day 8

data/examples/08.txt

@@ -0,0 +1,12 @@
+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............

src/bin/08.rs

@@ -0,0 +1,139 @@
+use std::cmp::Ordering;
+use std::collections::{HashMap, HashSet};
+
+advent_of_code::solution!(8);
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let mut antennas = Vec::new();
+
+    for (y, line) in input.lines().enumerate() {
+        for (x, c) in line.chars().enumerate() {
+            if c != '.' {
+                antennas.push((x as i32, y as i32, c));
+            }
+        }
+    }
+    let height = input.lines().count() as i32;
+    let width = input.lines().map(|l| l.chars().count()).max().unwrap_or(0) as i32;
+
+    let mut freq_map: HashMap<char, Vec<(i32, i32)>> = HashMap::new();
+    for (x, y, c) in antennas {
+        freq_map.entry(c).or_default().push((x, y));
+    }
+    let mut antinodes = HashSet::new();
+
+    for positions in freq_map.values() {
+        let n = positions.len();
+        for i in 0..n {
+            for j in (i + 1)..n {
+                let (x1, y1) = positions[i];
+                let (x2, y2) = positions[j];
+
+                antinodes.insert((2 * x1 - x2, 2 * y1 - y2));
+
+                antinodes.insert((2 * x2 - x1, 2 * y2 - y1));
+
+                let c_third_x = 2 * x1 + x2;
+                let c_third_y = 2 * y1 + y2;
+                if c_third_x % 3 == 0 && c_third_y % 3 == 0 {
+                    antinodes.insert((c_third_x / 3, c_third_y / 3));
+                }
+
+                let c_twothirds_x = x1 + 2 * x2;
+                let c_twothirds_y = y1 + 2 * y2;
+                if c_twothirds_x % 3 == 0 && c_twothirds_y % 3 == 0 {
+                    antinodes.insert((c_twothirds_x / 3, c_twothirds_y / 3));
+                }
+            }
+        }
+    }
+
+    antinodes.retain(|&(x, y)| x >= 0 && x < width && y >= 0 && y < height);
+
+    Some(antinodes.len() as u32)
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let mut antennas = Vec::new();
+
+    for (y, line) in input.lines().enumerate() {
+        for (x, c) in line.chars().enumerate() {
+            if c != '.' {
+                antennas.push((x as i32, y as i32, c));
+            }
+        }
+    }
+
+    let height = input.lines().count() as i32;
+    let width = input.lines().map(|l| l.chars().count()).max().unwrap_or(0) as i32;
+
+    let mut freq_map: HashMap<char, Vec<(i32, i32)>> = HashMap::new();
+
+    for (x, y, c) in antennas {
+        freq_map.entry(c).or_default().push((x, y));
+    }
+
+    let mut antinodes = HashSet::new();
+
+    for positions in freq_map.values() {
+        if positions.len() < 2 {
+            continue;
+        }
+        for i in 0..positions.len() {
+            for j in (i + 1)..positions.len() {
+                let (x1, y1) = positions[i];
+                let (x2, y2) = positions[j];
+
+                let dx = x2 - x1;
+                let dy = y2 - y1;
+                let g = gcd(dx, dy);
+                let step_x = dx / g;
+                let step_y = dy / g;
+
+                {
+                    let (mut cx, mut cy) = (x1, y1);
+                    while cx >= 0 && cy >= 0 && cx < width && cy < height {
+                        antinodes.insert((cx, cy));
+                        cx += step_x;
+                        cy += step_y;
+                    }
+                }
+
+                {
+                    let (mut cx, mut cy) = (x1 - step_x, y1 - step_y);
+                    while cx >= 0 && cy >= 0 && cx < width && cy < height {
+                        antinodes.insert((cx, cy));
+                        cx -= step_x;
+                        cy -= step_y;
+                    }
+                }
+            }
+        }
+    }
+    Some(antinodes.len() as u32)
+}
+
+fn gcd(a: i32, b: i32) -> i32 {
+    if b == 0 {
+        a.abs()
+    } else {
+        gcd(b, a % b)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(14));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(34));
+    }
+}