Don't move robot

Author: maneatingape

README.md

@@ -86,7 +86,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 12 | [Garden Groups](https://adventofcode.com/2024/day/12) | [Source](src/year2024/day12.rs) | 289 |
 | 13 | [Claw Contraption](https://adventofcode.com/2024/day/13) | [Source](src/year2024/day13.rs) | 14 |
 | 14 | [Restroom Redoubt](https://adventofcode.com/2024/day/14) | [Source](src/year2024/day14.rs) | 74 |
-| 15 | [Warehouse Woes](https://adventofcode.com/2024/day/15) | [Source](src/year2024/day15.rs) | 332 |
+| 15 | [Warehouse Woes](https://adventofcode.com/2024/day/15) | [Source](src/year2024/day15.rs) | 303 |
 
 ## 2023
 

src/year2024/day15.rs

@@ -35,8 +35,10 @@ pub fn parse(input: &str) -> Input<'_> {
 pub fn part1(input: &Input<'_>) -> i32 {
     let (grid, moves) = input;
 
+    // We don't need to move the robot symbol so mark as empty space once located.
     let mut grid = grid.clone();
     let mut position = grid.find(b'@').unwrap();
+    grid[position] = b'.';
 
     // Treat moves as a single string ignoring any newline characters.
     for b in moves.bytes() {
@@ -57,6 +59,7 @@ pub fn part2(input: &Input<'_>) -> i32 {
 
     let mut grid = stretch(grid);
     let mut position = grid.find(b'@').unwrap();
+    grid[position] = b'.';
 
     // Reuse to minimize allocations.
     let mut todo = Vec::new();
@@ -78,7 +81,7 @@ pub fn part2(input: &Input<'_>) -> i32 {
 
 fn narrow(grid: &mut Grid<u8>, start: &mut Point, direction: Point) {
     let mut position = *start + direction;
-    let mut size = 2;
+    let mut size = 1;
 
     // Search for the next wall or open space.
     while grid[position] != b'.' && grid[position] != b'#' {
@@ -89,7 +92,7 @@ fn narrow(grid: &mut Grid<u8>, start: &mut Point, direction: Point) {
     // Move items one space in direction.
     if grid[position] == b'.' {
         let mut previous = b'.';
-        let mut position = *start;
+        let mut position = *start + direction;
 
         for _ in 0..size {
             swap(&mut previous, &mut grid[position]);
@@ -110,12 +113,7 @@ fn wide(
     id: usize,
 ) {
     // Short circuit if path in front of robot is empty.
-    let position = *start;
-    let next = position + direction;
-
-    if grid[next] == b'.' {
-        grid[position] = b'.';
-        grid[next] = b'@';
+    if grid[*start + direction] == b'.' {
         *start += direction;
         return;
     }
@@ -151,8 +149,8 @@ fn wide(
         }
     }
 
-    // Move boxes in reverse order.
-    for &point in todo.iter().rev() {
+    // Move boxes in reverse order (skipping the robot as an optimization).
+    for &point in todo[1..].iter().rev() {
         grid[point + direction] = grid[point];
         grid[point] = b'.';
     }