hash_intersection.rb

class Hash
  # Returns a nested intersection of two Hashes.
  #
  # This draws a parallel to &/intersect on Array and Set.
  #
  # FIXME: However, those methods are not recursive, so it may make more sense
  #        to give this another name.
  #
  # Examples:
  #
  #    {a: 1} & {a: 2}                                    #=> {}
  #    {a: 1} & {b: 2}                                    #=> {}
  #    {a: 1} & {a: 1, b: 2}                              #=> {a: 1}
  #    {a: 1, b: {c: 3, d: 10}} & {a: 1, b: {c: 3, d: 9}} #=> {a: 1, b: {c: 3}}
  #    {a: 1, b: {c: 3, d: 10}} & {a: 2, b: {c: 3, d: 9}} #=> {b: {c: 3}}
  #
  def &(other)
    return {} unless other.is_a?(Hash)

    shared_keys = keys & other.keys
    shared_keys.each_with_object({}) do |key, intersection|
      a, b = self[key], other[key]
      if a == b
        intersection[key] = a
      elsif a.is_a?(Hash) && b.is_a?(Hash)
        intersection[key] = a & b
      end
    end
  end
  alias intersection &
end