require "active_storage/previewer"

class ActiveStorage::Previewer::OfficePreviewer < ActiveStorage::Previewer
  ACCEPTABLE_CONTENT_TYPES = [
    "application/msword", # .doc
    "application/vnd.openxmlformats-officedocument.wordprocessingml.document", # .docx

    "application/vnd.ms-powerpoint", # .ppt
    "application/vnd.openxmlformats-officedocument.presentationml.presentation", # .pptx

    "application/vnd.ms-excel", # .xls
    "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" # .xlsx
  ]

  class << self
    def accept?(blob)
      blob.content_type.in?(ACCEPTABLE_CONTENT_TYPES) && soffice_exists?
    end

    def soffice_exists?
      if @soffice_exists.nil?
        @soffice_exists = system(soffice_path, "--version", out: File::NULL, err: File::NULL)
      else
        @soffice_exists
      end
    end

    def soffice_path
      ActiveStorage.paths[:soffice] || "soffice"
    end
  end

  def preview
    download_blob_to_tempfile do |input|
      draw_poster_image_from input do |output|
        yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
      end
    end
  end

  private
    def draw_poster_image_from(file, &block)
      system soffice_path, "--headless", "--invisible", "--convert-to", "png",
        "--outdir", tmpdir, file.path, out: File::NULL, err: File::NULL
      consume "#{tmpdir}/#{File.basename(file.path, ".*")}.png", &block
    end

    def consume(path, &block)
      File.open(path, "rb", &block)
    ensure
      File.delete(path)
    end

    def soffice_path
      self.class.soffice_path
    end
end