require 'mini_magick'
require 'tempfile'

class Quality
  def initialize(path)
    @path = path
  end

  def black_pixels
    colours.fetch('#000000')
  end

  def percentage
    white_pixels.to_f / total_colour_pixels.to_f
  end

  def white_pixels
    colours.fetch('#FFFFFF')
  end

  private

  def colours
    @colours ||= Histogram.new(negated_image_path).colors
  end

  def negated_image_path
    Negated.new(@path).path
  end

  def total_colour_pixels
    colours.sum { |_colour, pixels| pixels }
  end

  # Retrieves a count of pixels per colour
  # magick output.png  -format %c histogram:info:-
  class Histogram
    HEX_REGEX = %r{(?<pixels>\d*)\:.*(?<hex>#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3}))}

    def initialize(path)
      @path = path
    end

    def colors
      raw.split("\n").each_with_object({}) do |colour, hash|
        colour.match(HEX_REGEX) { |m| hash[m['hex']] =  m['pixels'].to_i }
      end
    end

    private

    def image
      @image ||= MiniMagick::Image.open(@path)
    end

    def raw
      image.run_command('convert', image.path, '-format', '%c', 'histogram:info:')
    end

    Colour = Struct.new(:hex, :pixels)
  end

  # Creates a pure black and white image with no transparency. e.g. all coloured pixels become black.
  class Negated
    def initialize(path)
      @path = path
    end

    def path
      convert
      tempfile.path
    end

    private

    # magick convert images/e86500ca-8bb9-4e48-b884-d1f1bc684f25_1542111343_screenshot.png -negate -threshold -0 -negate output.png
    def convert
      MiniMagick::Tool::Convert.new do |convert|
        convert << @path
        convert.flatten
        convert.negate
        convert.threshold('-0')
        convert.negate
        convert << tempfile.path
      end
    end

    def tempfile
      @tempfile ||= Tempfile.new
    end
  end
end