs

@@ -0,0 +1,60 @@
+use std::io::Read;
+
+use crate::common::read_char_grid;
+use crate::Solution;
+
+#[derive(Default)]
+pub struct Day03;
+
+fn walk(right: usize, down: usize, forrest: &[Vec<u8>]) -> usize {
+    let mut trees = 0;
+    let mut x = 0;
+
+    for line in forrest.iter().step_by(down) {
+        if line[x] == b'#' {
+            trees += 1;
+        }
+
+        x = (x + right) % line.len();
+    }
+
+    trees
+}
+
+impl Solution for Day03 {
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let forrest = read_char_grid(input);
+
+        walk(3, 1, &forrest).to_string()
+    }
+
+    fn part2(&mut self, input: &mut dyn Read) -> String {
+        let forrest = read_char_grid(input);
+        let mut product = 1;
+
+        for &(right, down) in &[(1, 1), (3, 1), (5, 1), (7, 1), (1, 2)] {
+            product *= walk(right, down, &forrest);
+        }
+
+        product.to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("../samples/03.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation!(Day03, 1, SAMPLE, 7);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation!(Day03, 2, SAMPLE, 336);
+    }
+}

2020/src/lib.rs

@@ -17,6 +17,7 @@ pub fn get_implementation(day: usize) -> Box<dyn Solution> {
     match day {
         1 => Box::new(day01::Day01::default()),
         2 => Box::new(day02::Day02::default()),
+        3 => Box::new(day03::Day03::default()),
         _ => panic!("Unsupported day {}", day),
     }
 }