From 28a7a098e38713f09af56849c7025586623c176f Mon Sep 17 00:00:00 2001
From: sloane <1699281+sloanelybutsurely@users.noreply.github.com>
Date: Sat, 9 Dec 2023 13:37:06 -0500
Subject: [PATCH] solve 2023 9.2

---
 2023/lib/2023/9.ex | 14 +++++++++++++-
 README.md          |  2 +-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/2023/lib/2023/9.ex b/2023/lib/2023/9.ex
index 01da81e..500c4f7 100644
--- a/2023/lib/2023/9.ex
+++ b/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)
diff --git a/README.md b/README.md
index ec67642..e2f63b1 100644
--- a/README.md
+++ b/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