defmodule Tr do
  @moduledoc """
  functions to sum the numbers 0..n  both tail recursively and not tail recursively

  created: Oct 2022, gtowell
  """

  @doc """
  A tail recursive summ function. Uses a private version of the
  summ to introduce the extra variable needed for tail recursion.
  """
  def sumtr(b) do
    sumto(b,0)
  end

  #defp creates a function that is only accessible from within a module
  defp sumto(0,a) do
    a
  end
  defp sumto(b,a) do
    if rem(b,100 000 000)==0 do
      IO.puts b
    end
    sumto(b-1, a+b)
  end

  @doc """
  A non tail-recursive version of the summing function
  """
  def sumntr(a) do
    psumntr(a,a)
  end
  defp psumntr(0,_b) do
    0
  end

  #note b is not really important.  I is used only to
  # to make the IO look better
  defp psumntr(a,b) do
    if rem(a,10000000)==0 do
      IO.puts b-a
    end
    a+psumntr(a-1,b)
  end
end

defmodule Main do
  def main do
    limt = 100000000
    IO.puts Benchmark.measure(fn -> Tr.sumtr(limt) end )
    IO.puts Benchmark.measure(fn -> Tr.sumntr(limt) end )
  end
end

defmodule Benchmark do
  @moduledoc """
  Time a function
  """
  def measure(function) do
    function
    |> :timer.tc
    |> elem(0)
    |> Kernel./(1_000_000)
  end
end

Main.main()