diff.rb

class Diff
  def initialize(a, b)
    @a, @b = a, b
  end

  def left
    diff(@a, @b)
  end

  def right
    diff(@b, @a)
  end

private

  def diff(a, b)
    b.dup.delete_if { |k, v| a[k] == v }
      .merge!(a.dup.delete_if { |k, v| b.has_key?(k) })
  end
end

diff = Diff.new({a: 2, b: 1}, {a: 2, b: 3})
p diff.left  # => {:b => 3}
p diff.right # => {:b => 1}