set_cov_result.rb

require 'coverage'

module Coverage
  class << self
    alias :old_result :result
    alias :old_peek_result :peek_result

    attr_writer :result

    def result
      if @result
        diff_results @result, old_result
      else
        old_result
      end
    end

    def peek_result
      if @result
        diff_results @result, old_peek_result
      else
        old_peek_result
      end
    end

    def diff_results old, new
      (new.keys | old.keys).each_with_object({}) do |file, hash|
        old_cov = old.fetch(file, [])
        new_cov = new[file]
        hash[file] = new_cov.zip(old_cov).map { |n, o| n && o ? n - o : n }
      end
    end
  end
end

Coverage.start
require 'a'
p Coverage.peek_result
Coverage.result = Coverage.peek_result
require 'b'
p Coverage.peek_result