From 07fecf811d3a4f7e077b3404b6a5eb6231dad68b Mon Sep 17 00:00:00 2001
From: bluepython508 <bluepython508@users.noreply.github.io>
Date: Sat, 9 Dec 2023 12:03:00 +0000
Subject: [PATCH] Day 9

---
 lib/day9.ex      | 35 +++++++++++++++++++++++++++++++++++
 lib/mix_tasks.ex |  3 ++-
 tests/day9/1     |  3 +++
 tests/day9/1.1   |  1 +
 tests/day9/1.2   |  1 +
 5 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 lib/day9.ex
 create mode 100644 tests/day9/1
 create mode 100644 tests/day9/1.1
 create mode 100644 tests/day9/1.2

diff --git a/lib/day9.ex b/lib/day9.ex
new file mode 100644
index 0000000..347bc80
--- /dev/null
+++ b/lib/day9.ex
@@ -0,0 +1,35 @@
+defmodule Aoc2023.Day9 do
+  use Aoc2023
+
+  defp differences(lst) do
+    Enum.zip_with(lst, Enum.drop(lst, 1), &(&2 - &1))
+  end
+
+  defp prediction(lst) do
+    diffs = differences(lst)
+
+    if Enum.all?(diffs, &(&1 == 0)) do
+      List.first(lst)
+    else
+      List.last(lst) + prediction(diffs)
+    end
+  end
+
+  def parse(input) do
+    input
+    |> lines
+    |> Enum.map(pipe(String.split() |> Enum.map(&String.to_integer/1)))
+  end
+
+  def part1(input) do
+    input
+    |> Enum.map(&prediction/1)
+    |> Enum.sum
+  end
+
+  def part2(input) do
+    input
+    |> Enum.map(pipe(Enum.reverse |> prediction))
+    |> Enum.sum
+  end
+end
diff --git a/lib/mix_tasks.ex b/lib/mix_tasks.ex
index d7d0a41..834e38f 100644
--- a/lib/mix_tasks.ex
+++ b/lib/mix_tasks.ex
@@ -10,6 +10,7 @@ defmodule Mix.Tasks.Aoc do
   defp module(6), do: Aoc2023.Day6
   defp module(7), do: Aoc2023.Day7
   defp module(8), do: Aoc2023.Day8
+  defp module(9), do: Aoc2023.Day9
   # [MODULE INSERTION POINT]  
 
   defp base_dir(), do: System.get_env("AOC_BASE")
@@ -204,4 +205,4 @@ defmodule Mix.Tasks.Aoc do
       {:noreply, state}
     end
   end
-end
+end
\ No newline at end of file
diff --git a/tests/day9/1 b/tests/day9/1
new file mode 100644
index 0000000..539a763
--- /dev/null
+++ b/tests/day9/1
@@ -0,0 +1,3 @@
+0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45
diff --git a/tests/day9/1.1 b/tests/day9/1.1
new file mode 100644
index 0000000..dee79f1
--- /dev/null
+++ b/tests/day9/1.1
@@ -0,0 +1 @@
+114
diff --git a/tests/day9/1.2 b/tests/day9/1.2
new file mode 100644
index 0000000..0cfbf08
--- /dev/null
+++ b/tests/day9/1.2
@@ -0,0 +1 @@
+2