glitchylicious

defmodule Glitchylicious do
  @moduledoc """
  Produces magnificent glitches,
  by corrupting some jpg bytes.
  
  Pretty much stolen from: georg @ http://fishnation.de/

  Take a look at his js experiments, cool stuff!

  Example:

      alias Glitchylicious, as: G

      def glitch do
        G.glitch("./myimage.jpg", %{iter: 10, amount: 50, seed: 10}
      end
  """

  @doc """
  Glitches jpg data.

  Takes an input path (original image)
  as well as a config map. Returns corrupted binary data.
  
  Config map should include the following parameters:

      iter   - number of iterrations to perform.
      amount - corruption severety.
      seed   - seed value.
  """
  @spec glitch(String.t, map) :: binary
  def glitch(input, %{iter: iter, amount: amount, seed: seed}) do
    raw = File.read!(input)
    do_glitch(raw, header_size(raw), iter, amount, seed)
  end

  defp do_glitch(image, len, iter, amount, seed) do
    do_glitch(image, len, 0, iter, amount / 100, seed / 100)
  end
  defp do_glitch(image, _len, i, iter, _amount, _seed) when i == iter do
    image
  end
  defp do_glitch(image, len, i, iter, amount, seed) do
    max_index = byte_size(image) - len - 4
    px_min = round(max_index / iter * i)
    px_max = round(max_index / iter * (i + 1))
    delta = px_max - px_min
    px_i = round(px_min + delta * seed)
    
    byte_index = if px_i > max_index,
    do: len + max_index,
    else: len + px_i

    replace(image, byte_index, Float.floor(amount * 256) |> trunc)
    |> do_glitch(len, i + 1, iter, amount, seed)
  end
  
  defp header_size(raw), do: header_size(raw, 0)
  defp header_size(<< 255, 218, _data::binary >>, acc), do: acc + 2
  defp header_size(<< _a::8, b::8, data::binary >>, acc), do: header_size(<< b :: 8, data :: binary >>, acc + 1)
  
  defp replace(image, ind, replacement) do
    << head :: binary-size(ind), _ :: 8, rest :: binary >> = image
    << head :: binary, replacement, rest :: binary >>
  end
end