Day 3 - Part 2

tudorpavel · Dec 3, 2020 · 8a62c52 · 8a62c52
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diff --git a/day03/README.md b/day03/README.md
@@ -28,3 +28,7 @@ I assume Part 2 will ask us to find a slope which minimizes the number of trees
 The implementation was mostly straightforward, for the HashMap of tree positions I found [HashSet](https://doc.rust-lang.org/std/collections/struct.HashSet.html) in Rust's standard library which is perfect since I actually wanted a Set to easily check presence of a given Point.
 
 I thought about using a more functional style in `GeoMap#tree_count` but decided to go with this imperative approach. I was worried I couldn't easily generate a path Vector due to the wrapping of `x` by `self.width`.
+
+## Part 2
+
+The solution is simple enough, we just have to map `GeoMap#tree_count` over the given collection of Points and compute the product.
diff --git a/day03/src/main.rs b/day03/src/main.rs
@@ -14,7 +14,7 @@ struct GeoMap {
 }
 
 impl GeoMap {
-    fn build(lines: Vec<String>) -> GeoMap {
+    fn build(lines: &[String]) -> GeoMap {
         let mut trees = HashSet::new();
 
         for (y, line) in lines.iter().enumerate() {
@@ -32,7 +32,7 @@ impl GeoMap {
         }
     }
 
-    fn tree_count(&self, slope: Point) -> usize {
+    fn tree_count(&self, slope: &Point) -> usize {
         let mut current = slope.clone();
         let mut count = 0;
 
@@ -49,42 +49,63 @@ impl GeoMap {
     }
 }
 
-fn part1(lines: Vec<String>) -> usize {
+fn part1(lines: &[String]) -> usize {
     let geo_map = GeoMap::build(lines);
-    geo_map.tree_count(Point { x: 3, y: 1 })
+
+    geo_map.tree_count(&Point { x: 3, y: 1 })
+}
+
+fn part2(lines: &[String]) -> usize {
+    let geo_map = GeoMap::build(lines);
+
+    vec![
+        Point { x: 1, y: 1 },
+        Point { x: 3, y: 1 },
+        Point { x: 5, y: 1 },
+        Point { x: 7, y: 1 },
+        Point { x: 1, y: 2 },
+    ]
+    .iter()
+    .map(|p| geo_map.tree_count(p))
+    .product()
 }
 
 fn main() {
     let lines = utils::read_lines();
-    println!("Part 1: {}", part1(lines));
+    println!("Part 1: {}", part1(&lines));
+    println!("Part 1: {}", part2(&lines));
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    fn input() -> Vec<String> {
+        vec![
+            "..##.......",
+            "#...#...#..",
+            ".#....#..#.",
+            "..#.#...#.#",
+            ".#...##..#.",
+            "..#.##.....",
+            ".#.#.#....#",
+            ".#........#",
+            "#.##...#...",
+            "#...##....#",
+            ".#..#...#.#",
+        ]
+        .into_iter()
+        .map(|s| String::from(s))
+        .collect()
+    }
+
     #[test]
     fn part1_works() {
-        assert_eq!(
-            part1(
-                vec![
-                    "..##.......",
-                    "#...#...#..",
-                    ".#....#..#.",
-                    "..#.#...#.#",
-                    ".#...##..#.",
-                    "..#.##.....",
-                    ".#.#.#....#",
-                    ".#........#",
-                    "#.##...#...",
-                    "#...##....#",
-                    ".#..#...#.#",
-                ]
-                .into_iter()
-                .map(|s| String::from(s))
-                .collect()
-            ),
-            7
-        );
+        assert_eq!(part1(&input()), 7);
+    }
+
+    #[test]
+    fn part2_works() {
+        assert_eq!(part2(&input()), 336);
     }
 }