Day 6 - Slow

README.md

@@ -21,13 +21,14 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 
 | Day | Part 1 | Part 2 |
 | :---: | :---: | :---:  |
-| [Day 1](./src/bin/01.rs) | `39.0µs` | `49.2µs` |
-| [Day 2](./src/bin/02.rs) | `167.5µs` | `200.6µs` |
-| [Day 3](./src/bin/03.rs) | `31.5µs` | `35.2µs` |
-| [Day 4](./src/bin/04.rs) | `1.3ms` | `888.7µs` |
-| [Day 5](./src/bin/05.rs) | `78.1µs` | `164.6µs` |
-
-**Total: 2.95ms**
+| [Day 1](./src/bin/01.rs) | `37.4µs` | `51.0µs` |
+| [Day 2](./src/bin/02.rs) | `166.2µs` | `200.4µs` |
+| [Day 3](./src/bin/03.rs) | `34.6µs` | `33.7µs` |
+| [Day 4](./src/bin/04.rs) | `1.4ms` | `859.1µs` |
+| [Day 5](./src/bin/05.rs) | `75.8µs` | `166.7µs` |
+| [Day 6](./src/bin/06.rs) | `239.2µs` | `648.2ms` |
+
+**Total: 651.46ms**
 <!--- benchmarking table --->
 
 ---

data/examples/06.txt

@@ -0,0 +1,10 @@
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...

src/bin/06.rs

@@ -0,0 +1,104 @@
+use std::collections::HashSet;
+
+use advent_of_code::{Compass, Grid};
+
+advent_of_code::solution!(6);
+
+fn get_seen_positions(start: usize, grid: &Grid<char>) -> HashSet<usize> {
+    let mut seen = HashSet::new();
+    let mut d = Compass::N;
+    let mut p = start;
+
+    seen.insert(p);
+
+    while let Some(next) = grid.step_from_index(p, d) {
+        if grid.data[next] == '#' {
+            d = d.turn_right();
+        } else {
+            p = next;
+            seen.insert(p);
+        }
+    }
+
+    seen
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    let grid: Grid<char> = Grid::parse_lines(input);
+
+    let start = grid
+        .data
+        .iter()
+        .enumerate()
+        .find_map(|(i, c)| (*c == '^').then_some(i))
+        .unwrap();
+
+    Some(get_seen_positions(start, &grid).len())
+}
+
+fn has_cycle(start: usize, grid: &Grid<char>) -> bool {
+    let mut d = Compass::N;
+    let mut p = start;
+
+    let mut seen = HashSet::new();
+    seen.insert((p, d));
+
+    while let Some(next) = grid.step_from_index(p, d) {
+        if grid.data[next] == '#' {
+            d = d.turn_right();
+        } else {
+            p = next;
+            if !seen.insert((p, d)) {
+                return true;
+            }
+        }
+    }
+
+    false
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let mut grid: Grid<char> = Grid::parse_lines(input);
+    let start = grid
+        .data
+        .iter()
+        .enumerate()
+        .find_map(|(i, c)| (*c == '^').then_some(i))
+        .unwrap();
+
+    let possible_positions = get_seen_positions(start, &grid);
+
+    let result = possible_positions
+        .into_iter()
+        .filter(|ob| {
+            grid.data[*ob] = '#';
+
+            let result = has_cycle(start, &grid);
+
+            grid.data[*ob] = '.';
+
+            result
+        })
+        .count();
+
+    Some(result)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result =
+            part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(41));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result =
+            part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(6));
+    }
+}

src/lib.rs

@@ -105,6 +105,16 @@ impl Compass {
             D::S => D::N,
         }
     }
+
+    pub fn turn_right(&self) -> Self {
+        use Compass as D;
+        match self {
+            D::E => D::S,
+            D::N => D::E,
+            D::W => D::N,
+            D::S => D::W,
+        }
+    }
 }
 
 #[derive(Debug, PartialEq, Eq, Clone)]