From 379fc7f2e9754728aa1f7b886a39469a3f0d5965 Mon Sep 17 00:00:00 2001
From: Johnathon Slightham <slighthamj@gmail.com>
Date: Tue, 12 Dec 2023 00:44:21 -0500
Subject: [PATCH] Day 9

---
 2023/day9/p1.rb | 13 +++++++++++++
 2023/day9/p2.rb | 11 +++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 2023/day9/p1.rb
 create mode 100644 2023/day9/p2.rb

diff --git a/2023/day9/p1.rb b/2023/day9/p1.rb
new file mode 100644
index 0000000..ff852be
--- /dev/null
+++ b/2023/day9/p1.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+inputs = []
+File.foreach('./2023/day9/data') do |line|
+  inputs << line.split(' ').map(&:to_i)
+end
+
+def add_derivative(inputs)
+  return 0 if inputs.all?(&:zero?)
+  inputs[-1] + add_derivative(inputs.map.with_index { |x, i| i.zero? ? x : x - inputs[i - 1] }[1..inputs.length])
+end
+
+puts inputs.map { |input| add_derivative input }.sum
diff --git a/2023/day9/p2.rb b/2023/day9/p2.rb
new file mode 100644
index 0000000..3339ba2
--- /dev/null
+++ b/2023/day9/p2.rb
@@ -0,0 +1,11 @@
+inputs = []
+File.foreach('./2023/day9/data') do |line|
+  inputs << line.split(' ').map(&:to_i)
+end
+
+def add_derivative(inputs)
+  return 0 if inputs.all?(&:zero?)
+  inputs[-1] - add_derivative(inputs.map.with_index { |x, i| i.zero? ? x : inputs[i - 1] - x }[1..inputs.length])
+end
+
+puts inputs.map { |input| add_derivative input.reverse }.sum