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