defmodule LtiElixir do
  @moduledoc """
  A library to launch a LTI request
  """
  alias LtiElixir.{Credentials, OAuthData, LaunchParams}

  def credentials(url, key, secret) do
    %Credentials{url: url, key: key, secret: secret}
  end

  def oauth_params(%Credentials{key: key}) do
    %OAuthData{
      oauth_callback: "about:blank",
      oauth_consumer_key: key,
      oauth_version: "1.0",
      oauth_nonce: "11111111111",
      oauth_timestamp: timestamp(),
      oauth_signature_method: "HMAC-SHA1"
    }
  end

  def launch_data(%OAuthData{} = oauth, %LaunchParams{} = launch_params) do
    struct_to_list(oauth) ++ struct_to_list(launch_params)
    |> Enum.reduce(%{}, fn({key, value}, acc) -> Map.put(acc, key,  value) end)
    |> Enum.reduce([], fn({key, value}, acc) ->
      acc ++ ["#{key}=#{percent_encode(value)}"]
    end)
  end

  def signature(%Credentials{secret: secret} = creds, oauth_params, launch_params) do
    :sha
    |> :crypto.hmac(encode_secret(secret), base_string(creds, oauth_params, launch_params))
    |> Base.encode64()
  end

  defp encode_secret(secret) do
    "#{percent_encode(secret)}&"
  end

  defp base_string(%Credentials{url: url} = creds, oauth_params, launch_params) do
    query = oauth_params
            |> launch_data(launch_params)
            |> Enum.join("&")
            |> percent_encode()
    "POST&#{percent_encode(url)}&#{query}"
  end

  defp timestamp do
    {megasec, sec, _mcs} = :os.timestamp
    "#{megasec * 1_000_000 + sec}"
  end

  defp percent_encode({key, value}) do
    {percent_encode(key), percent_encode(value)}
  end

  defp percent_encode(other) do
    other
    |> to_string()
    |> URI.encode(&URI.char_unreserved?/1)
  end

  defp struct_to_list(struct), do: struct |> Map.from_struct() |> Map.to_list()
end