1 files changed, 31 insertions(+), 0 deletions(-)

A 10-2.nix

@@ 0,0 1,31 @@
+# SPDX-FileCopyrightText: 2020 Aluísio Augusto Silva Gonçalves <https://aasg.name>
+#
+# SPDX-License-Identifier: CC0-1.0
+
+{ pkgs ? import <nixpkgs> { }
+, aasg-nixexprs ? fetchTarball "https://git.sr.ht/~aasg/nixexprs/archive/master.tar.gz"
+}:
+with import "${aasg-nixexprs}/lib/extension.nix" { inherit (pkgs) lib; };
+let
+  input = pipe ./10-input.txt [
+    readFile
+    (splitString "\n")
+    (filter (s: s != ""))
+    (map toInt)
+  ];
+  ratings = pipe input [
+    (sort (a: b: a < b))
+    (xs: [ 0 ] ++ xs ++ [ (last xs + 3) ])
+  ];
+
+  # The input is a graph.  Count the paths from start to finish.
+  # Thanks to the folks at https://redd.it/ka9yj0 for pointing at the
+  # right direction, took me a few steps to find the correct algorithm.
+  reachableFrom = xs: n:
+    filter (x: x - n > 0 && x - n <= 3) xs;
+  pathsCount = foldr
+    (n: reachmap: reachmap // { ${toString n} = foldl' add 0 (map (n': reachmap.${toString n'}) (reachableFrom ratings n)); })
+    ((listToAttrs (map (n: nameValuePair (toString n) 0) ratings)) // { ${toString (last ratings)} = 1; })
+    (init ratings);
+in
+pathsCount."0"