add day9 part1 solution:

lib/day9.ex

@@ -0,0 +1,80 @@
+defmodule Day9 do
+  def init_state(p) when p == :part1 do
+    { {0,0}, {0,0}, MapSet.new() }
+  end
+
+  def execute(line, state) do
+    if String.trim(line) == "" do
+      state
+    else
+      [command, count] = line |> String.split(" ");
+      { count, _ } = Integer.parse(count);
+
+      move_rope = fn (movement, {head, tail, tail_positions}) ->
+        head = movement |> get_movement_tuple() |> add_2_tuple(head);
+        tail = tail |> keep_min_distance(head, 1)
+        tail_positions = MapSet.put(tail_positions, tail)
+        {head, tail, tail_positions}
+      end
+
+      Enum.reduce(
+        1..count,
+        state,
+        fn (_, state) -> 
+          #IO.puts(command)
+          #IO.puts(inspect(state))
+          new_state = move_rope.(command, state) 
+          #IO.puts(inspect(new_state))
+          new_state
+        end
+      );  
+    end
+  end
+
+  defp keep_min_distance(tail, head, min_distance) do
+    difference = distance_vector(tail, head)
+    #IO.puts(inspect(difference))
+
+    if max_coord_difference(tail, head) <= min_distance do 
+      tail 
+    else
+      {a, b} = difference
+      megan = fn (x) -> if x != 0 do
+          if x > 0 do 1 else -1 end
+        else
+          0
+        end
+      end
+      add_2_tuple(tail, {megan.(a), megan.(b)})
+    end
+  end
+
+  def get_answer(state) do
+    {_, _, tail_positions } = state
+    MapSet.size(tail_positions)
+  end
+
+  defp get_movement_tuple(direction) do
+    case direction do
+      "R" -> {1, 0}
+      "L" -> {-1, 0}
+      "U" -> {0, 1}
+      "D" -> {0, -1}
+    end
+  end
+
+  defp add_2_tuple(x, y) do
+    { elem(x,0) + elem(y,0), elem(x,1) + elem(y,1) }
+  end
+
+  defp distance_vector(x, y) do 
+    { elem(y,0) - elem(x,0), elem(y,1) - elem(x,1) }
+  end
+
+  defp max_coord_difference(x, y) do 
+    {a, b} = { abs(elem(x,0)-elem(y,0)), abs(elem(x,1)-elem(y,1)) }
+    #IO.puts("max_coord_difference: " <> inspect({a,b}))
+    max(a, b)
+  end
+end
+

main.exs

@@ -1 +1 @@
-AOCRunner.run(Day3, :part1)
+AOCRunner.run(Day9, :part1)