1 files changed, 62 insertions, 0 deletions
diff --git a/2015/day20.exs b/2015/day20.exs
new file mode 100644
index 0000000..331eda9
--- /dev/null
+++ b/2015/day20.exs
@@ -0,0 +1,62 @@
+{presentTarget, ""} =
+  File.read!("inputs/day20.txt")
+  |> String.trim()
+  |> Integer.parse()
+
+countPresents1 = fn houseNumber ->
+  sqrt = floor(:math.sqrt(houseNumber))
+
+  Enum.filter(1..sqrt, &(rem(houseNumber, &1) == 0))
+  |> Enum.map(fn lower ->
+    higher = div(houseNumber, lower)
+
+    if higher != lower do
+      (higher + lower) * 10
+    else
+      lower * 10
+    end
+  end)
+  |> Enum.sum()
+end
+
+[firstHouse1] =
+  Stream.filter(1..div(presentTarget, 10), &(countPresents1.(&1) >= presentTarget))
+  |> Enum.take(1)
+
+IO.puts("First house part 1: #{firstHouse1}")
+
+countPresents2 = fn houseNumber ->
+  sqrt = floor(:math.sqrt(houseNumber))
+
+  Enum.filter(1..sqrt, &(rem(houseNumber, &1) == 0))
+  |> Enum.map(fn lower ->
+    higher = div(houseNumber, lower)
+
+    lowerDeliveries =
+      if higher < 50 do
+        lower
+      else
+        0
+      end
+
+    higherDeliveries =
+      if lower < 50 do
+        higher
+      else
+        0
+      end
+
+    if higher != lower do
+      (lowerDeliveries + higherDeliveries) * 11
+    else
+      lowerDeliveries * 11
+    end
+  end)
+  |> Enum.sum()
+end
+
+[firstHouse2] =
+  Stream.filter(1..div(presentTarget, 10), &(countPresents2.(&1) >= presentTarget))
+  |> Enum.take(1)
+
+IO.puts("First house part 2: #{firstHouse2}")