[2025] Solution for Day 9

Author: kfarnung

2025/src/bin/day09.rs

@@ -0,0 +1,16 @@
+use aoc2025::days::day09::{part1, part2};
+use std::env;
+use std::fs;
+use std::process;
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    if args.len() < 3 {
+        println!("Missing required arguments");
+        process::exit(1);
+    }
+
+    let contents = fs::read_to_string(&args[1]).expect("Something went wrong");
+    println!("Part 1: {}", part1(&contents));
+    println!("Part 2: {}", part2(&contents));
+}

2025/src/days.rs

@@ -6,3 +6,4 @@ pub mod day05;
 pub mod day06;
 pub mod day07;
 pub mod day08;
+pub mod day09;

2025/src/days/day09.rs

@@ -0,0 +1,103 @@
+struct Point {
+    x: i64,
+    y: i64,
+}
+
+impl Point {
+    fn new(x: i64, y: i64) -> Self {
+        Point { x, y }
+    }
+
+    fn area(&self, other: &Point) -> i64 {
+        ((self.x - other.x).abs() + 1) * ((self.y - other.y).abs() + 1)
+    }
+}
+
+pub fn part1(contents: &str) -> i64 {
+    let points = parse_input(contents);
+    let mut max_area = 0;
+    for (i, p1) in points.iter().enumerate() {
+        for p2 in &points[i + 1..] {
+            let area = p1.area(p2);
+            if area > max_area {
+                max_area = area;
+            }
+        }
+    }
+    max_area
+}
+
+pub fn part2(contents: &str) -> i64 {
+    let points = parse_input(contents);
+    let mut max_area = 0;
+    for (i, p1) in points.iter().enumerate() {
+        for p2 in &points[i + 1..] {
+            let area = p1.area(p2);
+            if area > max_area && is_interior(p1, p2, &points) {
+                max_area = area;
+            }
+        }
+    }
+    max_area
+}
+
+fn parse_input(input: &str) -> Vec<Point> {
+    input
+        .lines()
+        .map(|line| {
+            let coords: Vec<i64> = line.split(',').map(|s| s.parse().unwrap()).collect();
+            Point::new(coords[0], coords[1])
+        })
+        .collect()
+}
+
+fn is_interior(p1: &Point, p2: &Point, points: &[Point]) -> bool {
+    let max_x = if p1.x > p2.x { p1.x } else { p2.x };
+    let min_x = if p1.x < p2.x { p1.x } else { p2.x };
+    let max_y = if p1.y > p2.y { p1.y } else { p2.y };
+    let min_y = if p1.y < p2.y { p1.y } else { p2.y };
+    for (i, p) in points.iter().enumerate() {
+        if (p.x > min_x && p.x < max_x) && (p.y > min_y && p.y < max_y) {
+            // Fully inside
+            return false;
+        } else {
+            let next = &points[(i + 1) % points.len()];
+            let min_line_x = if p.x < next.x { p.x } else { next.x };
+            let max_line_x = if p.x > next.x { p.x } else { next.x };
+            let min_line_y = if p.y < next.y { p.y } else { next.y };
+            let max_line_y = if p.y > next.y { p.y } else { next.y };
+            if (min_line_x <= min_x
+                && max_line_x >= max_x
+                && min_line_y > min_y
+                && max_line_y < max_y)
+                || (min_line_y <= min_y
+                    && max_line_y >= max_y
+                    && min_line_x > min_x
+                    && max_line_x < max_x)
+            {
+                // Line crosses rectangle
+                return false;
+            }
+        }
+    }
+    true
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part1() {
+        let input = vec!["7,1", "11,1", "11,7", "9,7", "9,5", "2,5", "2,3", "7,3"];
+
+        assert_eq!(part1(&input.join("\n")), 50);
+    }
+
+    #[test]
+    fn test_part2() {
+        let input = vec!["7,1", "11,1", "11,7", "9,7", "9,5", "2,5", "2,3", "7,3"];
+
+        assert_eq!(part2(&input.join("\n")), 24);
+    }
+}

2025/tests/day09.rs

@@ -0,0 +1,13 @@
+use aoc2025::days::day09::{part1, part2};
+
+#[test]
+fn test_part1() {
+    let content = std::include_str!("../../private/inputs/2025/day09.txt");
+    assert_eq!(part1(&content), 4749672288);
+}
+
+#[test]
+fn test_part2() {
+    let content = std::include_str!("../../private/inputs/2025/day09.txt");
+    assert_eq!(part2(&content), 1479665889);
+}