PDFSnapshotComparator.rb

# frozen_string_literal: true

class PDFSnapshotComparator
  def initialize(generated)
    @generated = generated
  end

  def same?(expectation)
    out1 = Tempfile.new(['', '.png'])
    out2 = Tempfile.new(['', '.png'])

    convert(@generated, out1.path)
    File.binwrite(expectation, File.binread(out1.path)) unless File.exist?(expectation)

    diff(out1.path, expectation_path, out2.path)
    histogram(out2.path).lines.count == 1
  end

  private

  def convert(pdf, output)
    MiniMagick::Tool::Convert.new do |convert|
      convert.density('300')
      convert << pdf
      convert.alpha('remove')
      convert.quality('150')
      convert.append.+ # rubocop:disable Lint/Void
      convert << output
    end
  end

  def diff(generated, expectation, output)
    MiniMagick::Tool::Convert.new do |convert|
      convert << generated
      convert << expectation
      convert.alpha('remove')
      convert.compose('difference')
      convert.composite
      convert << "png8:#{output}"
    end
  end

  def histogram(file)
    MiniMagick::Tool::Convert.new do |convert|
      convert << file
      convert.define('histogram:unique-colors=true')
      convert.format('%c') # rubocop:disable Style/FormatStringToken
      convert << 'histogram:info:-'
    end
  end
end

# USAGE:
PDFSnapshotComparator.new(generated_file.path).same?(expected_file.path)
# If expected file does not exists, it'll pass automatically, by creating it with generated file's content