Advent of Code 2023 - Day 11

- Solve day 11 puzzle

README.md

@@ -82,6 +82,11 @@ SKIP_SLOW_TESTS=1 bundle exec rake test
         <td><a href="lib/puzzles/2023/day10"><code>lib/puzzles/2023/day10</code></a></td>
         <td>🌟🌟</td>
       </tr>
+      <tr>
+        <td>1️⃣1️⃣ <a href="https://adventofcode.com/2023/day/11">Cosmic Expansion</a></td>
+        <td><a href="lib/puzzles/2023/day11"><code>lib/puzzles/2023/day11</code></a></td>
+        <td>🌟🌟</td>
+      </tr>
     </table>
   </p>
 </details>

lib/puzzles/2023/day11/README.md

@@ -0,0 +1,126 @@
+# [Day 11: Cosmic Expansion](https://adventofcode.com/2023/day/11)
+
+## Part One
+
+You continue following signs for "Hot Springs" and eventually come across an [observatory](https://en.wikipedia.org/wiki/Observatory). The Elf within turns out to be a researcher studying cosmic expansion using the giant telescope here.
+
+He doesn't know anything about the missing machine parts; he's only visiting for this research project. However, he confirms that the hot springs are the next-closest area likely to have people; he'll even take you straight there once he's done with today's observation analysis.
+
+Maybe you can help him with the analysis to speed things up?
+
+The researcher has collected a bunch of data and compiled the data into a single giant **image** (your puzzle input). The image includes **empty space** (`.`) and **galaxies** (`#`). For example:
+
+```
+...#......
+.......#..
+#.........
+..........
+......#...
+.#........
+.........#
+..........
+.......#..
+#...#.....
+```
+
+The researcher is trying to figure out the sum of the lengths of the **shortest path between every pair of galaxies**. However, there's a catch: the universe expanded in the time it took the light from those galaxies to reach the observatory.
+
+Due to something involving gravitational effects, **only some space expands**. In fact, the result is that **any rows or columns that contain no galaxies** should all actually be twice as big.
+
+In the above example, three columns and two rows contain no galaxies:
+
+```
+   v  v  v
+ ...#......
+ .......#..
+ #.........
+>..........<
+ ......#...
+ .#........
+ .........#
+>..........<
+ .......#..
+ #...#.....
+   ^  ^  ^
+```
+
+These rows and columns need to be **twice as big**; the result of cosmic expansion therefore looks like this:
+
+```
+....#........
+.........#...
+#............
+.............
+.............
+........#....
+.#...........
+............#
+.............
+.............
+.........#...
+#....#.......
+```
+
+Equipped with this expanded universe, the shortest path between every pair of galaxies can be found. It can help to assign every galaxy a unique number:
+
+```
+....1........
+.........2...
+3............
+.............
+.............
+........4....
+.5...........
+............6
+.............
+.............
+.........7...
+8....9.......
+```
+
+In these 9 galaxies, there are **36 pairs**. Only count each pair once; order within the pair doesn't matter. For each pair, find any shortest path between the two galaxies using only steps that move up, down, left, or right exactly one `.` or `#` at a time. (The shortest path between two galaxies is allowed to pass through another galaxy.)
+
+For example, here is one of the shortest paths between galaxies `5` and `9`:
+
+```
+....1........
+.........2...
+3............
+.............
+.............
+........4....
+.5...........
+.##.........6
+..##.........
+...##........
+....##...7...
+8....9.......
+```
+
+This path has length `9` because it takes a minimum of **nine steps** to get from galaxy `5` to galaxy `9` (the eight locations marked `#` plus the step onto galaxy `9` itself). Here are some other example shortest path lengths:
+
+- Between galaxy `1` and galaxy `7`: 15
+- Between galaxy `3` and galaxy `6`: 17
+- Between galaxy `8` and galaxy `9`: 5
+
+In this example, after expanding the universe, the sum of the shortest path between all 36 pairs of galaxies is **`374`**.
+
+Expand the universe, then find the length of the shortest path between every pair of galaxies. **What is the sum of these lengths?**
+
+Your puzzle answer was `10313550`.
+
+**The first half of this puzzle is complete! It provides one gold star:** 🌟
+
+## Part Two
+
+The galaxies are much **older** (and thus much **farther apart**) than the researcher initially estimated.
+
+Now, instead of the expansion you did before, make each empty row or column **one million times** larger. That is, each empty row should be replaced with `1000000` empty rows, and each empty column should be replaced with `1000000` empty columns.
+
+(In the example above, if each empty row or column were merely `10` times larger, the sum of the shortest paths between every pair of galaxies would be **`1030`**. If each empty row or column were merely `100` times larger, the sum of the shortest paths between every pair of galaxies would be **`8410`**. However, your universe will need to expand far beyond these values.)
+
+Starting with the same initial image, expand the universe according to these new rules, then find the length of the shortest path between every pair of galaxies. **What is the sum of these lengths?**
+
+Your puzzle answer was `611998089572`.
+
+**Both parts of this puzzle are complete! They provide two gold stars:** 🌟🌟

lib/puzzles/2023/day11/day11.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require_relative "part1"
+require_relative "part2"
+
+module AdventOfCode
+  module Puzzles2023
+    ##
+    # {include:file:lib/puzzles/2023/day11/README.md}
+    module Day11
+    end
+  end
+end