date_params

defmodule DateParams do
  @moduledoc """
  An Elixir library that converts specified values within maps into native
  `Date`, `DateTime`, `NaiveDateTime`, and `Time` structs.
  """

  @doc"""
  Convert the values within a map to DateTime structs for the given keys.
  The keys given follows the format for accessing nested structures like
  in `Kernel.get_in/2`.
  """
  def to_structs(map, keys) do
    Enum.reduce(keys, map, &transform_values/2)
  end

  @doc"""
  Converts the value to a DateTime struct
  """
  def to_struct(value), do: transform_value(value)

  # Converts a value to integer for creating dates
  defp to_integer(value) when is_atom(value),
    do: value |> Atom.to_string |> String.to_integer
  defp to_integer(value) when is_bitstring(value),
    do: String.to_integer(value)
  defp to_integer(value),
    do: value

  # ISO 8601 date and time without seconds (Chrome's datetime input submits this way)
  defp transform_value(value = <<_::4-bytes, ?-, _::2-bytes, ?-, _::2-bytes, sep,
                                 _::2-bytes, ?:, _::2-bytes>>) when sep in [?\s, ?T] do
    transform_value(value <> ":00")
  end

  # ISO 8601 date and time with seconds
  defp transform_value(value = <<_::4-bytes, ?-, _::2-bytes, ?-, _::2-bytes, sep,
                                 _::2-bytes, ?:, _::2-bytes, ?:, _::2-bytes>>) when sep in [?\s, ?T] do
    case Calendar.NaiveDateTime.Parse.iso8601(value) do
      {:ok, datetime, nil} -> {value, datetime}
      {:error, _}          -> {value, value}
    end
  end

  # Phoenix-style date-time param maps
  defp transform_value(value = %{"year" => year, "month" => month, "day" => day,
                                 "hour" => hour, "minute" => minute, "second" => second}) do
    datetime = NaiveDateTime.new(to_integer(year), to_integer(month), to_integer(day),
                                 to_integer(hour), to_integer(minute), to_integer(second))
    case datetime do
      {:ok, date} -> {value, date}
      {:error, _} -> {value, value}
    end
  end

   # ISO 8601 dates
  defp transform_value(value = <<_::4-bytes, ?-, _::2-bytes, ?-, _::2-bytes>>) do
    case Calendar.Date.Parse.iso8601(value) do
      {:ok, date} -> {value, date}
      {:error, _} -> {value, value}
    end
  end

  # Phoenix-style date param maps
  defp transform_value(value = %{"year" => year, "month" => month, "day" => day}) do
    case Date.new(to_integer(year), to_integer(month), to_integer(day)) do
      {:ok, date} -> {value, date}
      {:error, _} -> {value, value}
    end
  end
  
  # Fallback
  defp transform_value(value), do: {value, value}

  # Get and covert for the given keys
  defp transform_values(keys, map) do
    {_, transformed} = get_and_update_in(map, keys, &transform_value/1)
    transformed
  end
end