y2023/ex11 updated readme

y2023/ex11/README.md

@@ -1,15 +1,120 @@
-# Day 11: TODO: ADD TITLE HERE
+# Day 11: Cosmic Expansion
 
 [Check it out on adventofcode.com](https://adventofcode.com/2023/day/11)
 
 ## Part One
 
-TODO: ADD DESCRIPTION HERE
+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.
 
-Your puzzle answer was `?`. (TODO: )
+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 `9536038`.
 
 ## Part Two
 
-TODO: ADD DESCRIPTION HERE
 
-Your puzzle answer was `?`. (TODO: )
+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 `447744640566`