file_size.ex

defmodule Exfile.Processor.FileSize do
  @behaviour Exfile.Processor

  def call(file = %{ meta: %{ "file_size" => file_size }}, _, _) when is_integer(file_size) do
    { :ok, file }
  end

  def call(file, _, _) do
    local_file = Exfile.ProcessorChain.coerce_to_local_file(file)
    if File.exists?(local_file.path) do
      perform_file_cmd(file, local_file)
    else
      { :error, :unable_to_read_file }
    end
  end

  defp perform_file_cmd(file, local_file) do
    case System.cmd("wc", ["-c", local_file.path]) do
      { result, 0 } ->
        meta = Map.merge(local_file.meta, extract_meta(result))
        { :ok, %{ file | meta: meta }}
      _             -> { :error, :unable_to_read_file }
    end
  end

  # Processing `wc -c path` output
  defp extract_meta(result) do
    result
    |> String.trim
    |> String.split(" ")
    |> List.first
    |> String.to_integer
    |> (&%{ "file_size" => &1 }).()
  end
end