[2025] Day 9 draft

Author: connorslade

README.md

@@ -13,6 +13,7 @@ Thank you to [Eric Wastl](http://was.tl) for running this incredible yearly even
 - [Day 06: Trash Compactor](aoc_2025/src/day_06.rs)
 - [Day 07: Laboratories](aoc_2025/src/day_07.rs)
 - [Day 08: Playground](aoc_2025/src/day_08.rs)
+- [Day 09: Movie Theater](aoc_2025/src/day_09.rs)
 <!-- MARKER -->
 
 ## [2024](https://adventofcode.com/2024) [![aoc_2024](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2024.yml/badge.svg)](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2024.yml)

aoc_2025/src/day_09.rs

@@ -0,0 +1,103 @@
+use std::{cmp::Reverse, iter};
+
+use common::{Answer, solution};
+use itertools::Itertools;
+use nd_vec::{Vec2, vector};
+
+solution!("Movie Theater", 9);
+
+fn part_a(input: &str) -> Answer {
+    parse(input)
+        .iter()
+        .combinations(2)
+        .map(|x| (x[0].x().abs_diff(x[1].x()) + 1) * (x[0].y().abs_diff(x[1].y()) + 1))
+        .max()
+        .unwrap()
+        .into()
+}
+
+fn part_b(input: &str) -> Answer {
+    let red = parse(input);
+    red.iter()
+        .tuple_combinations()
+        .map(|(a, b)| {
+            (
+                (a.x().abs_diff(b.x()) + 1) * (a.y().abs_diff(b.y()) + 1),
+                (a, b),
+            )
+        })
+        .sorted_by_key(|(area, _)| Reverse(*area))
+        .find(|(_area, (a, b))| {
+            let min = vector!(a.x().min(b.x()), a.y().min(b.y()));
+            let max = vector!(a.x().max(b.x()), a.y().max(b.y()));
+
+            for (&α, &β) in red.iter().chain(iter::once(&red[0])).tuple_windows() {
+                let lmin = vector!(α.x().min(β.x()), α.y().min(β.y()));
+                let lmax = vector!(α.x().max(β.x()), α.y().max(β.y()));
+
+                if α.x() == β.x() {
+                    let x = β.x();
+                    if ((lmin.y() < min.y() && lmax.y() > min.y())
+                        || (lmin.y() < max.y() && lmax.y() > max.y())
+                        || (lmin.y() >= min.y() && lmax.y() <= max.y()))
+                        && x > min.x()
+                        && x < max.x()
+                    {
+                        return false;
+                    }
+                } else if α.y() == β.y() {
+                    let y = β.y();
+                    if ((lmin.x() < min.x() && lmax.x() > min.x())
+                        || (lmin.x() < max.x() && lmax.x() > max.x())
+                        || (lmin.x() >= min.x() && lmax.x() <= max.x()))
+                        && y > min.y()
+                        && y < max.y()
+                    {
+                        return false;
+                    }
+                }
+            }
+
+            true
+        })
+        .unwrap()
+        .0
+        .into()
+}
+
+// not: >1565730054
+
+fn parse(input: &str) -> Vec<Vec2<i64>> {
+    (input.lines())
+        .map(|x| {
+            let (x, y) = x.split_once(',').unwrap();
+            vector!(x.parse().unwrap(), y.parse().unwrap())
+        })
+        .collect::<Vec<_>>()
+}
+
+#[cfg(test)]
+mod test {
+    use indoc::indoc;
+
+    const CASE: &str = indoc! {"
+        7,1
+        11,1
+        11,7
+        9,7
+        9,5
+        2,5
+        2,3
+        7,3
+    "};
+
+    #[test]
+    fn part_a() {
+        assert_eq!(super::part_a(CASE), 50.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(super::part_b(CASE), 24.into());
+    }
+}

aoc_2025/src/lib.rs

@@ -8,6 +8,7 @@ mod day_05;
 mod day_06;
 mod day_07;
 mod day_08;
+mod day_09;
 // [import_marker]
 
 pub const SOLUTIONS: &[Solution] = &[
@@ -19,5 +20,6 @@ pub const SOLUTIONS: &[Solution] = &[
     day_06::SOLUTION,
     day_07::SOLUTION,
     day_08::SOLUTION,
+    day_09::SOLUTION,
     // [list_marker]
 ];