dreamega_ico_encoder

defmodule DreamegaIcoEncoder do
  @moduledoc """
  Generate multi-size ICO favicon files from PNG image data.

  The ICO format consists of a 6-byte header, 16-byte directory entries
  per image, and concatenated PNG data.
  """

  @default_sizes [16, 32, 48, 64, 128, 256]

  @doc """
  Returns the default icon sizes.
  """
  def default_sizes, do: @default_sizes

  @doc """
  Generate an ICO file from a list of `{size, png_binary}` tuples.

  ## Parameters

    - `png_images` - a list of `{size, png_data}` tuples where `size` is
      the pixel dimension and `png_data` is the raw PNG binary.

  ## Returns

  A binary containing the complete ICO file.

  ## Example

      png16 = File.read!("icon-16.png")
      png32 = File.read!("icon-32.png")
      ico = DreamegaIcoEncoder.generate([{16, png16}, {32, png32}])
      File.write!("favicon.ico", ico)
  """
  @spec generate([{non_neg_integer(), binary()}]) :: binary()
  def generate(png_images) when is_list(png_images) do
    count = length(png_images)

    # ICO header (6 bytes): reserved=0, type=1, count
    header = <<0::little-16, 1::little-16, count::little-16>>

    # Data offset base: header(6) + directory entries(16 * count)
    data_offset_base = 6 + 16 * count

    {directory, _final_offset} =
      Enum.reduce(png_images, {<<>>, data_offset_base}, fn {size, png_data}, {dir_acc, offset} ->
        w = if size >= 256, do: 0, else: size
        h = w
        data_size = byte_size(png_data)

        entry = <<
          w::8,
          h::8,
          0::8,
          0::8,
          1::little-16,
          32::little-16,
          data_size::little-32,
          offset::little-32
        >>

        {dir_acc <> entry, offset + data_size}
      end)

    # Concatenate all PNG data
    image_data =
      png_images
      |> Enum.map(fn {_size, png_data} -> png_data end)
      |> IO.iodata_to_binary()

    header <> directory <> image_data
  end
end