Day 3 part 2.

2020/src/day3.rs

@@ -49,6 +49,18 @@
 //! In this example, traversing the map using this slope would cause you to encounter 7 trees.
 //!
 //! Starting at the top-left corner of your map and following a slope of right 3 and down 1, how many trees would you encounter?
+//!
+//! --- Part Two ---
+//! Time to check the rest of the slopes - you need to minimize the probability of a sudden arboreal stop, after all.
+//!
+//! Determine the number of trees you would encounter if, for each of the following slopes, you start at the top-left corner and traverse the map all the way to the bottom:
+//!
+//! Right 1, down 1.
+//! Right 3, down 1. (This is the slope you already checked.)
+//! Right 5, down 1.
+//! Right 7, down 1.
+//! Right 1, down 2.
+//! In the above example, these slopes would find 2, 7, 3, 4, and 2 tree(s) respectively; multiplied together, these produce the answer 336.
 
 use std::ops::Index;
 
@@ -95,10 +107,19 @@ fn parse(input: &str) -> Bitmap {
 }
 
 #[aoc(day3, part1)]
-fn answer(map: &Bitmap) -> usize {
+fn answer_part1(map: &Bitmap) -> usize {
     (0..map.height).filter(|y| map[(*y * 3, *y)]).count()
 }
 
+#[aoc(day3, part2)]
+fn answer_part2(map: &Bitmap) -> usize {
+    (0..map.height).filter(|y| map[(*y, *y)]).count()
+        * (0..map.height).filter(|y| map[(*y * 3, *y)]).count()
+        * (0..map.height).filter(|y| map[(*y * 5, *y)]).count()
+        * (0..map.height).filter(|y| map[(*y * 7, *y)]).count()
+        * (0..map.height / 2).filter(|y| map[(*y, *y * 2)]).count()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -114,4 +135,15 @@ mod tests {
             }
         );
     }
+
+    const INPUT :&'static str="..##.......\n#...#...#..\n.#....#..#.\n..#.#...#.#\n.#...##..#.\n..#.##.....\n.#.#.#....#\n.#........#\n#.##...#...\n#...##....#\n.#..#...#.#\n";
+    #[test]
+    fn part1() {
+        assert_eq!(answer_part1(&parse(INPUT)), 7);
+    }
+
+    #[test]
+    fn part2() {
+        assert_eq!(answer_part2(&parse(INPUT)), 336);
+    }
 }