Add day 9

lib/aoc2020.ex

@@ -26,4 +26,7 @@ defmodule Aoc2020 do
 
   defdelegate day8_part1(), to: Aoc2020.Day8, as: :part1
   defdelegate day8_part2(), to: Aoc2020.Day8, as: :part2
+
+  defdelegate day9_part1(), to: Aoc2020.Day9, as: :part1
+  defdelegate day9_part2(), to: Aoc2020.Day9, as: :part2
 end

lib/aoc2020/day9.ex

@@ -0,0 +1,63 @@
+defmodule Aoc2020.Day9 do
+  def part1() do
+    part1(load_input())
+  end
+
+  def part1(input, preamble \\ 25) do
+    input
+    |> Stream.chunk_every(preamble + 1, 1)
+    |> Enum.find_value(fn list ->
+      {total, options} = List.pop_at(list, -1)
+
+      combinations =
+        for a <- options, b <- options, uniq: true do
+          [a, b] |> Enum.sort() |> List.to_tuple()
+        end
+
+      valid? = Enum.any?(combinations, fn {a, b} -> a + b == total end)
+
+      unless valid?, do: total
+    end)
+  end
+
+  def part2() do
+    input = load_input()
+    part2(input, part1(input))
+  end
+
+  def part2(input, search) do
+    list =
+      input
+      |> Stream.chunk_while([], chunk_fun(search), &after_fun/1)
+      |> Enum.at(0)
+
+    Enum.min(list) + Enum.max(list)
+  end
+
+  defp chunk_fun(total) do
+    fn element, acc ->
+      with_element = [element | acc]
+      check_sum(with_element, total)
+    end
+  end
+
+  defp check_sum(with_element, total) do
+    case Enum.sum(with_element) do
+      ^total -> {:cont, with_element, List.delete_at(with_element, -1)}
+      t when t < total -> {:cont, with_element}
+      t when t > total -> check_sum(List.delete_at(with_element, -1), total)
+    end
+  end
+
+  defp after_fun(acc), do: {:cont, acc}
+
+  @doc false
+  def load_input do
+    path = Application.app_dir(:aoc_2020, "priv/inputs/09/stream.txt")
+
+    path
+    |> File.stream!()
+    |> Stream.map(&String.trim/1)
+    |> Stream.map(&String.to_integer/1)
+  end
+end