https://www.codewars.com/kata/586c7cd3b98de02ef60001ab/train/elixir

solution.ex

defmodule UniqueStrings do
  @moduledoc """
  Combinatronics exercise
  reference: https://www.codewars.com/kata/586c7cd3b98de02ef60001ab/train/elixir
  Goal: Calculate permutations without repetition
  Formula: P(n,r)=n!(n−r)!
  """
  
  def uniq_count(string) do
    string = String.upcase(string)
    factorial_of(s_length(string)) / c_dupe_factorial(string)
  end
  
  def factorial_of(0), do: 1
  def factorial_of(n) when n > 0 do
    n * factorial_of(n-1)
  end
  
  def s_length(string), do: String.codepoints(string) |> length
  
  def c_dupe_factorial(string) do
    String.codepoints(string)
    |> count_letters()
    |> Enum.reduce(1,fn({_, x},acc) -> factorial_of(x) * acc end)
  end
  
  def count_letters(string) do
    increment = fn(word, map) ->
      Map.put(map, word, (map[word] || 0) + 1)
    end

    string |> Enum.reduce(%{}, increment) |> IO.inspect
  end
end