diff --git a/lib/day15/day15.ex b/lib/day15/day15.ex
new file mode 100644
index 0000000..02d9671
--- /dev/null
+++ b/lib/day15/day15.ex
@@ -0,0 +1,135 @@
+defmodule Day15 do
+  def part1(example \\ false) do
+    find_number_spoken_at(example, 2020)
+  end
+
+  def part2(example \\ false) do
+    find_number_spoken_at(example, 30_000_000)
+  end
+
+  def find_number_spoken_at(example, final_turn) do
+    inputs = parse_input(example)
+
+    # inputs
+    # |> Enum.reverse()
+    # |> step_times(final_turn - length(inputs))
+    # |> List.first()
+
+    # numbers = parse_input(example)
+
+    inputs
+    |> Enum.with_index()
+    |> Enum.map(fn {e, i} -> {e, i + 1} end)
+    |> Map.new()
+    |> do_van_eck(List.last(inputs), length(inputs) + 1, final_turn)
+
+    # |> step_until_turn(final_turn, length(numbers) + 1)
+    # |> what_was_spoken_on_turn(final_turn)
+  end
+
+  def parse_input(example) do
+    case example do
+      1 -> "0,3,6"
+      2 -> "1,3,2"
+      3 -> "2,1,3"
+      4 -> "1,2,3"
+      5 -> "2,3,1"
+      6 -> "3,2,1"
+      7 -> "3,1,2"
+      _ -> "9,12,1,4,17,0,18"
+    end
+    |> String.split(",")
+    |> Enum.map(&String.to_integer/1)
+  end
+
+  def do_van_eck(_, prev, turn, count) when turn == count + 1 do
+    prev
+  end
+
+  def do_van_eck(map, prev, turn, count) do
+    # IO.inspect(map)
+
+    new_val =
+      if Map.has_key?(map, prev) do
+        turn - map[prev]
+      else
+        0
+      end
+
+    # IO.inspect(map)
+    IO.puts("turn: #{turn} \tprev: #{prev} \tnew: #{new_val}")
+
+    do_van_eck(Map.put(map, prev, turn - 1), new_val, turn + 1, count)
+  end
+
+  def step_van_eck([prev | rest] = sequence) do
+    next =
+      case Enum.find_index(rest, &(&1 == prev)) do
+        nil -> 0
+        index_in_rest -> index_in_rest + 1
+      end
+
+    [next | sequence]
+  end
+
+  def step_times(sequence, 0) do
+    sequence
+  end
+
+  def step_times(sequence, count) do
+    step_times(step_van_eck(sequence), count - 1)
+  end
+
+  # def step(map, turn) do
+  #   # IO.puts("current turn: #{turn}")
+
+  #   {prev_spoken, prev_spoken_on_turns} = what_was_spoken_on_turn(map, turn - 1)
+
+  #   # IO.puts("prev spoken: #{prev_spoken}")
+  #   # IO.puts("prev spoken on turns: #{inspect(prev_spoken_on_turns)}")
+
+  #   number_to_speak =
+  #     case prev_spoken_on_turns do
+  #       [_] ->
+  #         0
+
+  #       [most_recent, second_most_recent | _] ->
+  #         most_recent - second_most_recent
+  #         # List.last(prev_spoken_on_turns) - Enum.at(prev_spoken_on_turns, -2)
+  #     end
+
+  #   # IO.puts("will speak #{number_to_speak}")
+
+  #   Map.update(map, number_to_speak, [turn], fn turns -> [turn | turns] end)
+  # end
+
+  # def step_until_turn(map, final_turn, turn) when final_turn + 1 == turn do
+  #   map
+  # end
+
+  # def step_until_turn(map, final_turn, turn) do
+  #   # IO.inspect(map)
+  #   step_until_turn(step(map, turn), final_turn, turn + 1)
+  # end
+
+  # # def step(numbers, turn) do
+  # #   # prev = Map.get(map, cur - 1)
+  # #   {prev, prev_turn} = Enum.find(numbers, &(elem(&1, 1) == turn - 1))
+
+  # #   IO.puts("turn #{turn}")
+  # #   IO.puts("prev #{prev}")
+
+  # #   new =
+  # #     case Enum.find(numbers, &(elem(&1, 0) == prev)) |> IO.inspect() do
+  # #       nil -> 0
+  # #       {_, ^prev_turn} -> 0
+  # #       {_, last_spoken_turn} -> turn - last_spoken_turn
+  # #     end
+
+  # #   # IO.inspect({prev, new})
+  # #   IO.puts("speak #{new}")
+
+  # #   [{new, turn} | Enum.reject(numbers, fn {n, _} -> n == new end)]
+  # #   |> IO.inspect()
+  # # end
+end
diff --git a/lib/day15/test.swift b/lib/day15/test.swift
new file mode 100644
index 0000000..83a7993
--- /dev/null
+++ b/lib/day15/test.swift
@@ -0,0 +1,22 @@
+import Foundation
+
+func vanEckSequence(start: [Int], maxLength: Int) -> [Int] {
+    let toCompute = maxLength - start.count
+    var seq = start + Array(repeating: 0, count: toCompute)
+    // print(seq.count)
+    
+    for i in start.count..<maxLength {
+        // print("i: \(i)")
+        if let prevIndex = seq[0..<(i - 1)].lastIndex(of: seq[i - 1]) {
+            seq[i] = (i - 1) - prevIndex
+        } else {
+            seq[i] = 0
+        }
+    }
+    
+    return seq
+}
+
+let seq = vanEckSequence(start: [9,12,1,4,17,0,18], maxLength: 30000000)
+// print(seq)
+print(seq.last!)