defmodule DataUrl do
  @moduledoc """
  Struct and functions for representing and parsing data: URLs
  """

  @default_mime_type "text/plain;charset=US-ASCII"
  defstruct mime_type: @default_mime_type, data: ""

  @type t() :: %DataUrl{
          mime_type: String.t(),
          data: binary()
        }

  @doc """
  Parse a data: URL into a %DataUrl{} struct
  """
  @spec parse(String.t()) :: {:ok, DataUrl.t()} | {:error, any()}
  def parse("data:" <> rest) do
    String.split(rest, ",", parts: 2)
    |> parse_parts
  end

  def parse(_), do: {:error, "missing \"data:\" scheme"}

  defp parse_parts([_]), do: {:error, "missing comma"}

  defp parse_parts([prefix, data]) do
    with {:ok, data} <- parse_data(String.ends_with?(prefix, ";base64"), data),
         prefix <- String.replace_suffix(prefix, ";base64", "") do
      {:ok, %DataUrl{mime_type: get_mime_type(prefix), data: data}}
    else
      :error -> {:error, "invalid base64"}
      x -> {:error, {"unknown error", x}}
    end
  end

  defp get_mime_type(""), do: @default_mime_type
  defp get_mime_type(mime_type) when is_binary(mime_type), do: mime_type

  defp parse_data(true, data), do: Base.decode64(data)
  defp parse_data(false, data), do: {:ok, URI.decode(data)}
end