Add solution for "Day 8: Playground", part 1

README.md

@@ -20,7 +20,11 @@ This project does not contain the puzzle or example inputs as per the [copyright
 
 ### Day 6: Trash Compactor
 
-:mag_right: Puzzle: <https://adventofcode.com/2025/day/6>, :white_check_mark: Solver: [`Trash Compactor`](src/solvers/trash_compactor.rs)
+:mag_right: Puzzle: <https://adventofcode.com/2025/day/6>, :white_check_mark: Solver: [`TrashCompactor`](src/solvers/trash_compactor.rs)
+
+### Day 8: Playground
+
+:mag_right: Puzzle: <https://adventofcode.com/2025/day/8>, :white_check_mark: Solver: [`Playground`](src/solvers/playground.rs)
 
 ## Tests
 

src/main.rs

@@ -2,7 +2,12 @@ use advent_of_code_2025::solvers;
 
 fn main() {
     println!("### Advent of Code 2025 ###\n");
+    // SecretEntrance
+    // GiftShop
+    // Lobby
     solvers::run(solvers::printing_department::PrintingDepartment {});
     solvers::run(solvers::cafeteria::Cafeteria {});
     solvers::run(solvers::trash_compactor::TrashCompactor {});
+    // Laboratories
+    solvers::run(solvers::playground::Playground::new(None));
 }

src/solvers.rs

@@ -6,6 +6,7 @@ use std::path;
 use std::path::Path;
 
 pub mod cafeteria;
+pub mod playground;
 pub mod printing_department;
 pub mod trash_compactor;
 

src/solvers/playground.rs

@@ -0,0 +1,95 @@
+use crate::solvers::Solver;
+use std::collections::{BTreeMap, HashSet};
+use std::io::BufRead;
+
+pub struct Playground {
+    connections: usize,
+}
+
+impl Playground {
+    pub fn new(connections: Option<usize>) -> Playground {
+        Playground {
+            connections: connections.unwrap_or(1000),
+        }
+    }
+
+    // Collects all junction box coordinates.
+    fn junction_boxes<R: BufRead>(reader: R) -> Vec<(u64, u64, u64)> {
+        let mut junction_boxes = Vec::new();
+        for line in reader.lines().map_while(Result::ok) {
+            let j: Vec<u64> = line.split(',').map(|s| s.parse().unwrap()).collect();
+            junction_boxes.push((j[0], j[1], j[2]));
+        }
+        junction_boxes
+    }
+
+    // Maps the closest pairs of junction boxes to their distances, up to the maximum number of
+    // connections.
+    fn distances<'a>(
+        &self,
+        junction_boxes: &'a Vec<(u64, u64, u64)>,
+    ) -> BTreeMap<u64, (&'a (u64, u64, u64), &'a (u64, u64, u64))> {
+        let mut distances = BTreeMap::new();
+        for i in 0..junction_boxes.len() {
+            let (_, right) = junction_boxes.split_at(i + 1);
+            for b in right {
+                let d = Self::distance_sqr(&junction_boxes[i], b);
+                distances.insert(d, (&junction_boxes[i], b));
+                if distances.len() > self.connections {
+                    distances.pop_last();
+                }
+            }
+        }
+        distances
+    }
+
+    fn circuit_sizes(distances: &BTreeMap<u64, (&(u64, u64, u64), &(u64, u64, u64))>) -> u64 {
+        let mut circuits: Vec<HashSet<&(u64, u64, u64)>> = Vec::new();
+        for junction_boxes in distances.values() {
+            let index_a = circuits.iter().position(|c| c.contains(&junction_boxes.0));
+            let index_b = circuits.iter().position(|c| c.contains(&junction_boxes.1));
+            if let Some(index_a) = index_a {
+                if let Some(index_b) = index_b {
+                    if index_a != index_b {
+                        let (low, high) = if index_a < index_b {
+                            (index_a, index_b)
+                        } else {
+                            (index_b, index_a)
+                        };
+                        let mut c = circuits.remove(high);
+                        circuits[low].extend(c.drain());
+                    }
+                } else {
+                    circuits[index_a].insert(junction_boxes.1);
+                }
+            } else {
+                if let Some(index_b) = index_b {
+                    circuits[index_b].insert(junction_boxes.0);
+                } else {
+                    let mut set = HashSet::new();
+                    set.insert(junction_boxes.0);
+                    set.insert(junction_boxes.1);
+                    circuits.push(set);
+                }
+            }
+        }
+        circuits.sort_by(|a, b| b.len().cmp(&a.len()));
+        circuits.iter().take(3).map(|c| c.len() as u64).product()
+    }
+
+    fn distance_sqr(a: &(u64, u64, u64), b: &(u64, u64, u64)) -> u64 {
+        a.0.abs_diff(b.0).pow(2) + a.1.abs_diff(b.1).pow(2) + a.2.abs_diff(b.2).pow(2)
+    }
+}
+
+impl Solver for Playground {
+    const PUZZLE_INDEX: u8 = 8;
+    const PUZZLE_NAME: &'static str = "Playground";
+
+    fn process_data<R: BufRead>(&self, reader: R) -> (u64, u64) {
+        let junction_boxes = Self::junction_boxes(reader);
+        let distances = self.distances(&junction_boxes);
+
+        (Self::circuit_sizes(&distances), 0)
+    }
+}