solve 2015 day 20

2015/README.md

@@ -17,7 +17,7 @@
 |      |      | [1]  | [2] | [3]  | [4]  | [5]  |
 |  [6] | [7]  | [8]  | [9] | [10] | [11] | [12] |
 | [13] | [14] | [15] | [16]| [17] | [18] | [19] |
-|  20  | 21   | 22   | 23  | 24   | 25   |      |
+| [20] | 21   | 22   | 23  | 24   | 25   |      |
 
 [1]: ./lib/2015/1.ex
 [2]: ./lib/2015/2.ex
@@ -38,3 +38,4 @@
 [17]: ./lib/2015/17.ex
 [18]: ./lib/2015/18.ex
 [19]: ./lib/2015/19.ex
+[20]: ./lib/2015/20.ex

2015/lib/2015/20.ex

@@ -0,0 +1,56 @@
+import AOC
+
+aoc 2015, 20 do
+  def p1 do
+    goal = input()
+
+    Stream.iterate(1, &(&1 + 1))
+    |> Enum.find(fn house ->
+      gifts_for_house_1(house) >= goal
+    end)
+  end
+
+  def p2 do
+    goal = input()
+
+    Stream.iterate(1, &(&1 + 1))
+    |> Enum.find(fn house ->
+      gifts_for_house_2(house) >= goal
+    end)
+  end
+
+  def gifts_for_house_1(house) do
+    elves =
+      house
+      |> divisors()
+      |> Enum.sum()
+
+    elves * 10
+  end
+
+  defp gifts_for_house_2(house) do
+    elves =
+      house
+      |> divisors()
+      |> Enum.filter(&(div(house, &1) <= 50))
+      |> Enum.sum()
+
+    elves * 11
+  end
+
+  defp divisors(n) do
+    e = n |> :math.sqrt() |> trunc
+
+    Enum.flat_map(1..e, fn
+      x when rem(n, x) != 0 -> []
+      x when x != div(n, x) -> [x, div(n, x)]
+      x -> [x]
+    end)
+  end
+
+  defp input do
+    input_string()
+    |> String.trim()
+    |> String.to_integer()
+  end
+end

README.md

@@ -6,7 +6,7 @@
 
 | Year | Stars | Languages |
 | - | - | - |
-| [2015] | **38/50** 🌟 | Elixir |
+| [2015] | **40/50** 🌟 | Elixir |
 | 2016 | | |
 | [2017] | **18/50** 🌟 | Haskell |
 | 2018 | | |