defmodule WeChat.Utils do
  use Timex

  @random_alphanumeric Enum.concat([?a..?z, ?A..?Z, 48..57])

  alias WeChat.{JSSDKSignature, CardSignature}

  def random_string(length) when length > 0 do
    @random_alphanumeric
    |> Enum.take_random(length)
    |> List.to_string()
  end

  def now_unix() do
    Timex.to_unix(Timex.now())
  end

  @doc """
  To configure and load WeChat JSSDK in the target page's url properly, use `jsapi_ticket` and `url` to generate an signature for this scenario.
  """
  @spec sign_jssdk(jsapi_ticket :: String.t(), url :: String.t()) :: JSSDKSignature.t()
  def sign_jssdk(jsapi_ticket, url) do
    url = String.replace(url, ~r/\#.*/, "")
    noncestr = random_string(16)
    timestamp = now_unix()
    str_to_sign = "jsapi_ticket=#{jsapi_ticket}&noncestr=#{noncestr}&timestamp=#{timestamp}&url=#{url}"
    signature = :crypto.hash(:sha, str_to_sign) |> Base.encode16(case: :lower)
    %JSSDKSignature{value: signature, timestamp: timestamp, noncestr: noncestr}
  end

  @doc """
  To initialize WeChat Card functions via JSSDK, use `wxcard_ticket`, `card_id` to generate an signature for this scenario.
  https://developers.weixin.qq.com/doc/offiaccount/OA_Web_Apps/JS-SDK.html#65
  """
  @spec sign_card(list :: [String.t()]) :: CardSignature.t()
  @spec sign_card(wxcard_ticket :: String.t(), card_id :: String.t()) :: CardSignature.t()
  @spec sign_card(wxcard_ticket :: String.t(), card_id :: String.t(), openid :: String.t()) :: CardSignature.t()
  def sign_card(wxcard_ticket, card_id), do: sign_card([wxcard_ticket, card_id])
  def sign_card(wxcard_ticket, card_id, openid), do: sign_card([wxcard_ticket, card_id, openid])
  def sign_card(list) do
    noncestr = random_string(16)
    timestamp = now_unix()
    timestamp_str = Integer.to_string(timestamp)
    str_to_sign = Enum.sort([timestamp_str, noncestr | list]) |> Enum.join()
    signature = :crypto.hash(:sha, str_to_sign) |> Base.encode16(case: :lower)
    %CardSignature{value: signature, timestamp: timestamp, noncestr: noncestr}
  end

end