2024 day 18

Author: ictrobot

crates/year2024/src/day18.rs

@@ -0,0 +1,84 @@
+use std::collections::VecDeque;
+use utils::point::Point2D;
+use utils::prelude::*;
+
+/// Finding when a path through a grid is blocked.
+#[derive(Clone, Debug)]
+pub struct Day18 {
+    positions: Vec<Point2D<usize>>,
+}
+
+impl Day18 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        Ok(Self {
+            positions: parser::number_range(0u32..=70)
+                .repeat_n(",")
+                .map(|[x, y]| Point2D {
+                    x: x as usize,
+                    y: y as usize,
+                })
+                .parse_lines(input)?,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u32 {
+        self.minimum_steps(1024).expect("no solution found")
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> String {
+        let mut left = 0;
+        let mut right = self.positions.len();
+
+        while left < right {
+            let mid = (left + right) / 2;
+            match self.minimum_steps(mid + 1) {
+                Some(_) => left = mid + 1,
+                None => right = mid,
+            }
+        }
+
+        if left >= self.positions.len() {
+            panic!("no solution found");
+        }
+
+        format!("{},{}", self.positions[left].x, self.positions[left].y)
+    }
+
+    fn minimum_steps(&self, fallen: usize) -> Option<u32> {
+        let mut blocked = [[false; 71]; 71];
+        for pos in self.positions.iter().take(fallen) {
+            blocked[pos.x][pos.y] = true;
+        }
+
+        let mut queue = VecDeque::new();
+        queue.push_back((Point2D::new(0, 0), 0));
+
+        while let Some((pos, steps)) = queue.pop_front() {
+            if pos == Point2D::new(70, 70) {
+                return Some(steps);
+            }
+            if pos.x > 0 && !blocked[pos.x - 1][pos.y] {
+                queue.push_back((Point2D::new(pos.x - 1, pos.y), steps + 1));
+                blocked[pos.x - 1][pos.y] = true;
+            }
+            if pos.x < 70 && !blocked[pos.x + 1][pos.y] {
+                queue.push_back((Point2D::new(pos.x + 1, pos.y), steps + 1));
+                blocked[pos.x + 1][pos.y] = true;
+            }
+            if pos.y > 0 && !blocked[pos.x][pos.y - 1] {
+                queue.push_back((Point2D::new(pos.x, pos.y - 1), steps + 1));
+                blocked[pos.x][pos.y - 1] = true;
+            }
+            if pos.y < 70 && !blocked[pos.x][pos.y + 1] {
+                queue.push_back((Point2D::new(pos.x, pos.y + 1), steps + 1));
+                blocked[pos.x][pos.y + 1] = true;
+            }
+        }
+
+        None
+    }
+}
+
+examples!(Day18 -> (u32, &'static str) []);

crates/year2024/src/lib.rs

@@ -19,4 +19,5 @@ utils::year!(2024 => year2024, ${
     15 => day15::Day15<'_>,
     16 => day16::Day16,
     17 => day17::Day17,
+    18 => day18::Day18,
 });