solve 2023 9.2

2023/lib/2023/9.ex

@@ -9,7 +9,11 @@ aoc 2023, 9 do
     |> Enum.sum()
   end
 
-  def p2(_input) do
+  def p2(input) do
+    input
+    |> lines_of_integers()
+    |> Enum.map(&prev_in_sequence/1)
+    |> Enum.sum()
   end
 
   def next_in_sequence(xs) do
@@ -19,6 +23,14 @@ aoc 2023, 9 do
     |> Enum.sum()
   end
 
+  def prev_in_sequence(xs) do
+    xs
+    |> differentiate()
+    |> Enum.map(&List.first/1)
+    |> Enum.reverse()
+    |> Enum.reduce(&(&1 - &2))
+  end
+
   def differentiate(xs) do
     xs
     |> Stream.iterate(&deltas/1)

README.md

@@ -14,7 +14,7 @@
 1. [2020] **17/50** 🌟
 1. [2021] **43/50** 🌟
 1. [2022] **14/50** 🌟
-1. [2023] **14/50** 🌟
+1. [2023] **15/50** 🌟
 
 [2015]: ./2015
 [2017]: ./2017