model.exs

defmodule Model do
  @doc """
  A function that given the current value calculates the
  difference between the current and next value.
  """
  @type t :: (value() -> dvalue())

  @doc """
  The value the model will mutate.
  """
  @type value :: integer()

  @doc """
  The difference in the value returned by `t:t()`.
  """
  @type dvalue :: value()
  @type points :: [value()]
  
  @type run_opts :: [
    start_value: integer(),
    start_at: integer(),
    max_t: integer(),
    dt: integer()
  ]

  @spec run(t(), run_opts()) :: value()
  def run(model, opts \\ []) do
    start_value = Keyword.get(opts, :start_value, 0)
    start_at = Keyword.get(opts, :start_at, 0)

    run(model, start_value, start_at, opts)
  end

  @spec run(t(), value(), integer(), run_opts()) :: value()
  def run(model, value, i, opts) do
    max_t = Keyword.get(opts, :max_t, 100)
    dt = Keyword.get(opts, :dt, 1)

    if dt <= max_t do
      value = value + model(value) * dt
      run(model, value, i + dt, opts)
    else
      value
    end
  end
end