solve 2015 19.1

2015/.envrc

@@ -1 +1 @@
-export ADVENT_OF_CODE_SESSION=53616c7465645f5f92dfbc249005a7e94ec3c6e62cd4e5e9ddf8e368a48c0c80440b6c5e37d836cc0b294e19a9e8a7f1
+export ADVENT_OF_CODE_SESSION=53616c7465645f5f18112545c7b5290961cf3f2c965e90c1b68415326ef1f99790e4dde4ef0cebc7abd120ea1b537aada76420516a0ceb883a722a84b81e7a02

2015/.tool-versions

@@ -1,2 +1,2 @@
-erlang 24.1.4
-elixir 1.12.3-otp-24
+erlang 26.1.2
+elixir 1.15.7-otp-26

2015/README.md

@@ -16,7 +16,7 @@
 | :--: | :--: | :-:  | :-: | :-:  | :-:  | :-:  |
 |      |      | [1]  | [2] | [3]  | [4]  | [5]  |
 |  [6] | [7]  | [8]  | [9] | [10] | [11] | [12] |
-| [13] | [14] | [15] | [16]| [17] | [18] | 19   |
+| [13] | [14] | [15] | [16]| [17] | [18] | [19] |
 |  20  | 21   | 22   | 23  | 24   | 25   |      |
 
 [1]: ./lib/2015/1.ex
@@ -37,3 +37,4 @@
 [16]: ./lib/2015/16.ex
 [17]: ./lib/2015/17.ex
 [18]: ./lib/2015/18.ex
+[19]: ./lib/2015/19.ex

2015/lib/2015/19.ex

@@ -18,25 +18,27 @@ aoc 2015, 19 do
     {replacement_mappings, starting_molecule}
   end
 
-  def splits(xs) do
-    for i <- 1..(length(xs) - 1) do
-      Enum.split(xs, i)
-    end
+  def possible_substitutions("", _, _), do: []
+
+  def possible_substitutions(str, source, replacement) do
+    <<curr::binary-size(1), next::binary>> = str
+
+    this =
+      if String.starts_with?(str, source),
+        do: [String.replace(str, source, replacement, global: false)],
+        else: []
+
+    this ++ for r <- possible_substitutions(next, source, replacement), do: curr <> r
   end
 
-  def possible_replacements(mappings, str) do
-    for {source, replacements} <- mappings,
-        replacement <- replacements,
-        {l, r} <- splits(String.split(str, source)),
-        uniq: true do
-      Enum.join(
-        [
-          Enum.join(l, source),
-          Enum.join(r, source)
-        ],
-        replacement
-      )
+  def possible_replacements(mappings, molecule) do
+    for {source, replacements} <- mappings, replacement <- replacements do
+      {source, replacement}
     end
+    |> Enum.flat_map(fn {source, replacement} ->
+      possible_substitutions(molecule, source, replacement)
+    end)
+    |> Enum.uniq()
   end
 
   def p1 do

README.md

@@ -6,7 +6,7 @@
 
 ## Years
 
-1. [2015] **36/50** 🌟
+1. [2015] **37/50** 🌟
 1. 2016
 1. [2017] **18/50** 🌟
 1. 2018