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) | 913 |
 
 ## 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 +1,208 @@
+//! # Guard Gallivant
+//!
+//! Part two is sped up by pre-computing the next obstacle in each direction from any point in
+//! the grid. If there is nothing left in the way then coordinates outside the grid are used.
+//! One dimensional example:
+//!
+//! ```none
+//!     .#...
+//!     Left: (-1, 2, 2, 2, 2)
+//!     Right: (1, 1, 5, 5, 5)
+//! ```
+//!
+//! This allows us to "shortcut" to each obstacle when looking for cycles. The remaining tricky
+//! part is including the extra obstacle which is different for each point on the guard's path.
+use crate::util::grid::*;
+use crate::util::hash::*;
+use crate::util::point::*;
+
+pub fn parse(input: &str) -> Grid<u8> {
+    Grid::parse(input)
+}
+
+/// Count distinct positions in the guard's path, which will eventually leave the grid.
+pub fn part1(grid: &Grid<u8>) -> u32 {
+    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();
+            continue;
+        }
+        position += direction;
+
+        // Avoid double counting when the path crosses itself.
+        if grid[position] == b'.' {
+            result += 1;
+        }
+        grid[position] = b'^';
+    }
+
+    result
+}
+
+/// Follow the guard's path, checking every step for a potential cycle.
+pub fn part2(grid: &Grid<u8>) -> u32 {
+    let mut grid = grid.clone();
+    let mut position = grid.find(b'^').unwrap();
+    let mut direction = UP;
+    let mut result = 0;
+
+    let shortcut = Shortcut::from(&grid);
+    let mut seen = FastSet::new();
+
+    while grid.contains(position + direction) {
+        if grid[position + direction] == b'#' {
+            direction = direction.clockwise();
+        }
+
+        let obstacle = position + direction;
+
+        // Avoid double counting when the path crosses itself.
+        if grid[obstacle] == b'.' {
+            if is_cycle(&shortcut, &mut seen, obstacle, position, direction) {
+                result += 1;
+            }
+            // Reuse `seen` to minimize allocations.
+            seen.clear();
+            grid[obstacle] = b'^';
+        }
+
+        position = obstacle;
+    }
+
+    result
+}
+
+fn is_cycle(
+    shortcut: &Shortcut,
+    seen: &mut FastSet<(Point, Point)>,
+    obstacle: Point,
+    mut position: Point,
+    mut direction: Point,
+) -> bool {
+    while shortcut.up.contains(position) {
+        // Reaching the same position in the same direction is a cycle.
+        if !seen.insert((position, direction)) {
+            return true;
+        }
+
+        // The tricky part is checking for the new time travelling instigated obstacle.
+        position = match direction {
+            UP => {
+                let next = shortcut.up[position];
+                if position.x == obstacle.x && position.y > obstacle.y && obstacle.y >= next.y {
+                    obstacle - UP
+                } else {
+                    next
+                }
+            }
+            DOWN => {
+                let next = shortcut.down[position];
+                if position.x == obstacle.x && position.y < obstacle.y && obstacle.y <= next.y {
+                    obstacle - DOWN
+                } else {
+                    next
+                }
+            }
+            LEFT => {
+                let next = shortcut.left[position];
+                if position.y == obstacle.y && position.x > obstacle.x && obstacle.x >= next.x {
+                    obstacle - LEFT
+                } else {
+                    next
+                }
+            }
+            RIGHT => {
+                let next = shortcut.right[position];
+                if position.y == obstacle.y && position.x < obstacle.x && obstacle.x <= next.x {
+                    obstacle - RIGHT
+                } else {
+                    next
+                }
+            }
+            _ => unreachable!(),
+        };
+
+        direction = direction.clockwise();
+    }
+
+    false
+}
+
+struct Shortcut {
+    up: Grid<Point>,
+    down: Grid<Point>,
+    left: Grid<Point>,
+    right: Grid<Point>,
+}
+
+impl Shortcut {
+    fn from(grid: &Grid<u8>) -> Self {
+        let mut up = copy(grid);
+        let mut down = copy(grid);
+        let mut left = copy(grid);
+        let mut right = copy(grid);
+
+        for x in 0..grid.width {
+            let mut last = Point::new(x, -1);
+
+            for y in 0..grid.height {
+                let point = Point::new(x, y);
+                if grid[point] == b'#' {
+                    last = Point::new(x, y + 1);
+                }
+                up[point] = last;
+            }
+        }
+
+        for x in 0..grid.width {
+            let mut last = Point::new(x, grid.height);
+
+            for y in (0..grid.height).rev() {
+                let point = Point::new(x, y);
+                if grid[point] == b'#' {
+                    last = Point::new(x, y - 1);
+                }
+                down[point] = last;
+            }
+        }
+
+        for y in 0..grid.height {
+            let mut last = Point::new(-1, y);
+
+            for x in 0..grid.width {
+                let point = Point::new(x, y);
+                if grid[point] == b'#' {
+                    last = Point::new(x + 1, y);
+                }
+                left[point] = last;
+            }
+        }
+
+        for y in 0..grid.height {
+            let mut last = Point::new(grid.width, y);
+
+            for x in (0..grid.width).rev() {
+                let point = Point::new(x, y);
+                if grid[point] == b'#' {
+                    last = Point::new(x - 1, y);
+                }
+                right[point] = last;
+            }
+        }
+
+        Shortcut { up, down, left, right }
+    }
+}
+
+fn copy(grid: &Grid<u8>) -> Grid<Point> {
+    Grid {
+        width: grid.width,
+        height: grid.height,
+        bytes: vec![ORIGIN; (grid.width * grid.height) as usize],
+    }
+}

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);
+}