2024 day 14

Author: ictrobot

crates/year2024/src/day14.rs

@@ -0,0 +1,90 @@
+use utils::point::Point2D;
+use utils::prelude::*;
+
+/// Finding when robots arrange themselves into a picture.
+///
+/// Assumes that the picture of the Christmas tree will involve a horizontal line of at least 10
+/// robots, and that doesn't happen in any prior iterations.
+#[derive(Clone, Debug)]
+pub struct Day14 {
+    robots: Vec<Robot>,
+}
+
+#[derive(Copy, Clone, Debug)]
+struct Robot {
+    position: Point2D<i32>,
+    velocity: Point2D<i32>,
+}
+
+const WIDTH: i32 = 101;
+const HEIGHT: i32 = 103;
+
+impl Day14 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        Ok(Self {
+            robots: parser::number_range(0..=WIDTH - 1)
+                .with_prefix("p=")
+                .then(parser::number_range(0..=HEIGHT - 1).with_prefix(","))
+                .then(parser::i32().with_prefix(" v="))
+                .then(parser::i32().with_prefix(","))
+                .map(|(px, py, vx, vy)| Robot {
+                    position: Point2D::new(px, py),
+                    velocity: Point2D::new(vx, vy),
+                })
+                .parse_lines(input)?,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        let mut counts = [0; 4];
+        for &(mut r) in self.robots.iter() {
+            r.position += r.velocity * 100;
+            r.position.x = r.position.x.rem_euclid(WIDTH);
+            r.position.y = r.position.y.rem_euclid(HEIGHT);
+
+            if r.position.x == WIDTH / 2 || r.position.y == HEIGHT / 2 {
+                continue;
+            }
+
+            let mut quadrant = 0;
+            if r.position.x > WIDTH / 2 {
+                quadrant += 2;
+            }
+            if r.position.y > HEIGHT / 2 {
+                quadrant += 1;
+            }
+            counts[quadrant] += 1;
+        }
+        counts.iter().product()
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u32 {
+        let mut robots = self.robots.clone();
+        for i in 1..=u32::MAX {
+            let mut grid = [false; (WIDTH + 1) as usize * HEIGHT as usize];
+            for r in robots.iter_mut() {
+                r.position += r.velocity;
+                r.position.x = r.position.x.rem_euclid(WIDTH);
+                r.position.y = r.position.y.rem_euclid(HEIGHT);
+                grid[r.position.y as usize * (WIDTH + 1) as usize + r.position.x as usize] = true;
+            }
+
+            let mut count = 0;
+            for b in grid {
+                if b {
+                    count += 1;
+                    if count >= 10 {
+                        return i;
+                    }
+                } else {
+                    count = 0;
+                }
+            }
+        }
+        unreachable!("no solution found")
+    }
+}
+
+examples!(Day14 -> (u32, u32) []);

crates/year2024/src/lib.rs

@@ -15,4 +15,5 @@ utils::year!(2024 => year2024, ${
     11 => day11::Day11,
     12 => day12::Day12,
     13 => day13::Day13,
+    14 => day14::Day14,
 });