defmodule Exexif do

  alias Exexif.Decode
  alias Exexif.Tag

  @max_exif_len          2*(65536+2)

  @image_start_marker   0xffd8
  # @image_end_marker     0xffd9 # NOT USED

  @app1_marker 0xffe1


  def exif_from_jpeg_file(name) when is_binary(name) do
    {:ok, buffer} = File.open(name,
                              [:read],
                              fn (file) ->
                                IO.binread(file, @max_exif_len)
                              end)
    exif_from_jpeg_buffer(buffer)
  end

  def exif_from_jpeg_file!(name) when is_binary(name) do
    case exif_from_jpeg_file(name) do
      {:ok, result}   -> result
      {:error, error} -> raise(Exexif.ReadError, type: error, file: name)
    end
  end

  def exif_from_jpeg_buffer(<< @image_start_marker :: 16, rest :: binary>>) do
    read_exif(rest)
  end

  def exif_from_jpeg_buffer(_), do: {:error, :not_a_jpeg_file}

  def exif_from_jpeg_buffer!(buffer) do
    case exif_from_jpeg_buffer(buffer) do
      {:ok, result}   -> result
      {:error, error} -> raise Exexif.ReadError, type: error, file: nil
    end
  end

  def read_exif(<<
                  @app1_marker :: 16,
                  _len         :: 16,
                  "Exif"       :: binary,
                  0            :: 16,
                  exif         :: binary
                >>) do

    <<
      byte_order   :: 16,
      forty_two    :: binary-size(2),
      offset       :: binary-size(4),
      _rest        :: binary          >> = exif

    endian = case byte_order do
               0x4949 -> :little
               0x4d4d -> :big
             end

    read_unsigned = fn (value) -> :binary.decode_unsigned(value, endian) end

    42     = read_unsigned.(forty_two)  # sanity check
    offset = read_unsigned.(offset)

    { :ok, reshape(read_ifd({exif, offset, read_unsigned})) }
  end

  def read_exif(<< 0xff :: 8, _number :: 8, len :: 16, data :: binary>>) do
    # IO.puts "Skip segment #{number}"
    skip_segment(len-2, data)
    |> read_exif()
  end

  def read_exif(_) do
    { :error, :no_exif_data_in_jpeg }
  end

  def skip_segment(len, data) do
    << _segment :: size(len)-unit(8), rest :: binary >> = data
    rest
  end


  def read_ifd({exif, offset, ru} = context) do
    << _ :: binary-size(offset), tag_count :: binary-size(2), tags :: binary  >> = exif
    read_tags(ru.(tag_count), tags, context, :tiff, [])
  end


  def read_tags(0, _tags, _context, _type, result) do
    Enum.into(result, %{})
  end

  def read_tags(count,
                <<
                  tag :: binary-size(2),
                  format :: binary-size(2),
                  component_count :: binary-size(4),
                  value :: binary-size(4),
                  rest :: binary
                >>,
                {_exif, _offset, ru} = context,
                type,
                result) do
    tag    = ru.(tag)
    format = ru.(format)
    component_count = ru.(component_count)
    value = Tag.value(format, component_count, value, context)
    {name, description} = Decode.tag(type, tag, value)
    kv = case name do
      :exif      -> { :exif, read_exif(value, context) }
      :gps       -> { :gps,  read_gps(value, context) }
      _          -> { name,  description }
    end
    read_tags(count-1, rest, context, type, [ kv | result ])
  end

  def read_tags(_, _, _, _, result) do # Handle malformed data
    Enum.into(result, %{})
  end

  def read_exif(exif_offset, {exif, _offset, ru} = context) do
    << _ :: binary-size(exif_offset), count :: binary-size(2), tags :: binary >> = exif
    count = ru.(count)
    read_tags(count, tags, context, :exif, [])
  end

  def read_gps(gps_offset, {gps, _offset, ru} = context) do
    << _ :: binary-size(gps_offset), count :: binary-size(2), tags :: binary >> = gps
    struct(Exexif.Data.Gps, read_tags(ru.(count), tags, context, :gps, []))
  end

  defp reshape(result) do
    result
    |> extract_thumbnail
  end

  defp extract_thumbnail(result) do
    exif_keys = Map.keys(result.exif)
    result =  if Enum.all?(Exexif.Data.Thumbnail.fields, fn e -> Enum.any?(exif_keys, & &1 == e) end) do
                Map.put(
                  result,
                  :thumbnail,
                  struct(
                    Exexif.Data.Thumbnail,
                    Exexif.Data.Thumbnail.fields
                    |> Enum.map(fn e -> {e, result.exif[e]} end)
                    |> Enum.into(%{})
                  )
                )
              else
                result
              end
    %{result | exif: Map.drop(result.exif, Exexif.Data.Thumbnail.fields)}
  end
end