optimize day 21 part 1

README.md

@@ -32,9 +32,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 18](./src/bin/18.rs) |  `5.0ms`  | `41.7µs`  |
 | [Day 19](./src/bin/19.rs) | `239.3µs` | `265.0µs` |
 | [Day 20](./src/bin/20.rs) |  `1.8ms`  |  `8.1ms`  |
-| [Day 21](./src/bin/21.rs) | `10.3ms`  |  `2.5ms`  |
+| [Day 21](./src/bin/21.rs) |  `2.4ms`  |  `2.6ms`  |
 
-**Total: 1988.52ms**
+**Total: 1980.72ms**
 
 <!--- benchmarking table --->
 

src/bin/21.rs

@@ -8,9 +8,15 @@ pub fn part_one(input: &str) -> Option<usize> {
 
 fn part_one_with_step_count(input: &str, step_count: u8) -> Option<usize> {
     let garden = Garden::from_input(input);
-    let positions = garden.get_possible_positions(step_count);
+    let distance_map = garden.get_distance_map();
+    let step_count_modulo = step_count % 2;
 
-    Some(positions.len())
+    Some(
+        distance_map
+            .values()
+            .filter(|&&v| v <= step_count && v % 2 == step_count_modulo)
+            .count(),
+    )
 }
 
 // Solution from https://github.com/villuna/aoc23/wiki/A-Geometric-solution-to-advent-of-code-2023,-day-21
@@ -95,31 +101,6 @@ impl Garden {
         }
     }
 
-    fn get_possible_positions(&self, step_count: u8) -> HashSet<Coordinate> {
-        let mut positions = HashSet::from([self.start.clone()]);
-
-        for _ in 0..step_count {
-            let mut next_positions = HashSet::new();
-
-            for coordinate in positions {
-                next_positions.extend(
-                    [
-                        Direction::Up,
-                        Direction::Down,
-                        Direction::Left,
-                        Direction::Right,
-                    ]
-                    .iter()
-                    .filter_map(|direction| self.get_next_coordinate(&coordinate, direction)),
-                );
-            }
-
-            positions = next_positions;
-        }
-
-        positions
-    }
-
     fn get_distance_map(&self) -> HashMap<Coordinate, u8> {
         let mut distance_map = HashMap::new();