Year 2024 Day 18

Author: maneatingape

README.md

@@ -89,6 +89,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 15 | [Warehouse Woes](https://adventofcode.com/2024/day/15) | [Source](src/year2024/day15.rs) | 303 |
 | 16 | [Reindeer Maze](https://adventofcode.com/2024/day/16) | [Source](src/year2024/day16.rs) | 390 |
 | 17 | [Chronospatial Computer](https://adventofcode.com/2024/day/17) | [Source](src/year2024/day17.rs) | 2 |
+| 18 | [RAM Run](https://adventofcode.com/2024/day/18) | [Source](src/year2024/day18.rs) | 61 |
 
 ## 2023
 

benches/benchmark.rs

@@ -88,5 +88,5 @@ benchmark!(year2023
 
 benchmark!(year2024
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17
+    day14, day15, day16, day17, day18
 );

src/lib.rs

@@ -68,5 +68,5 @@ library!(year2023 "Restore global snow production."
 
 library!(year2024 "Locate the Chief Historian in time for the big Christmas sleigh launch."
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17
+    day14, day15, day16, day17, day18
 );

src/main.rs

@@ -142,5 +142,5 @@ run!(year2023
 
 run!(year2024
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17
+    day14, day15, day16, day17, day18
 );

src/year2024/day18.rs

@@ -0,0 +1,79 @@
+//! # RAM Run
+//!
+//! We use a trick to speed things up. Instead of storing `#` and `.` in the grid, we store
+//! the time when a block arrives. For example:
+//!
+//! ```none
+//!            ...#...    ∞ ∞ ∞ 3 ∞ ∞ ∞
+//!     5,4    ..#....    ∞ ∞ 4 ∞ ∞ ∞ ∞
+//!     4,2    ....#..    ∞ ∞ ∞ ∞ 1 ∞ ∞
+//!     4,5 => ....... => ∞ ∞ ∞ ∞ ∞ ∞ ∞
+//!     3,0    .....#.    ∞ ∞ ∞ ∞ ∞ 0 ∞
+//!     2,1    ....#..    ∞ ∞ ∞ ∞ 2 ∞ ∞
+//!            .......    ∞ ∞ ∞ ∞ ∞ ∞ ∞
+//! ```
+//!
+//! Now we can [BFS](https://en.wikipedia.org/wiki/Breadth-first_search) from any arbitrary
+//! start time. Squares are blocked only if the grid time is less than or equal to the start time.
+//!
+//! A [binary search](https://en.wikipedia.org/wiki/Binary_search) is much faster than a
+//! linear search with complexity `O(log₂n)` vs `O(n)`. For example `log₂(3450) = 12`.
+use crate::util::grid::*;
+use crate::util::iter::*;
+use crate::util::parse::*;
+use crate::util::point::*;
+use std::collections::VecDeque;
+
+pub fn parse(input: &str) -> Grid<u16> {
+    let mut grid = Grid::new(71, 71, u16::MAX);
+
+    for (i, [x, y]) in input.iter_signed::<i32>().chunk::<2>().enumerate() {
+        grid[Point::new(x, y)] = i as u16;
+    }
+
+    grid
+}
+
+pub fn part1(grid: &Grid<u16>) -> u32 {
+    bfs(grid, 1024).unwrap()
+}
+
+pub fn part2(grid: &Grid<u16>) -> String {
+    let mut lower = 0;
+    let mut upper = 5041;
+
+    while lower < upper {
+        let middle = (lower + upper) / 2;
+        if bfs(grid, middle).is_some() {
+            lower = middle + 1;
+        } else {
+            upper = middle;
+        }
+    }
+
+    let index = grid.bytes.iter().position(|&time| time == lower).unwrap() as i32;
+    format!("{},{}", index % grid.width, index / grid.width)
+}
+
+fn bfs(grid: &Grid<u16>, time: u16) -> Option<u32> {
+    let mut todo = VecDeque::new();
+    let mut seen = grid.clone();
+
+    todo.push_back((ORIGIN, 0));
+    seen[ORIGIN] = 0;
+
+    while let Some((position, cost)) = todo.pop_front() {
+        if position == Point::new(70, 70) {
+            return Some(cost);
+        }
+
+        for next in ORTHOGONAL.map(|o| position + o) {
+            if seen.contains(next) && time < seen[next] {
+                todo.push_back((next, cost + 1));
+                seen[next] = 0;
+            }
+        }
+    }
+
+    None
+}

tests/test.rs

@@ -81,5 +81,5 @@ test!(year2023
 
 test!(year2024
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17
+    day14, day15, day16, day17, day18
 );

tests/year2024/day18.rs

@@ -0,0 +1,9 @@
+#[test]
+fn part1_test() {
+    // No test
+}
+
+#[test]
+fn part2_test() {
+    // No test
+}