Year 2024 Day 6

Author: maneatingape

README.md

@@ -82,6 +82,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 3 | [Mull It Over](https://adventofcode.com/2024/day/3) | [Source](src/year2024/day03.rs) | 8 |
 | 4 | [Ceres Search](https://adventofcode.com/2024/day/4) | [Source](src/year2024/day04.rs) | 77 |
 | 5 | [Print Queue](https://adventofcode.com/2024/day/5) | [Source](src/year2024/day05.rs) | 18 |
+| 6 | [Guard Gallivant](https://adventofcode.com/2024/day/6) | [Source](src/year2024/day06.rs) | 5500 |
 
 ## 2023
 

benches/benchmark.rs

@@ -297,4 +297,5 @@ mod year2024 {
     benchmark!(year2024, day03);
     benchmark!(year2024, day04);
     benchmark!(year2024, day05);
+    benchmark!(year2024, day06);
 }

src/lib.rs

@@ -296,4 +296,5 @@ pub mod year2024 {
     pub mod day03;
     pub mod day04;
     pub mod day05;
+    pub mod day06;
 }

src/main.rs

@@ -366,5 +366,6 @@ fn year2024() -> Vec<Solution> {
         solution!(year2024, day03),
         solution!(year2024, day04),
         solution!(year2024, day05),
+        solution!(year2024, day06),
     ]
 }

src/year2024/day06.rs

@@ -0,0 +1,82 @@
+//! # Guard Gallivant
+use crate::util::grid::*;
+use crate::util::hash::*;
+use crate::util::point::*;
+
+pub fn parse(input: &str) -> Grid<u8> {
+    Grid::parse(input)
+}
+
+pub fn part1(grid: &Grid<u8>) -> usize {
+    let mut grid = grid.clone();
+    let mut position = grid.find(b'^').unwrap();
+    let mut direction = UP;
+    let mut result = 1;
+
+    while grid.contains(position + direction) {
+        if grid[position + direction] == b'#' {
+            direction = direction.clockwise();
+        }
+        position += direction;
+
+        if grid[position] != b'^' {
+            result += 1;
+        }
+        grid[position] = b'^';
+    }
+
+    result
+}
+
+pub fn part2(grid: &Grid<u8>) -> usize {
+    let mut grid = grid.clone();
+    let mut position = grid.find(b'^').unwrap();
+    let mut direction = UP;
+    let mut result = 0;
+
+    let mut seen = FastSet::with_capacity(5_000);
+
+    while grid.contains(position + direction) {
+        if grid[position + direction] == b'#' {
+            direction = direction.clockwise();
+        }
+
+        let next = position + direction;
+
+        if grid[next] == b'.' {
+            grid[next] = b'#';
+
+            if check_cycle(&grid, &mut seen, position, direction) {
+                result += 1;
+            }
+
+            grid[next] = b'^';
+            seen.clear();
+        }
+
+        position = next;
+    }
+
+    result
+}
+
+fn check_cycle(
+    grid: &Grid<u8>,
+    seen: &mut FastSet<(Point, Point)>,
+    mut position: Point,
+    mut direction: Point,
+) -> bool {
+    while grid.contains(position + direction) {
+        if grid[position + direction] == b'#' {
+            if !seen.insert((position, direction)) {
+                return true;
+            }
+            direction = direction.clockwise();
+            continue;
+        }
+
+        position += direction;
+    }
+
+    false
+}

tests/test.rs

@@ -286,4 +286,5 @@ mod year2024 {
     mod day03_test;
     mod day04_test;
     mod day05_test;
+    mod day06_test;
 }

tests/year2024/day06_test.rs

@@ -0,0 +1,25 @@
+use aoc::year2024::day06::*;
+
+const EXAMPLE: &str = "\
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...";
+
+#[test]
+fn part1_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part1(&input), 41);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part2(&input), 6);
+}