Knights Dialer in Elixir.

KnightsDialer.ex

defmodule KnightsDialer do
  @nieghbors_map %{
    1 => [6, 8],
    2 => [7, 9],
    3 => [4, 8],
    4 => [3, 9, 0],
    5 => [],
    6 => [1, 7, 0],
    7 => [2, 6],
    8 => [1, 3],
    9 => [2, 4],
    0 => [4, 6]
  }

  def start, do: Agent.start_link(fn -> %{} end, name: __MODULE__)

  defp get_cache(key), do: Agent.get(__MODULE__, &Map.get(&1, key))

  defp put_cache(value, key) do
    Agent.update(__MODULE__, &Map.put(&1, key, value))
    value
  end

  def count_sequences(start, _) when start < 0 or start > 9 do
    raise "Start should be bewteen 0 and 9"
  end

  def count_sequences(_start, 0), do: 1

  def count_sequences(start, hops) do
    case get_cache({start, hops}) do
      nil ->
        @nieghbors_map
        |> Map.get(start)
        |> Enum.reduce(0, fn next, total -> total + count_sequences(next, hops - 1) end)
        |> put_cache({start, hops})

      cached ->
        cached
    end
  end
 end