Day 9

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

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

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

tests/day9/1.1

@@ -0,0 +1 @@
+114

tests/day9/1.2

@@ -0,0 +1 @@
+2