From 69ea94bec09e00c01d770a3624ce3ed266566a2b Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Fri, 12 Dec 2025 07:13:50 +0100
Subject: [PATCH] Solve 12

---
 src/bin/12.rs | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 src/bin/12.rs

diff --git a/src/bin/12.rs b/src/bin/12.rs
new file mode 100644
index 0000000..e0974d5
--- /dev/null
+++ b/src/bin/12.rs
@@ -0,0 +1,82 @@
+use aoc::*;
+use glam::USizeVec2;
+use itertools::Itertools;
+
+fn main() {
+    let input = include_str!("../../input/12");
+    println!("Part 1: {}", part1(input));
+}
+
+fn part1(input: &str) -> usize {
+    let (presents, regions) = parse_input(input);
+    regions.into_iter().filter(|r| r.fits(&presents)).count()
+}
+
+fn parse_input(input: &str) -> (Vec<Present>, Vec<Region>) {
+    let chunks = input.split("\n\n").collect_vec();
+    let (presents, regions) = chunks.split_at(chunks.len() - 1);
+    let presents = presents.iter().map(|p| Present::parse(p)).collect();
+    let regions = regions
+        .iter()
+        .flat_map(|chunk| chunk.lines())
+        .map(Region::parse)
+        .collect();
+    (presents, regions)
+}
+
+#[derive(Copy, Clone)]
+struct Present {
+    area: usize,
+}
+
+impl Present {
+    fn parse(input: &str) -> Self {
+        let area = input
+            .lines()
+            .skip(1)
+            .flat_map(|line| line.chars())
+            .filter(|&c| c == '#')
+            .count();
+        Self { area }
+    }
+}
+
+struct Region {
+    size: USizeVec2,
+    presents: Vec<usize>,
+}
+
+impl Region {
+    fn fits(&self, presents: &[Present]) -> bool {
+        let total_present_area: usize = self
+            .pick_presents(presents)
+            .into_iter()
+            .map(|(n, present)| n * present.area)
+            .sum();
+        total_present_area <= self.area()
+    }
+
+    fn pick_presents(&self, presents: &[Present]) -> Vec<(usize, Present)> {
+        self.presents
+            .iter()
+            .copied()
+            .enumerate()
+            .map(|(i, n)| (n, presents[i]))
+            .collect()
+    }
+
+    fn area(&self) -> usize {
+        self.size.x * self.size.y
+    }
+
+    fn parse(line: &str) -> Self {
+        let (size, presents) = line.split_once(": ").unwrap();
+
+        let (x, y) = size.split_once('x').unwrap();
+        let size = USizeVec2::new(x.parse().unwrap(), y.parse().unwrap());
+
+        let presents = parse_ws_separated(presents).collect();
+
+        Self { size, presents }
+    }
+}