Add solution for 2023 day11

2023/day11_cosmic_expansion/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "day11_cosmic_expansion"
+version = "0.1.0"
+edition = "2021"
+rust-version.workspace = true
+
+[dependencies]
+log.workspace = true
+env_logger.workspace = true
+ndarray.workspace = true
+itertools.workspace = true

2023/day11_cosmic_expansion/README.md

@@ -0,0 +1,19 @@
+My first thought reading this was [Floyd-Warshall](https://en.wikipedia.org/wiki/Floyd%E2%80%93Warshall_algorithm).
+But since this is a discrete matrix then the distances between the galaxies can
+be computed using a simple [Taxicab/Manhattan distance](https://en.wikipedia.org/wiki/Taxicab_geometry).
+Now, the crucial piece of information that makes this work is that "the
+shortest path between two galaxies is allowed to pass through another galaxy".
+Without this, the taxicab distance _does not_ work. I did not read that part
+initially so I had dismissed this solution, instead looking into how to
+implement [A\*](https://en.wikipedia.org/wiki/A*_search_algorithm).
+
+I remember seeing on reddit the expression "Advent of reading comprehension"
+and this is the perfect example of that. The constraint did eventually register
+with me so I went back to taxicab but I had spent quite a bit of time to reach
+that point.
+
+---
+
+Regarding the implementation itself, there's nothing special, this was just
+another one of those mathy problems and I lucked out on part 2 being easy
+purely because I didn't choose to go for a graph implementation in part 1. Meh.