fluxion

defmodule Fluxion.NewtonSolver do
  @moduledoc """
  A nonlinear equation solver which uses Newton's Method to find the zeros of
  an arbitrary function.
  """

  @doc """
  Find a zero of the provided function starting from an initial guess.

  # Example

      iex> defmodule XSq do
      ...>   @behaviour Fluxion.Function
      ...>   def f(x), do: x*x - 4
      ...>   def dfdx(x), do: 2*x
      ...> end
      iex> computed_zero = Fluxion.NewtonSolver.find_zero(XSq, 10)
      iex> Fluxion.Math.is_equal(computed_zero, 2)
      :true
      iex> computed_zero = Fluxion.NewtonSolver.find_zero(XSq, -0.1)
      iex> IO.puts computed_zero
      iex> Fluxion.Math.is_equal(computed_zero, -2)
      :true

  """
  @spec find_zero(Fluxion.Function.t(), number(), number()) :: number
  def find_zero(function, guess, max_iterations \\ 100) do
    iterate(function, guess, 0, max_iterations)
  end

  # rase an error if the solution cannot be found within the max iteration
  # count
  defp iterate(function, _current_guess, max_iterations, max_iterations) do
    raise "cannot find the zero for #{Kernel.inspect(function)} in #{
            max_iterations
          } iterations"
  end

  # compute the next guess for the zero
  # return the guess if it's close enough to zero, otherwise iterate again
  defp iterate(function, current_guess, iteration, max_iterations) do
    next_guess =
      current_guess - function.f(current_guess) / function.dfdx(current_guess)

    if Fluxion.Math.is_equal(function.f(next_guess), 0) do
      next_guess
    else
      iterate(function, next_guess, iteration + 1, max_iterations)
    end
  end
end