Day 10 part 1

2019/input/day10.txt

@@ -0,0 +1,25 @@
+#..#.#.#.######..#.#...##
+##.#..#.#..##.#..######.#
+.#.##.#..##..#.#.####.#..
+.#..##.#.#..#.#...#...#.#
+#...###.##.##..##...#..#.
+##..#.#.#.###...#.##..#.#
+###.###.#.##.##....#####.
+.#####.#.#...#..#####..#.
+.#.##...#.#...#####.##...
+######.#..##.#..#.#.#....
+###.##.#######....##.#..#
+.####.##..#.##.#.#.##...#
+##...##.######..##..#.###
+...###...#..#...#.###..#.
+.#####...##..#..#####.###
+.#####..#.#######.###.##.
+#...###.####.##.##.#.##.#
+.#.#.#.#.#.##.#..#.#..###
+##.#.####.###....###..##.
+#..##.#....#..#..#.#..#.#
+##..#..#...#..##..####..#
+....#.....##..#.##.#...##
+.##..#.#..##..##.#..##..#
+.##..#####....#####.#.#.#
+#..#..#..##...#..#.#.#.##

2019/src/bin/day10.rs

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+use std::collections::{BTreeMap, BTreeSet};
+use std::f64::consts::PI;
+use std::{fs, io};
+
+type Angle = isize;
+type Distance = isize;
+
+const PRECISION: f64 = 1000.;
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+struct Asteroid {
+    x: usize,
+    y: usize,
+}
+
+fn main() -> io::Result<()> {
+    let data = fs::read_to_string("input/day10.txt")?;
+    let map = data
+        .lines()
+        .enumerate()
+        .flat_map(|(y, line)| {
+            line.trim().chars().enumerate().filter_map(move |(x, c)| {
+                if c == '#' {
+                    Some(Asteroid { x, y })
+                } else {
+                    None
+                }
+            })
+        })
+        .collect::<Vec<_>>();
+
+    // From a given asteroid find the angle and distance to every other asteroid
+    // only keep the closest one for each angle.
+    let maxsteroid = map
+        .iter()
+        .map(|&asteroid| {
+            let stats = angles_and_distances(asteroid, &map);
+            stats
+                .iter()
+                .map(|(_angle, distances)| distances.iter().next().unwrap())
+                .count()
+        })
+        .max()
+        .unwrap();
+    println!("Part 1: {}", maxsteroid);
+
+    Ok(())
+}
+
+fn angles_and_distances(
+    from: Asteroid,
+    asteroids: &[Asteroid],
+) -> BTreeMap<Angle, BTreeSet<(Distance, Asteroid)>> {
+    asteroids
+        .iter()
+        .fold(BTreeMap::new(), |mut result, &asteroid| {
+            let angle = from.angle_to(asteroid);
+            let distance = from.distance_to(asteroid);
+            result
+                .entry(angle)
+                .or_insert(BTreeSet::new())
+                .insert((distance, asteroid));
+            result
+        })
+}
+
+impl Asteroid {
+    // https://en.wikipedia.org/wiki/Atan2#Definition_and_computation
+    fn angle_to(&self, other: Asteroid) -> Angle {
+        let x = other.y as f64 - self.y as f64;
+        let y = other.x as f64 - self.x as f64;
+
+        let angle = if x > 0. {
+            (y / x).atan()
+        } else if y > 0. {
+            (PI / 2.) - (x / y).atan()
+        } else if y < 0. {
+            -(PI / 2.) - (x / y).atan()
+        } else if x < 0. {
+            (y / x).atan() + PI
+        } else {
+            // undefined but for our purposes 0 will do
+            0.
+        } * PRECISION;
+
+        angle.round() as Angle
+    }
+
+    fn distance_to(&self, other: Asteroid) -> Distance {
+        (((self.x as f64 - other.x as f64).powf(2.) + (self.y as f64 - other.y as f64).powf(2.))
+            .sqrt()
+            * PRECISION)
+            .round() as Distance
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_distance() {
+        let this = Asteroid { x: 3, y: 4 };
+        let other = Asteroid { x: 1, y: 0 };
+
+        assert_eq!(this.distance_to(other), 4472);
+    }
+
+    #[test]
+    fn test_angle() {
+        let this = Asteroid { x: 0, y: 0 };
+        let other = Asteroid { x: 0, y: 9 };
+        assert_eq!(this.angle_to(other), 0);
+
+        let this = Asteroid { x: 0, y: 0 };
+        let other = Asteroid { x: 9, y: 0 };
+        assert_eq!(
+            this.angle_to(other),
+            ((PI / 2.) * PRECISION).round() as Angle
+        );
+
+        let this = Asteroid { x: 3, y: 4 };
+        let other = Asteroid { x: 1, y: 0 };
+        assert_eq!(this.angle_to(other), -2678);
+    }
+}