001.rb

require 'mini_magick'

class Walker

  def initialize
    @filenames = []

  end

  def walk(dirname)
    # puts "directory: #{dirname}"

    Dir.foreach(dirname) do |filename|

      full_filename = File.join(dirname, filename)

      if File.directory? full_filename
        if filename != '.' && filename != '..'
          walk(full_filename)
        end
      else
        @filenames << full_filename
        puts "file: #{full_filename}"
      end

    end
  end

  def filenames
    @filenames
  end

end

class Watermaker

  def initialize(watermark_path)
    @watermark_image = MiniMagick::Image.open(watermark_path)
  end

  def place_watermark(file_names)
    puts "<Watermark.place_watermark> file_names.count = #{file_names.count}"

    file_names.each do |original_path|
      original_image = MiniMagick::Image.open(original_path)

      result = original_image.composite(@watermark_image) do |c|
        c.gravity 'Center'
      end

      puts "<Watermark.place_watermark> write #{original_path}."
      result.write original_path
    end

  end

end

walker = Walker.new
walker.walk('/home/scout/git/untitled/work/masterkottage/flow/20170215_watermark/all_together/files/uploads')

watermarker = Watermaker.new 'files/watermark.png'
watermarker.place_watermark walker.filenames