defmodule Meta.Captcha do
  # We can't use a graphical-only captcha library such as
  # https://hex.pm/packages/captcha since it would break accessibility for
  # blind people. Let's make it simple maths instead!

  # Captcha expression tuning:
  @terms 1..6
  @operands ["+", "-", "*"]

  def validate(changeset, expected_result) do
    result = Ecto.Changeset.get_field(changeset, :captcha)
    if result == expected_result do
      changeset
    else
      Ecto.Changeset.add_error(changeset, :captcha, "does not match")
    end
  end

  def generate do
    left = @terms |> Enum.random
    right = @terms |> Enum.random
    operation = @operands |> Enum.random

    text = "#{left} #{operation} #{right}"
    value = case operation do
      "+" -> left + right
      "-" -> left - right
      "*" -> left * right
    end

    {text, value}
  end
end