this post was submitted on 09 Dec 2025
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Advent Of Code

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An unofficial home for the advent of code community on programming.dev! Other challenges are also welcome!

Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

Everybody Codes is another collection of programming puzzles with seasonal events.

EC 2025

AoC 2025

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๐ŸŽ - 2025 DAY 9 SOLUTIONS - ๐ŸŽ (programming.dev)
submitted 1 month ago by CameronDev@programming.dev to c/advent_of_code@programming.dev
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Day 9: Movie Theater

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[โ€“] CameronDev@programming.dev 2 points 1 month ago* (last edited 1 month ago)

Rust

pt2: 71ms.

Wow. What a step up in difficulty. Using the geo library got me the right answer, and quickly in terms of code, but run time was terrible (2m+). Switching back to simply checking if a wall is entirely outside the rectangle got me a much faster win, and the code isn't too bad either.

Code and algorithm description(A wall is outside if its entirely to the left OR entirely to the right of the rect) OR (entirely above OR entirely below).

   fn check_wall_outside(
        c: &(&Coord, &Coord),
        top: usize,
        bottom: usize,
        left: usize,
        right: usize,
    ) -> bool {
        if (c.0.x <= left && c.1.x <= left) || (c.0.x >= right && c.1.x >= right) {
            return true;
        }
        if (c.0.y <= top && c.1.y <= top) || (c.0.y >= bottom && c.1.y >= bottom) {
            return true;
        }
        false
    }

   #[test]
    fn test_y2025_day9_part2_mine1() {
        let input = include_str!("../../input/2025/day_9.txt");
        let coords = input
            .lines()
            .map(|l| {
                let halfs = l.split_once(',').unwrap();
                Coord {
                    x: halfs.0.parse::<usize>().unwrap(),
                    y: halfs.1.parse::<usize>().unwrap(),
                }
            })
            .collect::<Vec<Coord>>();

        let mut walls = vec![];

        for i in 0..coords.len() {
            let first = &coords[i];
            let second = coords.get(i + 1).unwrap_or(coords.first().unwrap());

            walls.push((first, second));
        }

        let mut max_area = 0;
        for i in 0..coords.len() {
            let first = &coords[i];
            'next_rect: for j in i..coords.len() {
                let second = &coords[j];
                if first == second {
                    continue 'next_rect;
                }
                let area = (first.x.abs_diff(second.x) + 1) * (first.y.abs_diff(second.y) + 1);
                if area < max_area {
                    continue 'next_rect;
                }

                let (top, bottom) = if first.y > second.y {
                    (second.y, first.y)
                } else {
                    (first.y, second.y)
                };

                let (left, right) = if first.x > second.x {
                    (second.x, first.x)
                } else {
                    (first.x, second.x)
                };

                for wall in &walls {
                    if !check_wall_outside(wall, top, bottom, left, right) {
                        continue 'next_rect;
                    }
                }

                max_area = area;
            }
        }
        assert_eq!(max_area, 1542119040);
        println!("Part 2: {}", max_area);
    }