[2024] Day 20 initial solution

Author: connorslade

README.md

@@ -24,6 +24,7 @@ Thank you to [Eric Wastl](http://was.tl) for running this incredible yearly even
 - [Day 17: Chronospatial Computer](aoc_2024/src/day_17.rs)
 - [Day 18: RAM Run](aoc_2024/src/day_18.rs)
 - [Day 19: Linen Layout](aoc_2024/src/day_19.rs)
+- [Day 20: Race Condition](aoc_2024/src/day_20.rs)
 <!-- MARKER -->
 
 ## [2023](https://adventofcode.com/2023) [![aoc_2023](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2023.yml/badge.svg)](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2023.yml)

aoc_2024/src/day_20.rs

@@ -0,0 +1,121 @@
+use std::{collections::VecDeque, convert::identity, u32};
+
+use aoc_lib::{direction::cardinal::Direction, matrix::Grid};
+use common::{solution, Answer};
+use itertools::Itertools;
+use nd_vec::{vector, Vec2};
+
+solution!("Race Condition", 20);
+
+fn part_a(input: &str) -> Answer {
+    Problem::parse(input).solve(2).into()
+}
+
+fn part_b(input: &str) -> Answer {
+    Problem::parse(input).solve(20).into()
+}
+
+struct Problem {
+    board: Grid<char>,
+    start: Vec2<usize>,
+    end: Vec2<usize>,
+}
+
+impl Problem {
+    fn parse(input: &str) -> Self {
+        let board = Grid::parse(input, identity);
+
+        let start = board.find('S').unwrap();
+        let end = board.find('E').unwrap();
+
+        Self { board, start, end }
+    }
+
+    fn solve(&self, max_skip: i32) -> u32 {
+        let (sc, ec) = (self.cost_map(self.start), self.cost_map(self.end));
+        let base_cost = sc[self.end];
+
+        let mut out = 0;
+
+        for (pos, tile) in self.board.iter() {
+            if *tile == '#' || sc[pos] == u32::MAX {
+                continue;
+            }
+
+            for (x, y) in (-max_skip..=max_skip).cartesian_product(-max_skip..=max_skip) {
+                let offset = vector!(x, y);
+                let dist = offset.manhattan_distance(&Vec2::zero());
+                if dist > max_skip {
+                    continue;
+                }
+
+                let end = pos.try_cast::<i32>().unwrap() + offset;
+                if !self.board.contains(end) || self.board[end] == '#' || ec[end] == u32::MAX {
+                    continue;
+                }
+
+                let cost = sc[pos] + ec[end] + dist as u32;
+                out += (cost + 100 <= base_cost) as u32;
+            }
+        }
+
+        out
+    }
+
+    fn cost_map(&self, start: Vec2<usize>) -> Grid<u32> {
+        let mut costs = Grid::new(self.board.size, u32::MAX);
+        let mut queue = VecDeque::new();
+        queue.push_back((start, 0));
+
+        while let Some((pos, dist)) = queue.pop_front() {
+            if costs[pos] != u32::MAX {
+                continue;
+            }
+
+            costs[pos] = dist;
+            for dir in Direction::ALL {
+                let next = dir.wrapping_advance(pos);
+                if let Some(tile) = self.board.get(next) {
+                    if matches!(tile, '.' | 'E') {
+                        queue.push_back((next, dist + 1));
+                    }
+                }
+            }
+        }
+
+        costs
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use indoc::indoc;
+
+    const CASE: &str = indoc! {"
+        ###############
+        #...#...#.....#
+        #.#.#.#.#.###.#
+        #S#...#.#.#...#
+        #######.#.#.###
+        #######.#.#...#
+        #######.#.###.#
+        ###..E#...#...#
+        ###.#######.###
+        #...###...#...#
+        #.#####.#.###.#
+        #.#...#.#.#...#
+        #.#.#.#.#.#.###
+        #...#...#...###
+        ###############
+    "};
+
+    #[test]
+    fn part_a() {
+        assert_eq!(super::part_a(CASE), 44.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(super::part_b(CASE), ().into());
+    }
+}

aoc_2024/src/lib.rs

@@ -19,6 +19,7 @@ mod day_16;
 mod day_17;
 mod day_18;
 mod day_19;
+mod day_20;
 // [import_marker]
 
 pub const SOLUTIONS: &[Solution] = &[
@@ -41,5 +42,6 @@ pub const SOLUTIONS: &[Solution] = &[
     day_17::SOLUTION,
     day_18::SOLUTION,
     day_19::SOLUTION,
+    day_20::SOLUTION,
     // [list_marker]
 ];