From 2759b7e1f63c8777a7d52bc4d0f5cec71142d53d Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Sun, 7 Dec 2025 15:17:55 +0100
Subject: [PATCH] Solve 07

---
 src/bin/07.rs | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 src/bin/07.rs

diff --git a/src/bin/07.rs b/src/bin/07.rs
new file mode 100644
index 0000000..f93f8ee
--- /dev/null
+++ b/src/bin/07.rs
@@ -0,0 +1,95 @@
+use aoc::*;
+use cached::{proc_macro::cached, stores::UnboundCache};
+use std::collections::HashSet;
+
+const INPUT: &str = include_str!("../../input/07");
+
+fn main() {
+    println!("Part 1: {}", part1(INPUT));
+    println!("Part 2: {}", part2(INPUT));
+}
+
+#[test]
+fn example() {
+    assert_example!(part1, "07-test", 21);
+    assert_example!(part2, "07-test", 40);
+}
+
+fn part1(input: &str) -> usize {
+    let (start, splitters) = parse_input(input);
+
+    let mut sum = 0;
+    let mut beams = HashSet::from([start]);
+
+    for splitters in splitters {
+        let (count, new) = split_count(&splitters, beams);
+        beams = new;
+        sum += count;
+    }
+
+    sum
+}
+
+fn part2(input: &str) -> usize {
+    let (start, splitters) = parse_input(input);
+    timelines(start, 0, &splitters)
+}
+
+fn split_count(splitters: &HashSet<i32>, beams: HashSet<i32>) -> (usize, HashSet<i32>) {
+    let mut result = HashSet::new();
+    let mut count = 0;
+
+    for beam in beams {
+        if splitters.contains(&beam) {
+            result.insert(beam + 1);
+            result.insert(beam - 1);
+            count += 1;
+        } else {
+            result.insert(beam);
+        }
+    }
+
+    (count, result)
+}
+
+#[cached(
+    type = "UnboundCache<(i32, usize), usize>",
+    create = "{ UnboundCache::new() }",
+    convert = r#"{ (beam, depth) }"#
+)]
+fn timelines(beam: i32, depth: usize, splitters: &[HashSet<i32>]) -> usize {
+    if depth >= splitters.len() {
+        return 1;
+    }
+
+    let splitter = &splitters[depth];
+
+    if splitter.contains(&beam) {
+        timelines(beam - 1, depth + 1, splitters) + timelines(beam + 1, depth + 1, splitters)
+    } else {
+        timelines(beam, depth + 1, splitters)
+    }
+}
+
+/// Returns (start, lines of splitter positions)
+fn parse_input(input: &str) -> (i32, Vec<HashSet<i32>>) {
+    let mut lines = input.lines();
+
+    let first_line = lines.next().unwrap();
+    let start = first_line.chars().position(|c| c == 'S').unwrap() as i32;
+
+    let splitters = lines
+        .map(parse_line)
+        .filter(|line| !line.is_empty())
+        .collect();
+
+    (start, splitters)
+}
+
+fn parse_line(line: &str) -> HashSet<i32> {
+    line.chars()
+        .enumerate()
+        .filter(|&(_, c)| c == '^')
+        .map(|(i, _)| i as i32)
+        .collect()
+}