From 7df6f2ab51558230a3b41848bf7769458461b8bf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20M=C3=BCller?= <stefan.muller@ubisoft.com>
Date: Tue, 10 Feb 2026 10:41:56 +0100
Subject: [PATCH] Add solution for "Day 7: Laboratories", part 1 and 2

---
 Cargo.lock                  |  2 +-
 Cargo.toml                  |  2 +-
 README.md                   |  6 +++++
 src/main.rs                 |  2 +-
 src/solvers.rs              |  1 +
 src/solvers/laboratories.rs | 45 +++++++++++++++++++++++++++++++++++++
 tests/examples.rs           |  8 +++++++
 tests/full_data.rs          |  8 +++++++
 8 files changed, 71 insertions(+), 3 deletions(-)
 create mode 100644 src/solvers/laboratories.rs

diff --git a/Cargo.lock b/Cargo.lock
index f0e465d..5ca4e79 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -7,8 +7,8 @@ name = "advent_of_code_2025"
 version = "0.1.0"
 dependencies = [
  "grid",
+ "memchr",
  "primes",
- "regex",
  "sanitise-file-name",
  "scan_fmt",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 2b1ae2f..679764e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,7 +5,7 @@ edition = "2024"
 
 [dependencies]
 grid = "1.0.0"
+memchr = "2.7.6"
 primes = "0.4.0"
-regex = "1.12.2"
 sanitise-file-name = "1.0.0"
 scan_fmt = "0.2.6"
diff --git a/README.md b/README.md
index 397a8a0..e2335fc 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,12 @@ From the input data, we construct a [`MultiInterval`](src/common/interval.rs), w
 
 We implement two different ways to collect numbers from the input. For part 1, numbers separated by space are read line by line, stored in a two-dimensional array and then aggregated in columns. While for part 2, each column of characters in the input data represents a number, and empty columns are interpreted as the separators between blocks for aggregation.
 
+### Day 7: Laboratories
+
+:mag_right: Puzzle: <https://adventofcode.com/2025/day/7>, :white_check_mark: Solver: [`Laboratories`](src/solvers/laboratories.rs)
+
+Going row by row, for each column in the input data we count the number of different beam paths that reach that column in that row. When a beam is split, the count for that column is added to both columns the beam is split into. For part 1 we count the splits, and for parts 2 we sum the count for all columns of the final row.
+
 ### Day 8: Playground
 
 :mag_right: Puzzle: <https://adventofcode.com/2025/day/8>, :white_check_mark: Solver: [`Playground`](src/solvers/playground.rs)
diff --git a/src/main.rs b/src/main.rs
index eae05a1..a063ce1 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,7 +8,7 @@ fn main() {
     solvers::run(solvers::printing_department::PrintingDepartment {});
     solvers::run(solvers::cafeteria::Cafeteria {});
     solvers::run(solvers::trash_compactor::TrashCompactor {});
-    // Laboratories
+    solvers::run(solvers::laboratories::Laboratories {});
     solvers::run(solvers::playground::Playground::new(None));
     solvers::run(solvers::movie_theater::MovieTheater {});
     // Factory
diff --git a/src/solvers.rs b/src/solvers.rs
index 98136fb..f7de0de 100644
--- a/src/solvers.rs
+++ b/src/solvers.rs
@@ -7,6 +7,7 @@ use std::path::Path;
 
 pub mod cafeteria;
 pub mod gift_shop;
+pub mod laboratories;
 pub mod lobby;
 pub mod movie_theater;
 pub mod playground;
diff --git a/src/solvers/laboratories.rs b/src/solvers/laboratories.rs
new file mode 100644
index 0000000..589ce16
--- /dev/null
+++ b/src/solvers/laboratories.rs
@@ -0,0 +1,45 @@
+use crate::solvers::Solver;
+use memchr::{memchr, memchr_iter};
+use std::io::BufRead;
+
+pub struct Laboratories {}
+
+impl Laboratories {
+    const START_CHAR: u8 = b'S';
+    const SPLITTER_CHAR: u8 = b'^';
+
+    fn start_beam(line: &str) -> Vec<u64> {
+        let mut beams = vec![0; line.len()];
+        if let Some(s) = memchr(Self::START_CHAR, line.as_bytes()) {
+            beams[s] = 1;
+        }
+        beams
+    }
+
+    fn split_beams(line: &str, mut beams: Vec<u64>, mut splits: u64) -> (Vec<u64>, u64) {
+        for s in memchr_iter(Self::SPLITTER_CHAR, line.as_bytes()) {
+            if beams[s] > 0 {
+                splits += 1;
+                beams[s - 1] += beams[s];
+                beams[s + 1] += beams[s];
+                beams[s] = 0;
+            }
+        }
+        (beams, splits)
+    }
+}
+
+impl Solver for Laboratories {
+    const PUZZLE_INDEX: u8 = 7;
+    const PUZZLE_NAME: &'static str = "Laboratories";
+
+    fn process_data<R: BufRead>(&self, mut reader: R) -> (u64, u64) {
+        let line = reader.by_ref().lines().next().unwrap().unwrap();
+        let mut beams = Self::start_beam(&line);
+        let mut splits = 0;
+        for line in reader.lines().map_while(Result::ok) {
+            (beams, splits) = Self::split_beams(&line, beams, splits);
+        }
+        (splits, beams.iter().sum())
+    }
+}
diff --git a/tests/examples.rs b/tests/examples.rs
index 196a1bf..be6bab3 100644
--- a/tests/examples.rs
+++ b/tests/examples.rs
@@ -59,6 +59,14 @@ fn trash_compactor() {
     );
 }
 
+#[test]
+fn laboratories() {
+    assert_eq!(
+        Ok((21, 40)),
+        solvers::run_solver(solvers::laboratories::Laboratories {}, &EXAMPLE_PATHS)
+    );
+}
+
 #[test]
 fn playground() {
     assert_eq!(
diff --git a/tests/full_data.rs b/tests/full_data.rs
index 63f0bb7..e815d75 100644
--- a/tests/full_data.rs
+++ b/tests/full_data.rs
@@ -57,6 +57,14 @@ fn trash_compactor() {
     );
 }
 
+#[test]
+fn laboratories() {
+    assert_eq!(
+        Ok((1541, 80158285728929)),
+        solvers::run_solver(solvers::laboratories::Laboratories {}, &solvers::DATA_PATHS)
+    );
+}
+
 #[test]
 fn playground() {
     assert_eq!(