Day 9 Part 1

2020/day9/day9.ex

@@ -0,0 +1,50 @@
+defmodule SetTools do
+  def combinations(_, 0) do
+    [[]]
+  end
+
+  def combinations([], _) do
+    []
+  end
+
+  def combinations(list = [head | tail], size) do
+    taken = Stream.map(combinations(tail, size - 1), fn subset -> [head | subset] end)
+    left = combinations(tail, size)
+    Stream.concat(taken, left)
+  end
+end
+
+defmodule Day9 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp parse_lines(text) do
+    text
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&String.to_integer/1)
+  end
+
+  def part1 do
+    preamble_size = 25
+
+    read_input()
+    |> parse_lines()
+    |> Stream.chunk_every(preamble_size + 1, 1, :discard)
+    |> Enum.find(fn numbers ->
+      preamble = Enum.take(numbers, preamble_size)
+      next = List.last(numbers)
+
+      has_match =
+        SetTools.combinations(preamble, 2)
+        |> Enum.any?(fn subset -> next == Enum.sum(subset) end)
+
+      !has_match
+    end)
+    |> Enum.at(preamble_size)
+  end
+end
+
+Day9.part1() |> IO.inspect()